A Systems Engineering Approach to Engine Cooling Design
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A Systems Engineering Approach to Engine Cooling Design...
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ABOUT THE AUTHORS PETER KANEFSKY Peter Kanefsky is the Supervisor of Vehicle Cooling Technology, Research & Vehicle Technology, Ford Motor Company. He has worked in the automotive industry 26 years and has 23 years experience in cooling system design and development. Mr. Kanefsky began his career as a student apprentice with British Leyland Truck and Bus Division, England in 1973. After graduating from University of Manchester Institute of Science and Technology with a Bachelors Degree in Mechanical Engineering in 1977, began working on heavy truck cooling systems. In 1980, Mr. Kanefsky joined Ford of Britain and worked as a cooling system development engineer on car programs, where he worked on the development of computational techniques for system performance optimization. In 1989, after completing a Masters Degree in Advanced Automotive Engineering at Loughborough University, Mr. Kanefsky, began an International Service assignment with Ford’s Alpha Simultaneous Engineering group in the U.S. where he worked on the development of Computational Fluid Dynamics techniques for cooling system airflow analysis. In 1991, Mr. Kanefsky returned to Europe as a Technical Specialist where he worked on the deployment of the tools and methodologies previously developed in the U.S. In 1994, he co-authored a cooling system design guide for use internally within Ford, and as a “subject matter expert” was key member of a Ford2000 global re-engineering team for Cooling, Thermal Management and Aerodynamics. In 1995, Mr. Kanefsky returned to the U.S. and become Supervisor of CAE tools and methodology within a newly formed Thermal and Aerodynamic Systems Engineering group. In 1997, Mr.Kanefsky took his current assignment where he has responsibility for core and advanced vehicle cooling technology.
VALERIE NELSON Valerie Nelson P.E., is a Technical Specialist in engine cooling systems for Ford. She has 17 years experience in engine cooling system design. Ms. Nelson began her career in 1982 at Buick Motor Division, responsible for engine cooling design and development for the full size Buicks. After 5 years Ms. Nelson moved to Ford to work on the cooling system for the Ford Probe. While at Ford, she has pioneered the use of 1-D CAE tools in the development of the cooling system hydraulic circuit, and has authored an SAE paper "A Model to Simulate the Behavior of an Automotive Thermostat". In 1996, she acquired a Masters Degree from the University of Michigan-Dearborn in Mechanical Engineering with a specialization in heat transfer and fluid flow. Ms. Nelson was appointed Technical Specialist - Cooling Systems in 1996. As a Technical Specialist, she provides subject matter expertise to vehicle programs, develops new tools and methodology, leads the deployment of new design concepts and provides technical mentoring to over 60 cooling design engineers worldwide. As part of the technical mentoring, she has developed cooling system design guides, conducted technical seminars and has recently written a self-guided web-based cooling design course. Ms. Nelson is a licensed professional engineer (State of Michigan, 1992), an 18 year member of SAE, and an active member of the SAE cooling standards committee.
MARY RANGER Mary Ranger is a Senior Product Design Engineer at Ford, where she has 16 years experience, including three years as a subject matter expert in engine coolant technology. Ms. Ranger has a Bachelor of Science degree in Metallurgy and Materials Science Engineering and Bachelor of Arts degree in French from Lehigh University. She is currently working on a Masters degree in Quality Engineering, at Eastern Michigan University. Ms. Ranger began her career at Ford’s heat exchanger manufacturing facility in Connersville, Indiana in 1983, where she held positions as manufacturing engineer, production supervisor and plant metallurgist. As plant metallurgist, Ms. Ranger was involved with evaluating the effect of engine coolant on the corrosion resistance of brazed aluminum radiators. In 1995, she moved to Dearborn, Michigan where she held positions as a Materials Engineer and as a Product Design Engineer, within Ford’s Climate Control Division. Throughout this period Ms. Ranger continued to provide technical expertise in the field of engine coolant technology and corrosion protection. Her expertise in this area was recently acknowledged when transferred to Ford’s Research & Vehicle Technology group, where she has responsibility for the development and deployment of engine coolant technology. Ms. Ranger is a member of SAE and the ASTM D-15 subcommittee on engine coolants.
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Table of Contents
Abstract . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .1 Part 1 - Systems Engineering Fundamentals . . . . . . . . . . . . . . . . . . . . . . . . . .3 Part 2 - Engine Cooling Design from a Systems Engineering Perspective Section 1 - Vehicle and System Level Requirements. . . . . . . . . . . . . . . .21 Section 2 - Engine Internal Flow Subsystem . . . . . . . . . . . . . . . . . . . . . .29 Section 3 - External Flow Subsystem . . . . . . . . . . . . . . . . . . . . . . . . . . . .33 Section 4 - Heat Dissipation Subsystem . . . . . . . . . . . . . . . . . . . . . . . . . .39 Section 5 - Airflow Subsystem . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .63 Section 6 - Pressure Control Subsystem . . . . . . . . . . . . . . . . . . . . . . . . .79 Section 7 - Temperature Control Subsystem . . . . . . . . . . . . . . . . . . . . . .83 Section 8 - Coolant Requirements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .89 Section 9 - Coolant Fill and Drain Subsystem . . . . . . . . . . . . . . . . . . . . .99 Section 10 - Deaeration Subsystem . . . . . . . . . . . . . . . . . . . . . . . . . . . .103 Section 11 - Coolant Containment & Sealing Subsystem . . . . . . . . . . .105 Section 12 - Heater Subsystem . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .107 References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 111 Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 116 Appendix A - The Making of An Automobilist, by H.A. Grant - 1906. . . . . . 118
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1999-01-3780
A Systems Engineering Approach to Engine Cooling Design Peter Kanefsky Valerie Nelson Mary Ranger Ford Motor Company
Copyright © 1998 Society of Automotive Engineers, Inc.
ABSTRACT
INTRODUCTION - PART 1
This paper is divided into two parts:
Systems Engineering has been extensively used in the aviation and aerospace industries for many years. Its use in the automotive industry is relatively new and derives largely from the increased complexity of the vehicle being designed.
•
Part 1 - Systems engineering fundamentals
•
Part 2 - Engine cooling design from a systems engineering perspective
EARLY VEHICLE DESIGN
In Part 1, we explain how the task of designing a complex system can be made easier by the application of Systems Engineering principles. (This part is self contained and may be of general interest to those who have no special interest in engine cooling).
Engineers have successfully designed and built vehicles for both commercial and personal use for over 100 years. Initially, these vehicles were highly individualistic, custom built and assembled by craftsmen, with individual components being tailored to fit. As time progressed, the dimensions and specifications of individual components became standardized, allowing parts to be fitted together without adjustment. This standardization ultimately led to mass production techniques.
Systems Engineering provides three key benefits: •
•
•
It facilitates communication: •
Requirements define the problem, they allow team members to see their own work in context
•
Key information is standardized and made easier to visualize and verify.
•
An “audit trail” is maintained ensuring that important information is documented, and human memory is no longer relied on for important decisions.
Over time vehicles were refined; shapes changed, performance increased, but there was little functional difference between a vehicle of the 1970’s and one built 50 years earlier*. MODERN VEHICLE DESIGN
Translates requirements into design. •
Ensures that all requirements are specified in common terms and none are missed
•
Ensures that requirements are consistent and linked from the customer to the vehicle and all the way down to components.
•
Ensures that manufacturing and service issues are addressed up-front in the design process.
Within the last 20 years, the picture has changed significantly. Economic, environmental and social conditions have combined with technological advances, driven by the computer and electronics revolution, to dramatically change the modern vehicle and the process by which it is designed. There can be little doubt that today’s vehicle is a more sophisticated and complex product than its counterpart of the late 1970’s. It contains such innovations as:
Defines the interactions between the OEM's and the Suppliers
In part 2, we examine cooling system design using System Engineering principles.
*. Appendix A, reproduces the text and illustrations of a book entitled “The Making of An Automobilist” published in 1906. Readers may find it interesting to see how little has changed with time.
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•
Electronic engine management
•
Emission control system
•
Anti-lock braking system
•
Energy absorbing structures
•
Automatic temperature control
•
Extensive use of plastics
Throughout the design and development of a product as complex as a vehicle, these trade-off decisions are made on a daily basis. Such decisions number in the thousands before the product is finally sold to a customer.
•
Light weight, high strength alloys
•
Composites and other materials that would have been considered exotic in the past
In this paper we will show how Systems Engineering can be used to manage this trade-off process and ensure that the final product will meet the needs of the customer, throughout the product’s life cycle.
Specialization To meet the challenge of new technology, engineers have by necessity, become more technically qualified than an engineer was 20 years ago. As the human mind is knowledge limited, there is a tendency to specialize. With this increased specialization has come an inevitable narrowing, in knowledge breadth. Engineering Teams To compensate for the narrowing in knowledge, we leverage the skills of these engineer through the use of design teams, in which each member brings a different perspective and specialized skill to the problem. As the problems become more complex, the number of teams increases and the scope of their work narrows. For these teams to be effective they must be able to see their work in context, so that components and subsystems are not optimized at the expense of the overall product. In Part 1 of this paper we will show how Systems Engineering provides a consistent and structured framework where information and knowledge can be exchanged in a coordinated manner. Full Service Suppliers As the level of design complexity increases, the vehicle manufacturers are increasingly delegating the design task to specialized suppliers. These suppliers are frequently required to take total responsibility for the design, including warranty. For a supplier to accept such responsibility, they must be sure that all of the information that they require to complete the design, is both accurate and available in a timely manner. Systems Engineering provides the mechanism unambiguously communicating design information.
for
Compromise and Trade-Off Among Teams Even when good team management occurs, compromise and trade-off decisions are necessary. The design objectives of each of the sub-teams will almost certainly depend on another team's outcome, or may even be mutually incompatible with another team’s objectives. 2
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Part 1 Systems Engineering Fundamentals OVERVIEW The purpose of this section is to introduce the reader to the fundamental concepts of Systems Engineering. Systems Engineering provides a framework for managing and integrating the effort of many specialized engineers working on different parts of a complex product, into a single design team with shared objectives. A systemic approach embrace the following concepts: •
The system is greater than the sum of the parts.
•
Optimizing the parts does not optimize the whole.
•
Interactions determine system’s performance.
•
A system cannot be fully understood by breaking it down and analyzing its parts.
•
decisions are tracked
•
Systems Engineering is team oriented and interdisciplinary: - Design, Manufacturing, Marketing, Sales, Service, Finance etc. are an integral part of the system and their requirements must be considered simultaneously.
•
Systems Engineering institutionalizes quality into the design process, by continuously validating functional performance.
•
Quality is determined by the product as a whole, not by any single part of the product, no matter how good that part may be.
System
System
Subsystem
Assembly
Encapsulated in the previous definition are some key concepts:
•
detailed specifications are written
Product
…Systems Engineering integrates all the disciplines and specialty groups into a team effort forming a structured development process that proceeds from concept to production to operation. Systems Engineering considers both the business and the technical needs of all customers with the goal of providing a quality product that meets the user needs.” [4]
•
•
A system is a collection of interdependent functional elements that, when brought together as a single unit, combine to meet a set of common objectives. As shown in Figure 1
Engineering
“… an interdisciplinary approach and means to enable the realization of successful systems. It focuses on defining customer needs and required functionality early in the development cycle, documenting requirements, then proceeding with design synthesis and system validation while considering the complete problem …
•
knowledge is documented
WHAT CONSTITUTES A SYSTEM
WHAT IS SYSTEMS ENGINEERING The International Council on Systems (INCOSE) defines System Engineering as:
•
Assembly
System
Subsystem
Assembly
Component
System Engineering is a requirements driven process, in which the voice of the customer is paramount. (A customer can be defined as anybody downstream of the process that has equity in its outcome).
Assembly
Component
Figure 1, Example of System Hierarchy
Systems Engineering requires that, before we start designing the physical hardware, we must first understand all the functions that the design must perform, i.e. “form-follows-function”
A system has both internal structure and context.
Systems Engineering does not rely on the memory or knowledge of an individual: 3
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•
systems. Deriving its name from the Gantt chart, it is characterized by a development process in which each task is performed sequentially, as shown in figure 2.
Internal structure - System may contain, or be contained by, other systems. As Jonathan Swift eloquently said, some 300 years ago: Big fleas have little fleas Upon their back to bite 'em And little fleas have lesser fleas And so ad infinitum.
•
Requirements Design/ Synthesis
Context – Systems are characteristically interwoven with and affected by other systems. Frequently they only have value because of relationships to another system. Systems have boundaries and interfaces with other systems
Verification
Production
Systems can have two types of interfaces physical or functional •
•
Figure 2, Waterfall Sequential Model
Physical interfaces – We deal with physical interfaces every day. When we look at an object, we have become accustomed to constructing a hierarchy of physical interfaces within our mind's eye. What child did not learn that:-”the hipbone is connected to the thighbone, the thighbone is connected to the” … well you know the rest.
Although it is traditional to complete each task before moving on to the next one, it is not absolutely necessary. There is frequently enough information to begin a task before completing the preceding activity. If you advance a task too much however, you risk having to repeat it, in the light of improved information from a preceding task.
Functional interfaces – These interfaces while sometimes less obvious are equally important as they deal with the interdependency of performance characteristics.
The Waterfall Sequential Model is generally suitable for simple products having clear and well defined requirements immediately available at the start of the project, and a single phase of prototype verification.
For example, the fan of a truck cooling system may be physically connected to the engine, but it is functionally connected to the radiator, through which it induces airflow. The concept of physical and functional interfaces is central to the implementation of systems engineering.
SPIRAL SEQUENCE MODEL The spiral sequence model, figure 3, assumes an incomplete understanding of the customer’s requirements at the start of the project. It further assumes that several iterations are required through the design process before a product of sufficient maturity is ready for the customer.
SYSTEMS ENGINEERING PROCESS MODELS A key strength of Systems Engineering is that it recognizes that all design problems are not the same. There are several ways to manage the design process, depending on the nature and complexity of the task to be performed. Each of three commonly used process models have different strengths: •
Waterfall Sequential Model
•
Spiral Sequence Model
•
V” Sequence Model
This type of model is ideally suited to a process using multiple phases of prototype to gain user feedback, before the final product is committed to production. The Spiral Sequence Model is used extensively in the computer software industry, and is somewhat similar to a traditional hardware based vehicle development process. The spiral sequence model is generally slower than the V sequence model because it limits the degree of concurrency that is possible in the process. There are circumstances however, where a rapid prototype built from an imperfect set of requirements, may be faster than waiting for an exhaustive set of requirements, especially if analytical methods are not available for target setting or verification.
WATERFALL SEQUENTIAL MODEL The waterfall model is perhaps the oldest and most commonly used model and is typically used for simple 4
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Customer Needs
TIME & DETAIL
Prototype
Test requirements map
System Requirements
Prototype
End Item Requirements
Prototype
Product
TIME
Subsystem Requirements
within the same level
Customer Requirements
System Verification
End Item Verification
Subsystem Verification
Production (Mk 1)
Component Requirements Production
Production (Mk 2)
Requirements Analysis
Component Verification
Verification
Design / Synthesis
Design / Synthesis
Figure 4, V Sequence Model
Figure 3, Spiral Sequence Model[5]
Similarly, the bottom-up verification of the hardware solution, starts with components, and integrates through the various assemblies and subsystems back to the full product.
Unlike the other models, spiral sequence model does capture two important concepts: •
The importance of continuous improvement
•
A product development cycle can loop indefinitely, with periodic launches of improved versions of the product
Analytical tools are key to the successful implementation of this type of model. They enable the decomposed requirement targets to be validated and checked for compatibility before they are cascaded to the next level down. Similarly, test and analytical methods must exist to enable verification at the appropriate design level, instead of delaying verification until the complete system level.
Although these features are normally associated with products which do not require large capital investment to make changes, i.e., computer software. It is similar to the automotive industry’s practice of model year changes, to introduce both major and minor product freshening.
Modified V Sequence Model “V” SEQUENCE MODEL
In a large complex system with many requirements and multiple interactions, it may not be possible or desirable to complete the requirements cascade process in a single pass. Requirements may need modification or reprioritization in a trade-off process designed to balance functional performance, robustness, affordability and value.
The V Sequence Model, figure 4, extends the waterfall model by subdividing the requirements and verification phases. This approach allows for a more complex system, consisting of end-items, sub-systems and components, to be studied in greater depth. Rearranging the model sequence into a “V”, clearly identifies the relationship between the “downstroke” requirement phase and the “upstroke” verification phase of the project.
Under these circumstances, an iterative loop may be embedded within each level of the requirement cascade contained in the “V” sequence model’s downstroke. This loop is typically comprised of analysis, synthesis and verification phases but omits the production phase. As shown in figure 5.
The top-down decomposition of requirements and their associated targets, begins with the customers needs for the system, and cascades through the various design layers, down to requirements for individual components. 5
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Requ
ire ment Gap
Verifi cation Requirements
Requirements Analysis
Verification
Increased verification at the component and subsystem level.
•
Fewer phases of prototype vehicles.
•
Fewer downstream changes.
•
Shorter product development time.
s ib
il i
n
SYSTEM PARTITIONING
ts en
Terms & Definitions: Product: What the customer purchases
De
ty
em
si g
Fea
u ir R eq
Des ign Ga ps
Iterate for Compatibility
•
Design / Synthesis
System:
A group of two or more systems & subsystem
Subsystem: A group of two or more assemblies
Figure 5, Analysis, Synthesis and Verification Loop
Assembly: A group of two or more components Component: A group of individual part The modified version of the V sequence process model, shown in figure 6, is ideally suited to automotive industry needs. It provides the mechanism to implement a strong customer focus in the design and development of new vehicles.
Custommer Needs
Element:
System partitioning is a method for dividing a system into simplified and manageable pieces. Partitions may be based on any convenient grouping. Typical groupings include:
Product
Test Requirements Map Within The Same Level
System Requirements
End Item Requirements
Subsystem Requirements
System Verification
End Item Verification
•
Function
•
Technology
•
Physical location.
•
Product attribute
•
Reusability and complexity
Whatever the choice of grouping the objective is to minimize the number of interfaces across the partition boundaries.
Subsystem Verification
As discussed earlier there are two primary types of interfaces in every system: physical and functional. These interfaces can be sub-divided into two additional categories: external and internal.
Component Verification
Component Requirements
Design / Synthesis
•
External interfaces are those that deal with the interaction between elements within a partition and those in any other partition, or any interaction with the external environment.
•
Internal interfaces are those that exist between design elements within a partition.
Figure 6, V Sequence Model with embedded Subsystem process
This is the process model used at Ford Motor Company and it represents an evolutionary rather than revolutionary change in the way engineers work. It facilitates: •
A generic term for any of the above
Whether the interfaces occur at system, subsystem or component level, if the interface has different people or activities responsible for the interaction end-points, there is a potential for communications to breakdown. Partition
More up-front planning. 6
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boundaries are therefore carefully selected to minimize the number of requirements with cross-partition interfaces. Having partitioned the system into a number of semiautonomous system or subsystems, each is assigned its own design specification. A Program Development Team (PDT) is then given the responsibility to deliver all of the requirements contained within the specification.
•
A complete list of the functional requirements and associated targets for that element.
•
A description of the methodology to be used to verify the target has been met.
•
An interface specification, that describes how that element interacts with other elements
•
A history of document revisions
Partitioning based on physical architecture alone, may be insufficient in large complex systems, as there are certain customer requirements that do not map well into a hardware-based specification.
When cross-partition interfaces occur, a formal mechanism for managing the “requirement end points” is essential. The responsibilities may be divided but they are not shared. The notation “required of” and “required by” is often used to eliminate any ambiguity: The “required by” activity is responsible for: •
Defining the requirement objectives.
•
Setting the functional target level.
•
Ensuring the target affordability and value.
•
Confirming that the target is met.
Vehicle
Body
Electrical
Powertrain
Chassis
Climate Control
Fill, Drain & Deaeration Subsystem
Pressure Control Subsystem
Cooling System
The “required of” activity is responsible for: •
Agreeing that the target is technical feasible.
•
Cascading the requirements to the next level down.
•
Developing a design that meets the target.
•
Performing target verification.
Temperature Contol Subsystem
Airflow Subsystem
A schematic diagram of the element
Clamps
One method of doing this is to group similar customer requirements together into customer attributes.
Each element within the hierarchical structure has its own design specification. This specification should contain:
•
Hoses
Coolant
Clearly, if the “voice of the customer” is to be one of the primary drivers of product design, we need a method and process that can map these customer requirements, from the functional domain into the physical domain.
Based on this partitioning a specification tree is developed, from the top down, as shown in Figure 7. The number of levels employed depends on the complexity of the physical system. The lowest level is determined by the level where you can make the decision to make, buy, or reuse an existing element.
A non-technical statement of element’s function
Containment & Sealing Subsystem
For example, if a customer states - “I want the highest payload for a truck in this class”. It is not clear who would be responsible for delivering that improvement to the customer, because many of the vehicle’s systems could be affected.
DESIGN SPECIFICATIONS
•
Heater Subsystem
Figure 7, Example Specification Tree
Vehicle partitioning, based on physical architecture has been found to give the fewest external interfaces. It is therefore the model used within Ford.
A non-technical description of the element
Heat Dissipation Subsystem
External Flow Subsystem
Rigid Pipes
For example, A target for the dynamic balance of an engine mounted cooling fan is “required of” the cooling design team and “required by” the accessory drive design team, who would set the target at a level compatible with the mounting system.
•
Internal Flow Subsystem
For example, commonly recognized customer attributes might include the following:
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•
Cost of Ownership
•
Customer Life Cycle
•
Emissions
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•
Interior Comfort
•
Noise, Vibration & Harshness
•
Package & Ergonomics
•
Performance & Economy
•
Safety
•
Security
•
Styling & Appearance
•
Vehicle Dynamics
REQUIREMENTS ANALYSIS Terms & Definitions Requirement: - “A statement identifying a capability, physical characteristic, or quality factor that bounds a product or process need.” [6]
Those who have responsibility fore delivering these customer attributes would have following responsibility: •
Gathering customer, corporate and external wants. (Requirements analysis)
•
Converting those wants into functional requirements and targets. (Functional analysis)
•
Developing those requirements into a specification for the overall product.
Goal: -
A desired capability, characteristic or quality factor
Target: -
A quantitative measure of the functional performance necessary to satisfy a requirement
Tracability: - The ability to track a cascaded requirement back to its original source
The Requirements Analysis process establishes the overall objectives of the system, through compiling and analyzing requirements that define:
DESIGN MANAGEMENT With so many empowered teams, we need a method for managing the design process. Typically, this is achieved by the use of a matrix management structure of the type shown in figure 8. Program Integration Teams have lead responsibility for product related decisions (function), and the Program Development Teams, have lead responsibility for technology related decisions (hardware). Customer Attribute Leadership Product Integration Team
Product Integration Team
Product Integration Team
Product Integration Team
Technology Leadership
Product Developement Team
•
How well the product must perform in quantitative terms
•
Direct communication with the customer
•
Market analysis
•
Purchase reasons
•
Customer likes/dislikes
•
Quality surveys
•
Customer satisfaction measures
•
The physical environment in which the product will operate
•
Any constraints that will affect the design solution
Requirements are written using the language of the customer, rather than a technical or scientific language. Each requirement has an associated quantitative measure of desired performance, including where appropriate, an assessment of the customer’s expectations for performance at high mileage/time-in-service. Early in the design process, these measures may be subjective, (e.g. 1-10 scale) or relative, (best-in-class). The measures must be quantitative and never qualitative (e.g. improve fuel economy, reduce noise).
Product Developement Team Product Developement Team Product Developement Team
These high level requirements usually come from one of three basic sources:
Figure 8, Generic matrix organization
8
•
Customer expectations
•
Project / corporate constraints
•
External constraints
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CUSTOMER EXPECTATIONS Completely Satisfied
Customer expectations include: •
What the customer wants to be able to do with the vehicle.
•
How well each function the vehicle performs must be accomplished
•
Details of the natural and induced environment in which the vehicle will be operated.
•
Constraints that the customer will be placing on the vehicle: •
Purchase cost
•
Operating cost
•
How the vehicle will be serviced
•
Frequency of use
•
Hours of use per day
Excitement (Unspoken Want) Did Not Achieve rfo Pe
Basic wants
•
Performance wants
•
Features that cause excitement
Fully Achieved
Basic Want (Unspoken unless violated)
Completely Dissatisfied
Figure 9, Kano Model of quality, customer satisfaction and performance
Over-time, excitement and performance wants often become so readily expected by the customer, that they cease to be excitement and performance wants and become basic wants, shown in Figure 10.
These customer expectations and desires can be divided into three categories of wants: •
Sp e( nc a rm
ts) an w en ok
Examples of performance wants that have over time become basic wants, include:
The level to which these requirements are achieved, will determine the degree of satisfaction that the customer will have with the product. Basic Wants are typically unspoken by the customer. They represent the minimum expected for the vehicle. Successful delivery of these requirements will, at best produce a neutral response from the customer, while failure to deliver will cause dissatisfaction. These requirements are generally regarded as the “price of admission” to the marketplace
•
Seat belts
•
Air-bags
•
Anti-theft immobilization devices
•
Intermittent screen wipers
Similarly, when first introduced the cup holder was a surprise and delight feature. Today, we have become so used to their presence that a vehicle without one is a source of annoyance.
Performance Wants are spoken wants. They are desirable performance features that the customer is willing to pay more money to obtain. Better product performance will yield a greater level of customer satisfaction.
Excitement
Excitement Wants surprise and delight the customer when they are first introduced to a new product. They can be used in advertisements to distinguish the product from its competitors. Customers do not ask for things that they have never experienced, so these requirements are unspoken. The presence of these features may increase customer satisfaction, but their absence will have no impact.
Performance
Basic
A useful tool for understanding and measuring customer appeal is the Kano model[7], which is shown in figure 9.
Figure 10, The effect of rising customer expectations
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contrast is a goal which if met, will increase customer satisfaction.
PROJECT AND CORPORATE CONSTRAINTS Project and corporate constraints should include any internal requirements that will impact or limit potential design solutions. Project and corporate constraints may include:
Classification:
Description:
Given
A mandatory requirement
•
Financial constraints on investment and variable cost.
Must
•
Project timing.
A requirement that defines the minimum competitiveness
•
Technology availability and maturity.
Want
•
Market trends and “futuring”.
•
Reuse of design elements.
A desirable requirement that potentially differentiates the vehicle from others
•
Facilities for design, test and manufacture.
Not Required
•
Human resource allocation and skills.
•
Corporate specifications, standards, guidelines and best practices.
The presence or absence of this requirement has no impact on the vehicle
Not Wanted
•
Policies and procedures.
•
Product and manufacturing complexity.
The presence of this requirement detracts from the vehicle
•
Established product and process life cycle objectives
Table 1: Prioritization of wants
EXTERNAL CONSTRAINTS External constraints include requirements that will impact or limit potential design solutions. •
Local and international laws and regulations.
•
Social, political and economic requirements.
•
The technical capability of the supply base.
•
Industry, international and general specifications, standards and guidelines
•
Competitor product capability
REQUIREMENTS ARE CONSTRAINTS All requirements place constraints on the system. As constraints they have the potential to limit design alternatives and adversely impact cost, quality, function and weight. Therefore, it is essential to validate each requirement to ensure that it adds value to the product, before accepting it as part of the product’s baseline requirements. It is not necessary to conduct verification* at this stage, as we are working in the customer domain rather than the engineering domain. A simple check for discrepancies and conflicts will be adequate if the list of requirement is complete.
REQUIREMENT PRIORITIZATION Each requirement should be assigned a relative or absolute priority using at least one of the classifications shown in table 1. These priorities are used for subsequent target trade studies.
Look for discrepancies and conflicts in the wording of the requirement and the performance objective. Do not attempt to resolve any conflict, but simply prioritize the importance of any apparently conflicting requirements. The resolution of conflict only begins after technical engineering targets are assigned and we start to select design concepts, determine value and affordability.
For example: • •
Meeting the U.S Government emission standards is a given. It is non-negotiable
After validation the requirements are added to the Validated Requirements Baseline document for the product. All subsequent steps in the design process use the Validated Requirements Baseline as the authoritative source in any requirement conflict, or audit of functional requirement traceability.
A fuel economy requirement might state that the performance must be at least 8.2 MPG, but also, want 8.5 MPG for highway driving.
While the “must” is a requirement as it defines the minimum competitive position of the product. (In this context “minimum” may not be simply a neutral customer response, the image of the product may dictate that the “minimum” may in fact be “best-in-class”). The “want” by
*.
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While validate and verify are sometimes used interchangeably, Merriam-Webster offers the following opinion: validate implies establishing validity by authoritative affirmation, whereas verify implies the establishing of correspondence of actual facts.
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specific knowledge at this point in time. (This process of target setting will be covered in detail later in this part).
CUSTOMER DOMAIN Customer Intent
Voice of the Customer
Perceived Results
REQUIREMENT VALIDATION Before each functional requirement is accepted, it is validated using the following fundamental principles to confirm that the requirement is properly defined[8][9][10]. A requirement must be:
Noise System Signal
(made up from sub-systems)
Necessary: Deletion of the requirement should create a deficiency that cannot be fulfilled by other capabilities of the product.
Response
Controls
ENGINEERING DOMAIN
Verifiable: The target associated with the requirements should be quantified in a manner that can be verified by inspection, analysis, demonstration or test.
Figure 11, Translation of requirements from the Customer to Engineering Domain
Attainable: The requirement should be capable of being achieved by one or more design concept that has been proven viable, within the constraints of the project. Concise: The requirement statement should contain only one requirement.
FUNCTIONAL ANALYSIS The Functional Analysis process translates high level, nontechnical requirements contained in the Validated Requirements Baseline from the “customer domain” to the “engineering domain” (figure 11). In the engineering domain they are converted to detailed technical functional requirements. Working on the principle that “form-followsfunction”, these requirements are generic and should state what is needed, rather than how to accomplish it.
Complete: The requirement should not need further elaboration. Consistent: A requirement should not contradict or duplicate another requirement. Unambiguous: The requirement should not be open to alternative interpretation.
The process of translating the voice of the customer uses top-down Functional Decomposition, which takes each requirement in the Validated Requirements Baseline and asks the question: “What function is needed to satisfy this requirement?” This question may be asked several times to separate multiple engineering functions contained within a single customer requirement.
Implementation Free: The requirement should state WHAT is needed, not HOW it is to be accomplished. Standard Construct: Requirements should used standard terminology and avoid subjective or relative expressions. For example, a poorly written requirement might state:
Each engineering function is examined to determine what factors directly influence the successful delivery of that requirement. These factors can be divided into two categories; control factors and noise factors.
“Cycling of the thermo-viscous fan clutch should be kept to a minimum”
Control factors are those parameters of a design concept that can be adjusted to optimize the function of that design.
The requirement is poor for a number of reasons:
Noise factors are the sources of variability that degrade the functional performance of the design. As each functional requirement is identified, the subjective and relative performance targets from the Validated Requirements Baseline are converted into objective and absolute targets for performance and reliability. This conversion may not be possible in all cases due to a lack of 11
•
The word minimum is ambiguous and cannot be verified. The expression “not less than” used in conjunction with a number would remove the ambiguity.
•
The requirement is not implementation free, as it assumes that a “thermo-viscous fan clutch” is the chosen method for driving the fan.
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•
•
•
developed that will be used later in the product verification phase of the design process to manage failure risk.
The requirement is incomplete as it not clear what represents “cycling”. Does it mean moving from fully disengaged, to fully engaged, and then back to fully disengaged, or does it mean any reversal in the state of clutch engagement?
NOTE: It is important to recognize that at this point in the process we are validating failure to meet functional targets and not the mechanical failure modes of a specific design solution.
The word “should”, has a special meaning in the standard nomenclature of specification writing and is used to denote a goal rather than a requirement. The word “shall” is used to denote a requirement.
After the system-level requirements are established the process is repeated. Functional Decomposition is applied to each system level functional requirement, to obtain the subsystem functional requirements. This process is recursively applied until all of the requirements necessary to either make or buy the design have been identified. (The depth of this process depends on each individual design element and could be anything from a complete system to an individual part.)
Finally, it is unclear that the requirement is necessary, since its wording is so vague that it is impossible to determine if deletion of the requirement will have any impact on the product.
In contrast, a well-written requirement might state: “When tested in accordance with key life test procedure KLT-8K616, the fan drive mechanism shall, at a 95% confidence level, have a B5 life of not less than 250k miles.”
The process of cascading requirements and targets from the top-down is relatively straightforward, if the product being designed begins with a clean sheet of paper. Realistically, most new products are refinements of existing products or make use of existing design elements.
Once all the functional requirements at the system-level have been identified, they are documented in a systemlevel functional specification. (In a large complex system it may be necessary to sub-divide the requirements into more manageable groupings based on either a physical or functional partitioning of the system, as described earlier).
REENGINEERED PRODUCTS In a reengineered product, the idealized top-down approach must be modified. Each element of the product that is to be reused already has a predetermined functional performance and set of interface constraints. The approach varies for reused design elements:
FUNCTIONAL INTERFACES The next step in the process is to establish the functional interfaces between the requirements and document them on an interface diagram, which is added to functional specification. Each requirement is examined for any interaction that it might have with other requirements. Relationships are assigned one of the following classifications:
•
If it is a component that is to be reused, then the cascade will be from the bottom-up
•
If it is a sub-system, then the cascade will most likely be middle-out, with the cascade being performed in both directions, as shown in figure 12.
The desire to reuse an existing design element may occur for a number of reasons: capital investment, product timing, manufacturing complexity etc. Elements should not be reused before demonstrating that they can satisfy the requirements contained in the Validated Requirements Baseline.
Independent: The functions are not linked in any way. (E.g. coolant temperature is independent of door closing effort). Dependent: The functions are linked such that if one is changed, the other is automatically changed. (E.g. Waterpump speed is dependent on engine speed).
Conduct a top-down Functional Decomposition of the highlevel requirements to identify those which impact the element to be reused. Divide these requirements into two categories, those that impact primary functions and those that create secondary constraints, such as unwanted side effects and potential failure modes, as shown in the figure 13.
Correlated: The functions are linked such that if one changes, a change in the other is implied. (E.g Radiator heat dissipation is correlated to fan airflow rate). Following validation, a Functional or Concept FMEA (Failure Mode Effect Analysis) is performed on each requirement to identify the consequences of not meeting the functional requirement. Standard FMEA techniques are used to determine the severity of risk associated with each potential failure mode. A design verification plan is
For example, the primary function of a cooling fan is to move air. Secondary constraints might include aerodynamic noise, power consumption, dynamic imbalance etc. 12
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process starts with identification of design specific failure modes for the reused element. Using historical reliability data, a design Failure Mode Analysis (FMA) is performed to identify existing failure modes. When the FMA is complete, a potential FMEA is performed, to identify any new design failure modes. (The fact that the design functioned satisfactorily in the past does not mean that with different boundary conditions, it will continue to do so in the future). As each failure mode is identified it is translated into a functional requirement and added to the high-level requirements that were cascaded by Functional Decomposition.
Custommer Needs System Requirements
Top-down
Subsystem Requirements
Middle-out
Assembly Requirements
These functional requirements are fed into a bottom-up Functional Composition process in which the question: “What is the importance of this sub-function to the function of the system?” is repeatedly asked, until all of the related higher-level design functions are identified. Additional requirements and functional dependencies not previously identified during Functional Decomposition are added to the appropriate interface diagram and functional specification.
Bottom-up Component Requirements
Design / Synthesis
Primary functional targets are assigned to the reused element, at the minimum level necessary to satisfy the system-level functional objectives. Targets for dependent or correlated secondary design function, are calculated and added to the various functional specifications. If the imposition of these secondary functional targets does not violate any primary design target for the entire product, then the reuse of the design element is feasible.
Figure 12, Requirements Cascade
A capability study, (ignoring secondary effects), is performed on the design element to determine if reuse is feasible. If design control factors can be adjusted to give a functional performance greater than that required by the cascaded functional target, then the element has the potential to be reused and a more extensive study is warranted. If, however, adjusting the control factors cannot yield an output that satisfies the functional target, (even in the absence of constraints), then either the customer-level assumption must be changed, or the element cannot be reused.
These targets are said to be compatible. Compatibility, however, does not imply that an optimum design solution has been reached. Greater value may still be achieved by adopting a new design. For example, the reuse of an existing radiator might require an increase in fan airflow, which is achieved by increasing the fan’s speed of rotation. This increase, however, might also generate so much aerodynamic noise that extra sound insulation is required. The incremental cost associated with the extra insulation might be greater than that saved by reuse of the radiator.
Noise Factors
Signal
Primary Function
Function
This process of determining the value of reusing existing design elements is a key part of the trade-off process, which is central to the development of robust product targets.
Side effects Control Factors
Secondary Failure Constraints modes
TARGET SETTING
Figure 13, P-Diagram showing side effects and failure modes
TARGET SETTING BACKGROUND Before we discuss the target setting process in detail we will first look at some of the important concepts that influence the way targets are set.
If the primary functional targets can be achieved, the impact of the secondary effects is investigated. This 13
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and deterioration noise is typically called “wear-out”. Examples of deterioration noise include the following:
To obtain a robust design that satisfies the customer’s requirements we must take into consideration the effect of variability when setting the target level for a functional requirement. This variability generally referred to as noise, can be split into three basic categories: •
Unit to unit noise.
•
Environmental (external) noise.
•
Deterioration (internal) noise.
•
Customer usage
•
Wear due to friction
•
Changes in material properties such as: compression set, creep, work hardening, fatigue, etc.
•
Erosion due to fluid flow
•
Internal corrosion
Three distinct failure modes, that characterize functional variability due to noise, can be combined to produce a graph commonly referred to as the “bathtub curve”, (figure 14).
•
Raw and processed material variations due to thickness, hardness, chemical composition etc.
•
Dimensional variations due to forming or machining operations.
•
Fabrication or assembly variations due to weld quality/ location, bolt torque etc.
•
Product surface finish.
•
Product handling damage.
Instanteous Failure Rate
Unit to unit noise is due to inherent variability in the manufacturing processes used to make and assemble the product. Problems related to this type of noise are typically seen early in the product life cycle and are frequently referred to as “infant mortality” failures. Examples of unit-tounit variability include:
•
Misuse of the product. (accidental or intentional)
•
Unintentional energy input (heat, vibration, radiation etc.)
•
Dust in the environment.
•
Chemical attack (corrosion, ozone, gasoline, etc.)
•
Electromagnetic interference.
Deterioration
Environmental
Limit of useful life
Figure 14, Reliability “bathtub curve”
Customer usage variation is due to the ways the customer may use the product.
Environmental noise is generated by sources that are external to the product. Sources of this type of noise include the following: Climatic variations (temperature, humidity, rain, wind etc.)
Unit to Unit
Wear-out
Time in service
Units that meet the minimum acceptable performance suffer from a loss of function that may cause subsequent failure in the presence of other noise sources.
•
Infant mortality
•
Accidental misuse of the product.
•
Intentional misuse of the product
It is important for the designer to remember that the customer will always find ways to use the vehicle that were never intended, when the specification was written.
ON-TARGET ENGINEERING Conformance to specification has traditionally been the primary method of assessing the quality of a product. Anything within specification is viewed as good, while anything outside specification is bad. In this approach, variation within the specification is both acceptable and unimportant.
This type of noise is always present, to some extent. When applied to units with marginal functional performance, it is responsible for the apparently “random” failures that occur throughout the product’s life.
Consider the example shown in figure 15, in which the unitto-unit variations of a single design are compared.
Deterioration noise comes from within the product and is associated with functional deterioration of the product due to age or use. The combined effect of environmental noise
The specification set by the designer called for a critical characteristic to be controlled within a range m ± ∆0 which 14
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difference between two samples, one on-target and one just inside specification, may be very large and highly significant.
is shown as an Upper and Lower Specification Limit, (USL / LSL) Sample “A” represents the initial random population sample taken from production. On examination, it was found that variability was causing some samples to fall outside the specification limits generating a process Capability Index, Cpk of less than 1. Production was halted and the equipment adjusted.
The traditional view of quality, where samples are viewed as either pass or fail, does not recognize that in most cases, loss of functional performance is a continuum, rather than a discrete event. It is important to recognize that two questions need to be answered when setting the targets and tolerances for a design: -
Sample “B” represents the population distribution after adjustment. The sample indicates that variability was halved and process capability reestablished with a Cpk index of 1.4.
Sample B
How far from the target, can functional performance deviate, before a customer perceived a loss of function that would cause them to act? How far from the target, can a dimensional or physical characteristic of the design deviate, before manufacturing should reject the unit? In both cases an economically measurable action occurs. (the customer action might be delayed, such as a decision not to purchase another product). Dr. Genichi Taguchi is generally acknowledged as the first to develop methodology for evaluating this financial impact, which he quantified in a Quality Loss Function.
Nominal Target
USL
LSL
Frequence
Sample A
QUALITY LOSS FUNCTION In developing an expression to quantify loss, Taguchi[11] defines quality loss as: “the loss a product causes to society after being shipped, other than any loss caused by its intrinsic functions”. This loss is made up from two parts: m - ∆0
m m + ∆0 Functional Performance
•
Loss caused by functional variability.
•
Loss caused by harmful side effects.
The cost associated with scrap or reworked products prior to shipment is deliberately excluded from this view of quality, as they represent a cost to the manufacturer, but not a quality loss.
Figure 15, Variation of Functional Performance for two production samples
From his studies, Taguchi concluded that the loss of quality for a product, due to deviation from target, could be approximated by a function, which increased quadratically with deviation from target. The approach was found applicable to the four types of targets listed below:
Viewed from a manufacturing perspective, the equipment setup used to produce sample “B” delivers significantly better quality, since all units produced meet specification. However, from the customer’s perspective, sample “A” has more units with on-target performance and would have produced greater customer satisfaction.
Nominal is best: The target has an optimal value ± a tolerance. Quality Loss Function is proportional to the magnitude of the deviation. (E.g. the alignment of a rubber isolator)
This apparent contradiction, where “worse” quality produces greater overall customer satisfaction, comes about because “within-specification” and “on-target” are not the same thing.
Smaller is better: The target response is ideally equal to zero, when the Quality Loss Function will also be zero. Tolerances are expressed only as an upper limit. (E.g. engine noise, aerodynamic drag, breaking distance, etc.)
The difference between two individual samples, one just inside specification and one just outside specification, may be very small and functionally insignificant. Whereas, the 15
k = Quality Loss Coefficient (an economic factor) A = Cost to society if tolerance is exceeded D0 = Customer functional tolerance
Equally Bad
Loss
Equally Bad
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Equally Good
Customer Functional Tolerance ∆0, is the point at which the product either fails, or 50% of the customers would decide that the functional performance was unacceptable and take an economically measurable action.
0 Target Specification Limits
Poor
Loss
The cost of this action, typically a repair, should include consideration of all related costs, some of which are listed below:
Poor
•
The cost of parts, labor and special tooling needed to make the repair, whether it is repaired under warranty, or at the customer’s expense.
•
The cost associated with loss of use, including lost earnings and/or the hire of a replacement product.
Best
•
The cost of transporting the product to the point of repair.
Target Specification Limits
•
The cost for disposal or recycling for failed parts.
•
The cost of spare part inventory.
•
The cost of any reduction in product residual value.
Good
Good
0
Figure 16, Comparison of traditional and quadratic Quality Loss Function
When developing a new product, it may be difficult to obtain accurate data for either the Customer Functional Tolerance or the Cost to Society. However, exact data is not required and it should be possible to make approximations that are close enough to evaluate the relative merits of different targets and design concepts.
Larger is better: The target response is ideally infinity, when the Quality Loss Function will be zero. Tolerances are expressed only as a lower limit. (E.g. weld strength, corrosion resistance, light bulb life, etc.)
Cost, is not the only consideration in developing a mathematical relationship to express a value for loss of function. Safety and security issues may also be involved.
Asymmetric Nominal is Best: The target has an optimal value but is subject to asymmetric tolerances. Loss of function varies with the direction of deviation. (E.g. hose joint clearance, fan tip clearance, etc.)
Severity of Failure: Not all failures have a benign outcome, so it is necessary to consider the severity of failure when setting functional targets. The severity of a failure can be categorized using the following classes:
Figure 16 shows a comparison between the Quadratic Quality Loss Function and a traditional Loss Function for a “nominal-is-best” function. Expressed mathematically the Quality Loss Function is intended to represent the loss experienced by an average customer, (rather than a specific customer), and is defined as: Loss ,L ( y ) = k ( y – m )
2
A k = --------2 ∆0
•
Safety, security and dependability related concerns, which might strand the customer
•
Failures and degradation in function which reduce the customer's confidence in the product
•
Failures that cause significant irritation to the customer
•
Failures that cause the cost of ownership to increase
In many cases, the severity of failure will have no impact on the target directly, but may influence it indirectly through the following:
(1) (2)
Where: y = measured response m = target value 16
•
The level of functional variability permitted
•
The required useful life.
•
The modes by which the design concept is permitted to fail.
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Figure 17 shows an over view of the target setting process beginning with a review of the Validated Requirements Baseline.
Cost: When using cost to calculate the value associated with a functional requirement, it is important to use the total cost of design. Total cost has three elements; Unit Manufacturing Cost, Life Cycle Cost and Quality Loss Cost:
Develop Product Strategy Unit Manufacturing Cost: A combination of all of the cost related to manufacturing the design element and includes variable cost, fixed cost, tooling costs and the costs associated with reworking or scrapping defective parts
We develop an outline Product Strategy using the quantitative measure of desired performance, and the requirement priority, that was assigned by the customer during the Requirements Analysis phase. The overall product requirements are prioritized, using the previously defined classifications, which are repeated in table 2.
Life Cycle Cost: The summation of costs related to operating the product that are directly attributable to the design element under investigation. In addition to the cost of repairs and routine maintenance, it should also include the cost of energy required to operate the design Quality Loss Cost: the cost associated with the Quality Loss Function
Validated Requirements Baseline
Develop Product Strategy
Select Comparitor Product
Compare product capabilities
Set Futured Functional Targets
Review Bechmark & Technology Bookshelf
Classification:
Description:
Given
A mandatory requirement
Must
A requirement that defines the minimum competitiveness
Want
A desirable requirement that potentially differentiates the vehicle from others
Not Required
The presence or absence of this requirement has no impact on the vehicle
Not Wanted
The presence of this requirement detracts from the vehicle Table 2: Prioritization of wants
Define Gaps
Select Design Concepts
Trade-off Studies
Select Comparator Product
Set Product Target Ranges
The next step in the process is to select an existing product that most closely matches the product to be developed. For reengineered products, the product being replace would most likely be chosen. while the closest available product should be chosen for a new market segment.
Modify Product Strategy
Review Benchmark and Technology Bookshelf
Figure 17, Target Setting Process
Benchmark data must be reviewed to establish the best-inclass and median performance of competitor products. This benchmark phase should also include a review of all significant technologies on the competitor vehicles, or available internally or from suppliers. After gathering the information from the previous three steps, the initial process of target setting can begin.
TARGET SETTING PROCESS After identifying all of the functional requirements for the entire system, we have to decide on the performance and importance in relative and absolute terms for each of the requirements. (Even in a very simple system, it is unreasonable to expect that every requirement could be fully satisfied). Target setting is by definition an iterative process, and involves trade-off decisions at every level of the product.
Target Ranges At the early stages of the design process, targets are not expressed as “point” targets but as a “ranges” in which the target is bounded on at least one side, by a value that is traceable back to the voice of the customer, through the Validated Requirements Baseline document. This range is 17
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Conclusion
generally created by the need to keep risk low, in the face of uncertain technological capability.
The target setting process is repeated continuously, progressively narrowing the safety margins, until the ranges become point targets and the point targets become firm product objectives. This process is repeated at each level of the design cascade.
Using the Validated Requirements Baseline, competitive benchmarking and comparator product data, target ranges are established for each of the customer requirements. For example: •
All functional requirements that are traceable back to a requirement designated as a given are assigned a non-negotiable target (Minimum or maximum value of the target range is set at the level specified in the Validated Requirements Baseline)
•
All functional requirements that are traceable back to a requirement designated as a must are assigned a target established through benchmarking to be competitive (This target may include a margin that accounts for any shift in the competitive position, which may take place before the product is launched)
Define Gaps System analysis is performed on carry-over, new or alternative design concepts throughout the target setting process to confirm the technical feasibility of satisfying the required target. Gaps in the required performance are noted, whether they are a shortfall or over achievement. The gaps drive the remainder of the target setting process. If the target can be achieved, then value analysis is performed to provide data that can be used in crossfunctional optimization studies. Where Function Value = -----------------------Cost
(3)
And Cost = A universal currency of negotiation (e.g. $. weight, quality etc.) Trade-off Studies Trade-off optimization studies can range from simple Pareto charts, through to a multi-dimensional simulation, depending on the magnitude and complexity of the functional gaps. If all of the gaps cannot be closed, even when a reasonable amount of “stretch” is applied to the target then it may be necessary to go back and modify the initial Product Strategy. When all these steps are completed, then the product will have a set of “compatible” targets at the system level.
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PART 2
ENGINE COOLING DESIGN FROM A SYSTEMS ENGINEERING PERSPECTIVE
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Part 2 - Section 1 Vehicle and System Level Requirements that the reader may better understand the requirement being discussed.
INTRODUCTION In this section, we will discuss the design of vehicle cooling systems in detail. Requirements will be developed using System Engineering principles to show how a systemic approach can maximize customer and shareholder value, by improving the function, quality, reliability and time-tomarket for a new or redesigned vehicle.
For each major functional requirement, a Parameter Diagram has been constructed to help the reader understand the relationship between the following: -
As the cooling system is a complex system with numerous subsystems, assemblies and components the number of functional requirements is considerable. Within Ford we have formally identified over 150 cooling requirements at or above subsystem level, never mind all of the individual component requirements. For practical purposes not all of these will be discussed in this paper. What we hope to do is construct a framework that will encourage the reader to use their experience and imagination to develop functional requirements that match their own needs. In this paper we will not be discussing Ford’s detail design targets. While some of that information is confidential, the main reason for excluding the detail is that targets are product and market specific, and may only serve to mislead the reader. Generalised or “rule-of-thumb” targets should always be critically questioned, as they are often non-value added constraints that simply drive cost up.
•
The Ideal Function of the design element, i.e. the requirement and its associated target.
•
Control Factors, which can be used to adjust the element’s functional performance. (These in turn, become the functional requirements for the next level down).
•
The Noise Factors, which introduce uncontrollable variability and potential failure modes, into the elements functional performance. (If the signal/noise ratio becomes too great, then it may be necessary change the design and make the noise factor a control factor).
•
Side Effects, which are secondary and unintended outputs of the design element. These frequently create undesirable external interfaces to the system and place constraints on the tunable range of the control factors.
•
Potential Failure Modes, which place constraints on the tunable range of the control factors and introduce additional functional requirements.
In general the failure modes identified and discussed in this paper are “common cause” failures, inherent to the design concepts or requirement targets, rather than “special cause” failures that result from defects in an individual component.
While requirements are often generic in nature, targets must be developed to match the customer expectations for that specific product and must be set at a level that adds value to the product.
VEHICLE LEVEL REQUIREMENTS To distinguish REQUIREMENTS from the text they are boxed with double lines like this.
OBJECTIVE
To distinguish GOALS from the text they are boxed with single lines like this.
The purpose of the vehicle cooling system is to provide reliable powertrain operation under all vehicle operating conditions.
In Part 1 of this paper, we made a particular point of explaining what makes a good or bad requirement. In this part of the paper however, we have not necessarily followed our own “rules”. In such cases we leave the reader to decide how the requirement should be worded.
In this section we will discuss the relationship of the cooling system to the customer and the vehicle as a whole. We will discuss the following: -
Although this paper is not intended to be a definitive or comprehensive design guide for the cooling system, alternative design concepts will be discussed. Where appropriate, the underlying principles will be explained so 21
•
Customer requirements/expectations
•
Business and project requirements
•
External requirements
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To better understand the customers needs, let us first look at vehicle operating conditions which can be split into two general categories:
CUSTOMER REQUIREMENTS/EXPECTATIONS In part 1 of this paper, we identified the following as potential sources customer requirements:
•
Extreme conditions.
•
What the customer wants to be able to do with the vehicle.
•
Normal conditions.
•
How well each function that the vehicle performs must be accomplished.
•
Details of the natural and induced environment in which the vehicle will be operated.
Traditionally, the focus of the cooling engineer has been on the extremes of operation; high vehicle load at high ambient temperatures and low engine loads at low ambient temperatures.
•
Constraints that the customer will be placing on the vehicle: •
Purchase cost.
•
Operating cost.
•
How the vehicle will be serviced.
•
Frequency of use.
•
Hours of use per day.
While these conditions are important as they test the boundaries of system performance, they do not represent the typical customer concerns, which tend to be durability or fuctional degradation related, rather than performance related. In a customer focused cooling system design process the emphasis is shifted away from these severe performance measures, towards robustness under normal operating condition, cost of ownership and customer life cycle issues.
These customer expectations and desires were divided into three categories:
The effects of customer usage and environmental conditions are treated as system noise factors rather than system design requirements.
•
Basic wants
•
Performance wants
•
Features that cause excitement
What the customer expects of the cooling system is robustness. Furthermore, that robustness should not just apply to the vehicle the day it leaves the factory. It must have consistent functional performance for all of its useful life.
From the customers perspective, the vehicle cooling system falls into the category of “basic wants” and according to the Kano model, shown in figure 18, it is a requirement that is “unspoken unless violated”.
INTERACTION WITH CUSTOMER ATTRIBUTES Although there are no direct customer requirements for the vehicle cooling system, the cooling system side-effects and failure modes interacts with many of the customer attributes shown in figure 19.
Completely Satisfied Excitement (Unspoken Want) Did Not Achieve
ce an rm o f r Pe
s) nt wa n ke po (S
Fully Achieved
Customer Attributes (Vehicle Level)
Basic Want (Unspoken unless violated)
Vehicle Dynamics
Completely Dissatisfied
Security
Interior Comfort
NVH
Styling
Perfomance & Economy
Emissions
Safety
Package & Ergonomics
Cost of Ownership
Customer Life-cycle
Figure 18, Kano model of quality, customer satisfaction and performance. Body
Electrical
Powertrain
Climate Control
Chassis
Cooling System
The customer simply wants a cooling system that always works regardless of the vehicle operating conditions or ambient temperature.
Figure 19, Relationship between customer attributes and the cooling system.
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The specific ways in which the cooling system design can be modified to effect these customer attributes will vary with: •
Vehicle design
•
The target customer
•
The environment in which the vehicle is to be used
•
The intended use of the vehicle.
•
The styling of the vehicle, including front-end openings, will normally be signed-off before verification testing is conducted.
•
Road based cooling system testing has seasonal dependencies, which can be impacted by project timing.
As the design of the cooling system frequently contains a substantial level of design reuse, particularly parts internal to the engine, much of the process requires bottom-up, rather than top-down approach.
These effects are product and company specific and are outside the scope of this paper. BUSINESS REQUIREMENTS
EXTERNAL REQUIREMENTS
There are many corporate and project requirements for the entire vehicle that place constraints on the design of the cooling system. These constraints are highly project and company specific and are therefore outside the scope of this paper.
At present there are no legislative requirements that apply specifically to the cooling system, except for some local market requirements with regard to coolant usage and disposal.
However, one of these constraints, project timing, is worthy of discussion as it frequently presents major problems to the cooling system engineer.
SYSTEM LEVEL REQUIREMENTS
PROJECT TIMING SYSTEM PARTITIONING As cooling is one of the major interfaces between the vehicle and the powertrain, it is frequently subject to project timing issues that make the designer’s job more difficult.
As explained in Part 1 of this paper, the choice of system partitioning is somewhat arbitrary. It can be performed using any convenient criteria. In this paper we have chosen to use a partitioning based on the physical hierarchy shown in figure 20.
Typical powertrain issues include the following: •
•
•
•
The powertrain and the vehicle must have timing plans that are synchronized, but they are not necessarily concurrent. The design of a new powertrain will frequently start before the vehicle.
Customer Attributes (Vehicle Level) Vehicle Dynamics
The parts of the cooling system internal to the engine are frequently designed without any knowledge of the vehicle installation components.
Security
Engine calibration is frequently one of the last vehicle items to be frozen. Changes to the calibration strategy and hence heat to coolant, frequently occur long after the cooling system has been not only sized, but also signed-off.
Body
Perfomance & Economy
NVH
Styling
Customer Life-cycle
Climate Control
Powertrain
Electrical
Cost of Ownership
Safety
Package & Ergonomics
Emissions
The engine may be carry-over or externally purchased, with little opportunity for customization or tuning.
Chassis
Cooling System
In addition to the powertrain issues there are a number of vehicle related issues that the cooling engineer must address: •
Interior Comfort
Temperature Contol Subsystem
When the initial underhood package is being defined and heat exchanger size fixed, there will frequently be no accurate data for engine heat dissipation or coolant flow rate.
Airflow Subsystem
Internal Flow Subsystem
Heat Dissipation Subsystem
External Flow Subsystem
Heater Subsystem
Fill, Drain & Deaeration Subsystem
Containment & Sealing Subsystem
Pressure Control Subsystem
Coolant
Figure 20, Relationship between Vehicle Level requirements and the cooling system
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Although we cannot control the wall temperature directly, we can control it indirectly, by adjusting the average coolant temperature and the local heat transfer coefficient, as shown in figure 22.
As can be seen in figure 20, the cooling system interacts with two of the major vehicle systems the Powertrain and Climate Control system. POWERTRAIN REQUIREMENTS Put simply, the primary objective of the powertrain designer is to maximize the quantity of useful work output from the engine, for any given quantity of fuel input.
Tg, Hg Twg
Whether the vehicle is powered by an internal combustion engine, electric motor, gas turbine or any other source of propulsion, it can be said with confidence that the conversion of fuel energy into useful work will not be 100% effective. Heat will always be generated as a side effect of the energy conversion process.
Legend T = Temperature H = Heat transfer coefficient L = Wall thickness k = Thermal conductivity Subscripts c = coolant
This heat can be divided into two components: •
Useful heat: - which can be used to warm the passenger compartment in cold ambient temperatures and maintain the engine at the optimum temperature for combustion and lubrication efficiency.
•
Unwanted heat: - which must be removed from the system once the optimum operating temperature has been reached.
k
Twc Tc, Hc L
g = gas
w = wall
Figure 22, Heat flow through a plain wall
In giving up control of the engine wall temperature we allow it to become a system noise factor, which increases the variability of useful work and heat output. The lack of wall temperature control also introduces some potential failure modes, shown in figure 23, which must be managed by the cooling system.
If we examine the mathematics of the combustion process we find that the temperature of the combustion chamber walls is one of the factors that controls combustion efficiency and useful work output.
Noise Factors Engine surface temperature
This process can be illustrated by the diagram in figure 21. Fuel
If the problem is approached from a theoretical perspective, it is possible for any given engine design, to determine an optimum temperature for the combustion chamber wall. However, from a practical perspective the temperature of the wall varies from point-to-point and moment-to-moment and we have no direct way to control the temperature. The definition of a requirement and target for temperature at the wall surface would be meaningless as the requirement would be unattainable.
Combustion
[Other factors] Average coolant temperature Local heat transfer coefficient
Control Factors
Useful Work Side effects Unwanted heat Useful heat Failure modes Structural failure Combustion pre-ignition Lubrication failure Poor hydrocarbon emissions Poor fuel consumption
Figure 23, Engine combustion process using coolant temperature as a control.
Noise Factors
Fuel
Combustion
[Other factors] Engine surface temperature
Useful Work
Rather than use heat transfer coefficient as a direct control of combustion, it is used to “manage” the level of noise that wall temperature can impose on the system.
Side effects Unwanted heat Useful heat
CONTROL FACTORS
Control Factors
Although many control factors exist for optimizing the combustion process and useful work output, in this paper we will limit discussion to average coolant temperature.
Figure 21, Engine combustion process with surface temperature as a control.
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In the case of today’s internal combustion engines, the thermal conductivity of the combustion chamber materials and the engine firing frequency combine to make the temperature, a time averaged requirement.
When discussing the effect of temperature on system function, it is important that we always distinguish between:•
Global and local temperature.
•
Time averaged and instantaneous temperature.
For example: The time averaged local metal temperature, shall not exceed TBD°C under any engine operating condition.
Average coolant temperature As discussed earlier, the average temperature of the coolant should be set for optimal combustion efficiency. It is the joint responsibility of the engine designer and calibration engineer to set the target.
Combustion Efficiency
Traditionally, as the temperature control system utilized is a mechanical device, with coolant temperature as the only feedback mechanism, it has only been possible to set a single nominal target, which is then applied to all operating conditions. Loss functions for engine performance, fuel economy, emissions and interior comfort should be used to determine the value of maintaining the temperature within a specified tolerance band.
The efficiency of the combustion process is bounded on two sides by temperature. Firstly, if the combustion walls are too cold, combustion will be incomplete leading to an increase in hydrocarbon emissions and poor fuel economy. This combustion quenching is a global and time averaged phenomenon. As both fuel-economy and emissions are evaluated on a cumulative rather than instantaneous basis, the temperature requirement is also a cumulative requirement, which is expressed as a function of time.
For example: The nominal operating temperature of the cooling system shall be TBD°C ± TBD° during any engine operating conditions.
For example: When tested in accordance with drive cycle XYZ, the engine shall achieve an average coolant temperature of TBD°C in not less than TBD minutes.
With the availability of enhanced powertrain electronics, it is now possible to use additional combustion control factors as feedback signals, which can be used to modify the temperature control strategy. The improved system performance, which is quantified in terms of a reduced loss function, must be offset against the increased cost of sensors, actuators and controller and that is required to implement a more sophisticated control strategy.
As the heat used to achieve this functional requirement, is “useful heat”, the requirement must compete for its “allocation” in a functional trade-off, which will be discussed later.
FAILURE MODES
Secondly, if the temperature is too high, the charge may spontaneously ignite or detonate causing noise and possibly structural damage to the engine. This type of failure and can be triggered by a single “hot-spot” within an individual cylinder, on an individual cycle of the engine.
In addition to the requirement created by the use of coolant temperature as a control function, temperature requirements are necessary to prevent the potential failure modes, shown in the P-Diagram (figure 23), from occurring.
The temperature requirement needed to protect against this type of failure has a target value that is both localized and instantaneous.
Structural failure As the heat generated by combustion is neither uniformly distributed throughout the engine structure, nor in the case of an internal combustion engine continuously generated, temperature within the engine varies with both location and time. These variations in temperature cause differential expansion of the engine structure, resulting in thermally induced stress.
For example: The instantaneous metal temperature shall not exceed TBD°C at any point within the engine, during any engine operating condition.
To prevent structural failure we require that a target be sets for the maximum permitted localized metal temperature for each different material used within the engine. 25
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It might seem strange to some reader that there is no requirement for “maximum coolant temperature” among those listed above.
Lubrication Efficiency Due to the close correlation that exists between engine metal temperature, lubricating oil temperature and cooling system temperature, the cooling system engineer frequently collaborates in the management of oil temperature requirements. While it is clearly the responsibility of the tribologist to define the target oil temperature range, (i.e. WHAT is required), the cooling engineer is often responsible for HOW it is to be achieved.
There is a requirement to limit the maximum coolant temperature, but it is a requirement that is imposed by the coolant, rather than by the powertrain. The powertrain only requires that the coolant remains liquid at all times and that the average coolant temperature is consistent with optimum combustion.
The efficiency of the lubrication process, like the combustion process, is bounded on two sides by temperature requirements. If the temperature is too low, high oil viscosity causes an increase in engine friction, which results in poor fuel economy.
Design Strategies for risk management To minimize the risk of engine failure due to temperature related problems it has become standard practice to fit vehicles with a coolant temperature gauge or warning light. Some manufacturers also equip their vehicles with an oil temperature gauge, although this is by no means standard.
For example: When tested in accordance with drive cycle XYZ, the engine shall have an average oil temperature of TBD°C after not more than TBD minutes.
For a passive warning system to be effective, it must accomplish two objectives: -
The process of warming-up the lubricating oil requires “useful heat” so this requirement must compete for its allocation in a functional trade-off.
It must display accurate information, in a timely manner. For example:
The weight or grade of lubricating oil utilized in an engine is carefully selected to have a viscosity, which matches the operating temperature of the vehicle. If operated at a temperature outside the specified range, the viscosity of the oil may become too low, leading to an increase in oil consumption. This increased oil consumption affects both the cost of ownership and the amount of hydrocarbon emissions.
Within TBD seconds of reaching the “maximum safe operating temperature” either a warning light shall be triggered, or the coolant temperature gauge shall indicate “hot”. It must be stable under normal operation, to avoid causing false concern.
It is the responsibility of the tribologist to supply to the cooling system engineer, both a target for the nominal oil temperature and two loss functions that the shows the effect on oil consumption and oil life of off-target operation.
The coolant temperature gauge shall have an arc of movement of no more than TBD degrees for all operating temperature between, TBD°C below thermostat fully closed temperature and TBD°C above the maximum continuous operating temperature.
For example: The nominal operating temperature of the engine lubricating oil shall be TBD°C ± TBD° for the 5th – 95th percentile customer.
In general, passive warning systems are most effective when the type of failure is both progressive and foreseeable. For example, on a hot day while climbing a steep-hill, the coolant temperature-gauge slowly increases and moves into the hot zone.
If the engine oil becomes too hot, it may breakdown and lead to a complete failure of the lubrication process. A maximum temperature limit is therefore required.
Passive warning systems are less effective when the failure mode is sudden or unexpected.
For example: The temperature of the engine lubricating oil, measured at the engine oil cooler inlet, (or dip stick if no cooler is fitted), shall not exceed TBD°C
For example, while cruising along the freeway on a cold wet night, a coolant hose fails and all of the coolant is lost from the engine.
A note on maximum coolant temperature 26
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surface “dry-out”. Typically, this will be the area of highest heat flux.
The effectiveness of a passive system can be enhanced by proving an audible warning that attracts the driver’s attention, but the effectiveness still relies on the driver hearing the warning and taking the correct action. As alphanumeric message centers with voice synthesis become more common, it will be possible to give the driver instructions on what action to take in case of a warning.
When subject to a loss of coolant flow, the engine shall enter fail-safe mode within TBD seconds of any location in the cylinder head temperature reaching TBD°C
The most effective system of risk management is an active system, which uses sensors to detect system failure and automatically intervene to prevent damage. A cooling system that uses this technology is known as a “fail-safe” system.
An alternative approach to preventing this particular failure mode might involve the use “of the rate of change of temperature”, rather than the absolute temperature as an indicator of failure.
For a fail-safe system to be effective, the coolant temperature sensor cannot be used as a method of detection. If the coolant is lost from the system, the output signal will be unreliable. A cylinder head metal temperature sensor is used instead, as it provides a direct measure of the critical requirement, engine metal temperature.
While these requirements may sound very similar, they are different and it may not be possible to achieve all three levels of protection with a single sensor.
Depending on the type of sensor and its location within the engine, several different levels of protection can be offered by fail-safe cooling:
•
The vehicle enters a “limp-home mode” which allows the vehicle to be driven a limited distance at reduced power and speed. Fuel to the cylinders is cut-off in a pattern that rotates each cylinder through a reduced fuel firing cycle, followed by a no fuel pumping cycle, which allows the air charge to cool the cylinder. The erratic firing immediately notifies the driver that a problem exists.
•
The vehicle can be brought to a complete stop by progressively reducing the fuel injected and hence the power available. Care must be taken not to cut-off fuel too suddenly, as an unexpected reduction in engine power may expose the occupants of the vehicle to danger.
On detection of a failure, two different strategies can be employed:
Protection against a sudden and total loss of coolant. (E.g. a split hose). This requires that the sensor is in the hottest location, or is in a location that can be consistently correlated to the hottest point in the engine. When subject to a complete loss of coolant, the engine shall enter fail-safe mode within TBD seconds of any location cylinder head temperature reaching TBD°C
If a limp-home strategy is employed additional component failures may occur as result of continuing to operate the vehicle, increasing the cost of repair. In particular, care must be taken not to overheat the cylinder head gasket or the waterpump seal, which without coolant will “run dry” and could sustain unseen damage.
Protection against a slow leak that progressively exposes sensitive metal surfaces. (E.g. a leaking hose joint). This requires that the sensor is located in an area of the engine that is the first to be uncovered. In a longitudinal mounted engine, which is inclined, this location is readily identified. However, in a transverse engine, the effect of vehicle dynamics may need to be considered.
In limp-home mode the vehicle shall be capable of being driven X miles at the maximum engine speed obtainable, without causing damage any component that was not already damaged prior to entering limp-home mode.
When subject to a loss of coolant, at the rate of TBD liters per minute, the engine shall enter failsafe mode within TBD seconds of any location in the cylinder head temperature reaching TBD°C
If additional component damage cannot be avoided it shall be explicitly stated in the owners handbook and service literature.
Protection against a loss of flow due to partial or complete waterpump failure. (E.g. a broken pump drive-belt).
When first introduced, fail-safe cooling was a “surprise and delight” feature. Today it has become a performance feature for which customers are prepared to pay additional money, but like many good ideas, it is quickly becoming a standard feature on new engine
This requires that the sensor is located in the area of the engine that is the first to experience vapor film boiling and
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Part 2 - Section 2 Engine Internal Flow Subsystem OVERVIEW In this section we will discuss: •
engine internal flow requirements,
•
system flow resistance and supply curves,
•
modes of heat transfer,
•
the onset of flow instability,
•
and methods for determining the minimum flow rate.
or surface lubrication fails,
•
or pre-ignition is triggered.
ENGINE INTERNAL FLOW REQUIREMENTS In all but evaporative cooling systems, adequate flow through the engine is required in order to prevent vaporization of the coolant within the heated passages of the engine. Under all engine operating conditions, the local coolant flow rate shall be TBD% greater than the flow rate at which the onset of flow instability (due to vaporization) occurs.
TERMS USED IN THIS CHAPTER Critical Heat Flux: The heat transfer rate at which any increase in metal temperature will result in a decrease in the amount of heat transferred due to vapor blanketing of the metal surface
In addition to a minimum local coolant flow rate, there is also a maximum local coolant flow rate which is determined by the stability of the corrosion protection layer.
Forced Convection: The transfer of heat from a solid to a fluid due to fluid flow across the surface of the solid produced by mechanical means (such as a fan or pump)
High local coolant velocities can cause erosion, resulting in significant localized pitting. Pitting can create regions of locally high stresses, reducing the overall strength of the cross-section.
Natural Convection: The transfer of heat from a solid to a fluid due to fluid flow across the surface of the solid produced from differences in density (induced by the heating of the fluid)
Engine coolant flow rate shall be less than the flow rate which will cause measurable erosion.
Nucleate Boiling: The name given to the boiling process when the vapor bubbles remain very small (microscopic scale)
The flow rate must also be sufficient to purge any accumulated gas which could interfere with function or cause corrosion. Any coolant temperature sensors must always be in contact with the coolant so gas cannot be allowed to accumulate at the location of the sensor. The coolant flow rate must be sufficient to purge any accumulated gas that would cause the sensor to read the wrong temperature.
Operating Point: That condition at which the flow rate and pressure rise provided by the pump is equal to the flow rate and pressure rise required by the system Pump Characteristic Curve: Plots the pressure rise across the pump versus flow rate as the flow restriction across the pump is varied
Coolant flow to all temperature sensors shall be sufficient to purge any accumulated air or vapor, under all engine operating conditions
Subcooling: Refers to the amount of temperature difference between the boiling point and the temperature of the fluid
These functional requirements can be expressed in a Pdiagram as shown in figure 24.
System Curve: Plots the pressure loss across the system as the flow rate through the system is varied. The purpose of the engine internal flow subsystem is to prevent the local metal temperature from exceeding the temperature at which: •
•
structural failure occurs, 29
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Localized Heat Flux
Internal Flow Subsystem
Local coolant velocity Surface area Coolant temperature Coolant pressure Coolant thermal properties Control Factors
Noise factor legend: V = Component variability U = Customer Usage E = Environmental
System resistance or demand curve Pressure Difference (kPa)
Noise Factors Engine structure thermal properties (V) Wall thickness variation (V) Engine load (U) Engine Speed (U)
Local Metal Temperature
Side effects Thermal stress Failure Modes Unstable heat transfer Cavitation Surface erosion
Operating point
Pump supply characteristic
Coolant flow rate (m^3/s)
Figure 24: P-Diagram for engine flow Figure 25: Coolant Operating Point
SYSTEM AND SUPPLY CURVES
While the engine is running, heat is transferred from the engine to the coolant by means of forced convection and sometimes by means of nucleate boiling at very high heat fluxes. After engine shut-down the heat is transferred away from the engine via coolant evaporation. Figure 26 shows the rate of heat transferred versus wall surface temperature for both nucleate boiling and forced convection
Surface Heat Flux (W/cm^2)
The pump characteristic curve (or supply curve) plots the pressure rise across the pump versus flow rate as the flow restriction across the pump is varied. This curve illustrates possible flow rate and pressure rise combinations that a given pump at a single rpm can supply. The pump characteristic curve is measured by connecting a variable valve to a pump and varying the valve position. This curve will tend downward as the flow rate increases, because the flow rate will increase as the restriction across the pump is decreased for a constant input in energy. Similarly, the possible combinations of flow rates and pressure rise required by a system is illustrated with the system or demand curve which plots the pressure loss across the system through which the fluid is being pumped. The system curve tends upward as the flow rate increases, because the frictional losses increase as the flow rate through the system is increased. As illustrated in figure 25, the flow rate and pressure loss through a cooling system graphically intersect at the operating point for a given pump and system. The operating point is that point at which the flow rate and pressure rise provided by the pump is equal to the flow rate and pressure rise required by the system.
F
D
D'
C
E
Velocity
B A Metal Surface Temperature (Log °C)
Figure 26: Pool boiling curve for water at atmospheric pressure[12]
The operating point in this example is a stable operating point, because the supply curve has a more negative slope at the operating point than the system curve. This means that if there is a perturbation, and the flow rate is decreased, then the driving pressure supplied by the pump will exceed the pressure required by the system, and the flow will increase back to the operating point. In the following paragraphs, you will see that the system curve shown here is a simplification of the system characteristic curve of a fluid in a firing engine, and under these conditions unstable operating points are possible
Figure 26 shows the change in heat transfer mode as the surface temperature of the solid increases. The region from point A to point B is convective heat transfer. At a given velocity a small change in heat flux will result in a relatively large change in the solid surface temperature. The region from point B to point C represents the nucleate boiling regime. Here the wall temperature change is relatively 30
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Critical Heat Flux (CHT) refers to the onset of vapor blanketing in an open channel, however in an engine, heat transfer takes place inside small diameter passages. Vigorous nucleate boiling can lead to blocking of small diameter passages at a heat flux that is lower than the CHT observed in an open channel. When vapor blocking occurs in a closed passage, the presence of macroscopic bubbles can result in not only an insulation of the solid surface, but also a blockage of the incoming coolant flow.
small as the surface heat flux is increased. Although this relative stability in the surface temperature is desirable, operating the cooling system in the vicinity of point D is risky. Point D is known as the critical heat flux, and it represents the point at which any increase in wall temperature will result in a decrease in the amount of heat transferred. If the heat flux increases beyond the value at point D, the wall temperature will jump from the surface temperature at point D to the surface temperature at point D’ (or higher). This excursion in surface temperature will generally result in the overheating of the solid surface, hence the term “burnout”, which is sometimes used to describe this condition. This decrease in heat flux beyond point D occurs because the very small vapor bubbles which are formed during nucleate boiling (which quickly collapse as they move away from the solid surface), have increased in size and energy to the point where they have begun to coalesce, and blanket the solid surface. The progression from convective heat transfer to film boiling is illustrated in figure 27.
Refer to Figure 25, which showed the system curve when the fluid is a single phase liquid and the passages are not heated. If a single phase gas were used, the curve would be steeper, representing a greater pressure loss for a given flow rate. Figure 28 shows two phases present in a heated passage. This two-phase curve represents the flow characteristics of coolant in a running engine.
Pressure difference across heated passage
Two phase flow
A-B Natural convection B Onset of nucleate boiling B-C Nucleate boiling low heat fluxes
A B Single phase - gas
C OFI Pump or supply curve Single phase - liquid Volumetric coolant flow rate
B-C Nucleate boiling high heat fluxes
Figure 28: Flow characteristics of coolant in a running engine
C Critical Heat Flux Onset of Flow Instability (OFI) The onset of flow instability (OFI) occurs on the two-phase flow curve at the local minimum where the curve begins to turn upward. At this point the flow rate in the engine coolant passage falls below a critical flow value. The vapor bubbles begin to block the flow of coolant to the extent that an increased driving pressure is required to maintain flow.
D-E Transition boiling
E-F Film boiling
At a flow rate that is below OFI, the slope of the system curve is more negative than the supply curve. This situation is always unstable as illustrated in the following scenario.
Figure 27: The various stages in the pool boiling curve[9]
In the system represented by the above curves, the system curve crosses the supply curve at points A, B, or C. If the flow rate falls below OFI, the system must operate at point A or point B, however at point B, any reduction in flow would result in an increase in pressure drop required, 31
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For new engines, the trend among engine manufacturers is to provide flow rates that correspond to an entirely convective heat transfer mode, with no excursion into the nucleate boiling mode. The advantage of this approach is that there is no danger of approaching OFI, which cannot as yet be analytically predicted. Furthermore, these high velocity cooling passages are smaller in cross-section, so the total fluid in the engine is reduced. This decreases the time required to heat up the coolant and therefore improves passenger compartment warm-up. In order to assure that the heat transfer within the cooling passages remains safely within the convective heat transfer mode, the local coolant velocities must be matched to the local heat flux so that a relatively constant surface temperature is maintained throughout the engine. This is sometimes called “precision cooling”. Computational fluid dynamics (CFD) is used to determine local flow velocities, and finite element models are used to determine local heat transfer rate.
which the pump cannot supply, and so the operating point would move to point A which represents all-vapor. Under these conditions the engine would overheat. The system design must supply a flow rate that is well above the critical flow rate to avoid the onset of flow instability. VERIFICATION Detecting Flow Instability To detect the onset of flow instability in an engine, run the engine in a dynamometer at the maximum heat rejection condition anticipated during vehicle operation, then slowly decrease the flow rate through the engine. On a vee engine, this best done by reducing the flow on only one bank at a time. When conducting this test it is necessary to devise a test set-up where the engine ignition can be shut off, and the flow rate rapidly increased when the critical flow rate is reached, to minimize the possibility of damage to the engine due to overheating.
Decreasing Minimum Flow If the minimum flow test indicates that the engine required minimum flow is close to or greater than the supplied flow rate, then the cooling system designer must increase the flow rate to the engine, or decrease the bulk coolant temperature. However if the engine design is under development, or is undergoing a change which will require new tooling, then any changes to the engine that will decrease the required coolant flow should be considered. These changes generally consist if increasing flow velocities at locations of relatively low flow and high heat flux.
The critical flow rate is reached when the flow rate becomes unstable, and the volume of the coolant increases (if the coolant temperature is held constant during testing). This critical flow rate is a function of heat rejected to the coolant, and the amount of subcooling at the engine exit or inlet. Subcooling refers to the amount of temperature difference between the boiling point and the temperature of the fluid. When a liquid is lower in temperature than its boiling point, it is sub-cooled. For example when water is at 95 degrees C, and atmospheric pressure, then the water is at 5 degrees C of subcooling. The closer the temperature of the exiting coolant is to boiling, the higher will be the critical flow rate (see figure 29).
CFD is often used to determine these areas of low coolant flow velocity. Magnesium borate has also been used as a means of determining the location of coolant vaporization. Magnesium borate is a substance that can be added to the coolant during an engine dynamometer test, which will deposit on the metal surfaces at locations of boiling and high coolant temperatures. The test engine is then cut apart, to observe the locations in the cooling passages where the magnesium borate has been deposited.[13]
critical one side flow (liters/min)
30 25 20 15 10 5 0 0
20
40
60
80
Exit subcooling (degrees C) (Exit subcooling = Boiling temperature minus the coolant temperature at engine outlet)
Figure 29: Critical flow rate vs. subcooling for a hypothetical engine
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Part 2 - Section 3 External Flow Subsystem OVERVIEW
Noise factors Pump assembly (V) Gasket variation (V) Deposit (A) Heat exchanger fouling (A) Loss of thermostat stroke (A) Engine speed (U) Vibration (E) NPSHA (E) Trapped air (E)
This chapter discusses: •
coolant flow requirements for the heat exchangers and degas reservoir,
•
system design alternatives,
•
and verification methods.
TERMS USED IN THIS CHAPTER
Engine Speed
Loss coefficient: The non-dimensional difference in pressure across a component
External Flow Subsystem
Flow to Componen
Pump geometry Pump Ratio Component hydraulic resistance Control Factors
Cavitation: Cavitation occurs when the pressure at a point within the fluid falls below the saturated vapor pressure causing the fluid to locally boil. Vapor bubbles form and subsequently collapse as they are carried into regions of higher pressure or lower temperature.
Legend V = Component varia A = Aging U = Customer usag E = Environmenta
Side effects Power consum Failure Modes Noise E i
Figure 30, P-diagram for flow to components
COOLANT FLOW REQUIREMENTS The cooling system must provide flow to the radiator, heater, engine, and any other heat exchangers in the coolant circuit at a rate which is adequate for performance, but does not exceed the flow rate that will cause an unacceptable amount of erosion. This requirement must be met under all operating conditions, for all customers, throughout the life of the vehicle
HEAT EXCHANGER FLOW REQUIREMENTS A typical fin and tube heat exchanger performance curve is shown in Figure 31. Note that the curve is steep at low flow coolant rates, and is flat at high coolant flow rates. At low flow rates large gains in performance can be achieved with small increases in flow rates. For radiators and heaters the program targets for flow rates must be determined by a cost benefits analysis, considering the following:
Figure 30 shows a parameter diagram for the external flow subsystem When existing production components are used, system design begins by determining the flow requirements of the components.
•
the cost of increased coolant and/or air flow rates,
•
cost and weight of larger radiators, and larger radiators,
•
and the consequences temperatures.
of
increased
coolant
Most heat exchangers will have some limitation on the maximum flow rate that can be tolerated. Excessive flow rates can lead to erosion damage of the heat exchanger tubes.
33
Heat transfer coefficient (kW/m²K)
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non-dimensional difference in pressure across a component. The hydraulic resistance of the component to the flow will cause a disruption in the flow, which will persist beyond the component and into the pipe beyond the component. The total pressure loss attributable to the component then must be measured across the entire distance that this flow disruption occurs.
Increasing Airflow
Degas Heater
Engine
Figure 31, Fin and tube heat exchanger performance
Radiator
Coolant flowrate (kg/s)
Bypass
Thermostat
Water Pump
Figure 32, Cooling System Schematic Example
DEGAS FLOW REQUIREMENTS De-aeration (or “degas”) reservoirs require a minimum flow rate for adequate performance. There is also a maximum flow limitation, above which the reservoir will cease to deaerate and begin to re-aerate the coolant. This maximum flow rate can be detected by installing a reservoir on a test rig and steadily increasing the flow rate. The critical flow rate at which the coolant is being re-aerated occurs when the coolant level rises (aeration of the coolant increases the volume), and air bubbles can be observed exiting the reservoir, (requires the installation of a clear hose at the reservoir exit).
To measure loss coefficient then, there must be a straight section upstream of the component to ensure uniform, fully developed flow into the component, and a straight section downstream of the component as well. The downstream pressure measurement must be taken sufficiently far downstream that the flow at the location of the pressure measurement is again fully developed. The pressure loss due to the pipe length between the upstream and downstream pressure measurements is subtracted from the pressure difference. In figure 33, the loss coefficient is the difference between the two parallel lines that represent the pressure versus length lines for the straight pipe with no component, and with the component in-line with the pipe.
NOTE: De-aeration requirements are discussed in a subsequent chapter. SYSTEM DESIGN FOR ADEQUATE FLOW
Developed friction gradient Non-dimensional Total Pressure
A simple schematic of the type shown in figure 32, can be used to indicate which components are in the system, and how the components are connected. Often more than one schematic will be considered in the initial stages of the system design. Given the schematic, and the hydraulic resistance of the components, you can determine the required output of the pump. The term “hydraulic resistance” is used here to indicate the pressure loss of the fluid as it flows through the component. More generally, it is the energy required to flow fluid through the component.
Friction gradient with Zero loss component K=
∆H 2
U / 2g
Non-dimensional Length
Redeveloped friction gradient
Component
The quantitative unit of measurement of hydraulic resistance is the loss coefficient. Loss coefficient is the
Figure 33, Loss Coefficient[14]
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The pressure drop across the two components in Figure 35 is given by the following equation:
Measuring loss coefficients is often appropriate where existing production parts are being utilized. Estimates of the loss coefficient (based on measurements of similar components) are appropriate for new components in the early stages of system design. When measuring loss coefficient, it is important to have fully developed flow upstream of each pressure measurement location, to eliminate variability in the measurement, and so that the loss associated with redevelopment of the flow is adequately accounted for. Figure 34 shows a schematic of a typical component flow measurement set up.
2
Q ρ ∆P = ---------- ( K A + K B ) 2A
Where: Q is volumetric flow rate, ρ is density, A is area, and KA and KB are component loss coefficients. Plotting this curve and determining the point of intersection with the pump curve will provide the operating point. However parallel circuits, component interactions, and the need to iterate the design will significantly increase the complexity of the task. This process lends itself to computer simulation that can be easily applied with the use of a one-dimensional axial flow simulation code, such as FLOWMASTER.
Pressure measurement locations
30 Diameters
Although this can be done mathematically, a computer code is extremely useful for this purpose. The input required and the computation time for a one-dimensional flow simulation are both significantly less than would be required for a 3-D or computational fluid dynamics code.
Component to be tested
Figure 34, Set up for component flow measurement
The pump must be matched to the system as well as provide enough output (head) for adequate flow. This means that the operating point must be near the peak efficiency of the pump. If the flow at the operating point exceeds the flow at best efficiency, then the risk of cavitation is significantly increased. Cavitation occurs when cavities or vapor bubbles form and subsequently collapse as they are carried into regions of higher pressure.
Calculate the loss associated with the flow through the pipe length after the test component and before the second pressure measurement. Then subtract this from the pressure loss measured. The pump output requirements can be determined after deciding on the flow arrangement and either measuring or estimating the loss coefficients for each component (as well as for the connecting hoses). The mathematical problem is easily defined as illustrated in the following example. Consider a simplified circuit with a schematic as in figure 35.
Pump
er Th
t sta mo
en Op
Flow Capacity
Figure 35, Simplified Circuit Schematic
Figure 36, Capacity Range
35
Hydraulic Efficiency
Best Efficiency Point
Capacity Range
KA
at Cl os ed
KB
Greater Than 80% BEP
m os t
Component A
Static Pressure Difference (Head)
Component B
As a general guideline, the operating point with the thermostat open should be at the point of best efficiency, and the operating point with the thermostat closed should correspond to a flow rate of no less than 80% of the flow with the thermostat open.
Th er
30 Diameters
(4)
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away from the reservoir as possible, as illustrated in figure 37. This is desirable for two reasons:
The requirement for pump characteristic curve (or range of curves) is passed along to the pump design engineer or supplier. After estimating loss coefficients and pump characteristics, the pump designer must verify that these estimates are feasible. If further investigation shows that the component requirements are not likely to be met, then it is necessary to modify the design of the system. Although the requirements process begins at the vehicle level, the process of setting targets is highly iterative and requirements are imposed on the system by the components just as the system imposes requirements on the components. The original function of the cooling system after all, is to provide an environment in which components can survive.
•
Flow as it leaves the restrictor has a high local velocity, but further downstream the flow redevelops at a lower velocity (the velocity of the fluid entering the reservoir should be as low as possible).
•
The hose and any hose joints downstream of the restrictor will be subjected to pressures that are lower than those locations which are upstream of the restrictor. Restrictors should NOT be added at these locations
Restrictors should be added at these locations
Degas
If the required flow rates are not achieved, the following system design alternatives can be considered: •
An external heater bypass (an additional flow branch) will increase flow through the engine, and decrease flow through the heater.
•
If heat exchangers are arranged in parallel rather than series, overall flow rate is increased. For example, if an oil cooler is in parallel with the heater rather than in series, this will increase flow through the engine and decrease flow through the heater.
•
Reducing hose bends and increasing hose diameters will increase flow.
•
A dual acting thermostat, which closes off the bypass as the radiator flow path is opened will increase flow through the radiator.
Engine
Figure 37, Flow restrictor location
VERIFICATION A validated one-dimensional computer simulation code can be relied upon for much of the product development process. To validate a flow simulation, the output of the flow model should be compared to the results of a bench test. The difference between the simulation and the bench test must be well below the safety margin applied.
COMPONENT DESIGN ALTERNATIVES
For example, if the critical flow rate through the heater is 20 gpm, and you have designed the system to deliver no more than 15 gpm, (a 25% safety margin), then a 5% to 10% degree of accuracy for the computer model is likely to be acceptable.
If system modification fails to produce adequate coolant flow, then the component design must be modified to: •
reduce the hydraulic resistance of the components,
•
or increase the output of the pump,
•
or decrease the flow requirements of the components.
Radiator
SYSTEM DESIGN ALTERNATIVES
When you conduct a flow test, the requirements of the flow measurement devices must be observed. The flow meter may require a length of straight section upstream (and sometimes downstream) in order to accurately measure flow. Furthermore most flow meters provide a measurable hydraulic resistance which will alter the resultant flow. For best results, set up a flow stand which accommodates all of the requirements of each of the flow and pressure measurement devices (there can be flow development, temperature, and vibration limitations). Then simulate the very same circuit (including all measurement devices) in the computer model.
In addition to minimum flow requirements, there are also maximum flow requirements, particularly with heat exchangers and degas reservoirs. Sometimes the minimum and maximum flow requirements for heat exchangers cannot both be accomplished without the addition of a diverter valve. The diverter valve causes some of the flow to bypass the heat exchanger at high pump speeds. For degas reservoirs, a restrictor is generally used to reduce the flow rate to the reservoir. Any restrictor in the degas reservoir inlet line should be added to the line as far 36
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If the output of the flow stand and the flow model are sufficiently close, then modify the computer model so that the hydraulic resistance of any measurement devices is removed. The results are then corrected flow stand data. If the computer model shows consistent agreement to the flow stand, then additional design alternatives can be evaluated with the computer model.
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Part 2 - Section 4 Heat Dissipation Subsystem OBJECTIVE
Noise Factors Air inlet temperature Air thermal properties Coolant properties Radiator surface fouling Radiator surface damage
The objective of the heat dissipation subsystem is to protect the coolant by keeping it liquid at all times. This is achieved by ensuring that the bulk coolant is not allowed to reach a temperature where a local heat flux, anywhere within the engine, is sufficient to cause the coolant to boil.
Waste Heat
In essence, the coolant protects the engine, and the heat dissipation system protects the coolant.
Coolant mass flowrate Airflow maldistribution Air mass flowrate Heat exchanger area Heat exchanger surface geometry
In this section we will discuss: •
Heat dissipation subsystem requirements
•
Heat exchanger selection
•
Business and project constraints
•
Heat exchanger construction
•
Manufacturing processes
•
Heat transfer considerations.
•
Heat exchanger design proceedure
•
Mechanical and structural considerations
Heat Dissipation Subsystem
Control Factors
Coolant Temperature Side effects Fluid pumping power Aerodynamic drag Underhood temperature Failure modes Radiator thermal fatigue Heat exchanger corrosion Heat exchanger erosion Fin damage
Figure 38, P-diagram for automotive heat exchanger
HEAT EXCHANGER SELECTION The selection of an optimum “heat transfer surface” is not easily made as it is always heavily influenced by the particular application and installation. The only thing we can say with certainty is that there is no single “best” heat exchanger design. Every design requires a trade off between both qualitative and quantitative design parameters.
HEAT DISSIPATION SUBSYSTEM REQUIREMENTS In order to keep the coolant liquid at all times, the heat dissipation subsystem must be capable of extracting heat from the cooling system at the same rate at which it is generated. It must accomplish this task at all engine operating conditions, in all environmental conditions, not a just a few discrete operating conditions at which the vehicle is normally tested.
BUSINESS AND PROJECT CONSTRAINTS In selecting an optimum heat exchanger a number of business and project constraints must be taken into consideration.
It covers any condition that the customer can impose the vehicle. Under all operating conditions, the heat dissipation subsystem shall be capable of removing heat from the cooling system at the rate that it is generated. This requirement must be satisfied under all environmental conditions for the market in which the vehicle was originally sold. The P-diagram in figure 38 identifies some of the key factors to be considered when designing a new automotive heat exchanger:
•
The heat exchanger is typically the most expensive component within the cooling system.
•
It can have a major impact on vehicle front-end appearance.
•
It is a bulky item and imposes a major constraint on front-end package, including cross member location, crash performance and engine accessory location, to name just a few.
•
It has one of the longest manufacturing lead-times of all vehicle components.
Clearly, the early selection of the heat exchanger is vital to successful vehicle design. 39
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be developed from first principles, using sizing* analysis techniques.
Investment / Design Reuse When developing the cooling system for a new vehicle, the first issue to be addressed is design reuse. As the heat exchanger is typically the most expensive item in the cooling system, the feasibility of reusing an existing design is generally one of the first decisions to be made.
The true loss function associated with the decision to reuse an existing design will be the difference between the actual loss function and the idealized loss function. Only after this initial trade-off analysis has been completed should the decision to reuse an existing design be taken.
Feasibility is generally performed by conducting a performance rating* analysis to assess the capability of any designs, which have the potential to be reused.
Variable Cost / Part Number Complexity
If a potential match is found, then a full capability analysis using functional composition and Potential Failure Mode and Effect Analysis should be performed. The design should be validated to ensure that the design can meet all of the functional requirements.
Within any vehicle product range, there are invariably a number of different engine and transmission options available to the customer. These different options may be combined to form a large number of different vehicle configurations.
If a partial match can be identified the engineer should develop a design based on the maximum reuse of existing parts and tooling. Table 3, show the effect of making a dimensional change on heat exchanger tooling.
To develop an optimized heat exchanger for each of these permutations may not be possible for complexity and investment reasons. It is important to establish early in the design process, how much complexity is affordable. Since each vehicle must meet a minimum level of performance in order to satisfy customer expectations, some of the vehicles will over-engineered and receive a heat exchanger that was designed for more demanding powertrain combination.
Fin Density
X
O
Tube mill
X
Header stamping Side support
Width (side to side)
Fin roll
Length (along tube)
Depth (front to back)
Parameter Changed
X X
Within the Systems Engineering philosophy this “overspecification” is as much a loss of function as “underspecification”.
X
To optimize the design, we must take a systemic view of this over-specification and look for offsetting cost reduction opportunities.
X
Design Control Parameters The heat exchanger design engineer has five basic parameters that can be tuned to produce an optimum design: -
Tanks
X
X
Tank gasket
X
X
Table 3: Effect of dimension change on heat exchanger tooling
The need for new tooling to support a fin density change will depend on the magnitude of the change. A single fin roll will be capable of producing a range of different fin densities.
•
Heat exchanger surface geometry
•
Heat exchanger size
•
Coolant mass flow rate
•
Air mass flow rate
•
Airflow maldistribution
Before the reuse of an existing or slightly modified design is accepted, a loss function for the primary function and each of the identified side effects should be developed.
These parameters impact other systems within the vehicle, both directly and indirectly, effecting the performance of other vehicle attributes. These effects may include the following: -
These loss functions should be compared against the loss function for an “idealized” design. This ideal design should
•
Engine parasitic losses due to coolant pump
•
Aerodynamic drag due to cooling airflow
•
Air-conditioning performance
•
Auxiliary cooler performance
*.
The techniques for Rating and Sizing will be discussed later in this section.
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•
Vehicle noise
•
Underhood component temperatures
•
Accessory drive durability
Flat Tube, Parallel Plate Fin Heat Exchangers For many years parallel plate fin heat exchangers of the type shown in figure 40, represented the state-of-the-art within the automotive industry. Constructed from solder coated brass tubes and copper fins, they were joined together by baking in an oven at temperatures around 300°C. The assembly required little sophisticated manufacturing technology and gave adequate thermal performance and moderate mechanical and corrosion durability.
The list above may not complete, but it does illustrate the range of issues that the cooling engineer must consider when conducting a trade-off study. To be able to perform this trade-off, the cooling system design engineer must be able to quantify the heat exchanger performance in terms that have universal value. There is little point trying to trade-off dBA and degrees Celsius. This measure of universal value may go through several intermediate iterations, such as weight, reliability, package size but it will always ends up as money. If we assume that fixed costs are constant for a particular method of heat exchange construction, we can develop a cost model of the heat exchanger based on the cost of the raw material used in its construction.
a) Plain Fin
HEAT EXCHANGER CONSTRUCTION
b) Slit Fin
c) Louvered Fin
Figure 40, Typical flat tube, parallel fin heat exchanger surfaces
In the early 1900’s automotive cooling heat exchangers comprised of simple loops of circular tubes with disc fins. These fins and tubes were coated with carbon black to enhance the heat transfer which was predominantly driven by radiation. (Figure 39)
Initially a plain plate fin was used, (Figure 40a), but this was soon augmented by the addition of simple scoops in the fin's surface (Figure 40b). As engine power increased, further refinement was necessary and the surfaces became fully louvered, (Figure 40c), yielding excellent thermal performance characteristics. Today, this type of heat exchanger is seldom used for anything other than special application, due to the high cost of manufacture. The high manufacturing costs associated with this type of design can be attributed to two factors. Firstly, the process of inserting the tubes through the slots in the plate fins is carried out by hand, making the assembly highly labor intensive and very costly. Secondly, material gauge used for both the fins and tubes is higher than functionally necessary, (from a thermal perspective), in order to withstand the physical handling that occurs during assembly.
Figure 39, Engine cooling “radiator” with alternative fin designs c.1900
In today’s vehicles, the heat exchanger may still called a “radiator” but almost all heat transfer takes place by forced convection rather than radiation.
This type of construction does, however, produce a extremely robust structure and can be used to produce cores much deeper than possible using the corrugated fin design. This capability is important for off-highway vehicles where heat exchanger frontal area is limited and fin density must be kept low to prevent fouling of the air passages. Under these circumstances, the only way to maintain heat transfer surface area is by increasing the core depth.
The high heat dissipation demands placed on the cooling system radiator have given rise to the development of three basic configurations of heat exchangers: •
Flat tube with parallel plate fins.
•
Mechanically bonded with parallel plate fins.
•
Flat tube with corrugated fins. 41
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Mechanically Bonded, Plate Fin Heat Exchangers This type of heat exchanger construction is a special adaptation of the tube and plate design concept, specifically developed for automated production using aluminum. Based on a concept first patented in the early 1900's, it does not require a thermal bonding process or the use of adhesives, for core assembly. (Figure 41).
a) Round tube
b) Elliptical Tube
Figure 42, Typical enhanced performance mechanically assembled heat exchanger fin geometry
Flat Tube, Corrugated Fin Heat Exchangers Corrugated fin construction, of the type shown in figure 43, has progressively replaced parallel plate designs and represents the current state-of-the-art in automotive heat exchanger design.
Figure 41, Typical mechanically bonded heat exchanger surface
Developed as a low cost alternative to parallel plate construction, corrugated fin designs utilized the same copper/brass and soft solder assembly techniques and had a similar degree of surface enhancement. Today, aluminum has replaced copper/brass as the preferred material of construction, for a number of reasons that will be discussed later.
Simple plate fins are stamped from strip aluminum using a multi-stage progression die. Tabs formed on the fins hold them apart at the required fin density. Extruded seamless circular tubes are inserted through the holes in the fins, (which are designed to have a clearance in order to facilitate easy assembly). The header plates are then added and the assembly inserted into a special press. Steel rods with hardened “bullet” expanders fitted to their ends are forced down the inside of the coolant tubes, which are expanded outward to form an interference joint with the fin. The simple nature of the fin surface augmentation and high thermal resistance of the fin to tube joints, produces a heat exchanger with modest thermal performance. Up to 60% more frontal area is required achieve the same level of performance as an equivalent corrugated fin design.[15] Furthermore, because of the use of circular tubes, rather than flat tubes, form drag is much higher causing a substantial increase in the air-side pressure drop.
a) Plain Fin
b) Slit Fin
c) Louvered Fin
Figure 43, Typical flat tube, corrugated fin heat exchanger surfaces
Various improvements have been made to this type of design using more sophisticated surface augmentation and elliptical, rather than round tubes, (figure 42). These enhancements have significantly improved performance relative to round tube designs by as much as 30% by weight[16], with minimal increase in cost.
The mechanical insertion of tubes through the fins used in the parallel plate construction, is replaced by a process that places alternate layers of corrugated fin and tubes loosely in a frame, which is then clamped together with the side supports. Tube header plates are fitted and the assembly baked in an oven to bond the fins, tubes, side supports and header plates.
They do not close the performance gap to corrugated fin designs significantly and are only suitable for low heat dissipation applications and therefore will not be discussed further in this paper.
This construction technique significantly reduces assembly forces and enables designers to utilize more sophisticated (and delicate) fin surface augmentation, along with thinner gauge material for both tube and fin. The resultant heat exchanger is lighter, has a higher thermal performance and is significantly cheaper than an equivalent plate fin design.
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•
While the thermal performance of corrugated fin heat exchangers is superior to that of the plate fin designs, the mechanical strength is inherently lower. In plate fin construction, the tubes pass through the fins producing an interlocking matrix, which does not rely upon the solder joint between the tube and the fin for structural integrity.
Entrapment of corrosive brazing flux residue left over from the manufacturing process.
The problem of salt spray induced corrosion can significantly reduced by chromating the surface, but problem of flux entrainment is completely tied into method of manufacture. (Chromating may make problem worse by locking-in flux residue left behind by brazing process).
In the corrugated fin design, however, the bonds between the coolant tubes and fins are load-bearing joints, which are subject to constant stress changes, as a result of differential thermal expansion, mechanical vibration and loads caused by internal pressurization of the tubes.
be the the the the
A variety of different brazing techniques were investigated employing different tubes, fluxes and tube surface cladding with varying degrees of success. Initially all of these designs used lock seam tubes of the type shown in figure 44. These seams acted as a natural trap for flux, distorted during the rapid rise to 600°C necessary for brazing, and formed a notch where the tube passed through the header plate that often failed to fill during brazing.
For passenger car applications, the poor fatigue strength of the soldered tube to fin joint in copper and brass radiators did not present a problem. For heavy duty and off highway vehicles it was not until the introduction of brazed aluminum construction that corrugated fin designs became universally viable. MANUFACTURING PROCESSES
Header Piercing Profile
The basic processes for the manufacture of an aluminum heat exchanger differ significantly only in respect to the technique of fabrication. Instead of a 300°C solder process a 600°C temperature is necessary to melt the braze filler. The brazing processes used for aluminum heat exchanger production can be broadly split into three categories: •
Atmospheric flux brazing
•
Fluxless vacuum brazing.
•
Controlled atmosphere flux brazing.
Figure 44, Section through seam-lock tube
Although impact formed tubes and tubes drawn from extrusion were both seamless and common place, neither could be used in this application. The impact formed tubes were both too short and dimensionally inaccurate, while the drawn tube were too slow to manufacture, too expensive and could not be clad, thus requiring the fins to be clad.
Atmospheric Flux Brazing (FBAR) The techniques for flux brazed aluminum radiators (FBAR) were first developed in the 1950’s but their uses were limited to low volume specialty vehicles like the D-type Jaguar and Corvette where weight considerations were of greater importance than the corrosion and financial constraints that prevented their widespread use.
It was not until a machine capable of seam butt welding strip into tubes was developed, in 1965, that tubes could be produced at a rate and price that made mass production viable.
Aluminum is susceptible to two basic types of corrosion; galvanic and localized pitting. The problems of galvanic corrosion can be easily prevented by avoiding the contact of dissimilar materials, but pitting corrosion is more difficult to prevent.
However, despite the use of tubes produced by this technique, excessively long flux cleaning cycle times of up to 40 minutes remained a problem[17]. Even the development of a low flux brazing process, coupled with the use of ultrasonic cleaning could only reduce wash times to six minutes, which was still to long for large scale production. A combination of hot water and nitric acid as a cleaner, with a final rinse of de-ionized water eventually enabled large scale production of atmospheric brazing.
Pitting corrosion can occur both internally and externally. Internally, the major risks comes from uncertainty about the long-term effectiveness and stability of the corrosion inhibitors in the coolant antifreeze solution.
Paquet[18] reports that heat exchangers using atmospheric flux brazing, now offer the best external corrosion resistance of all of the brazing processes. By using a flux that diffuses zinc into the core material, the serious
Externally, two major sources of corrosion exist: •
Exposure to sea-coast salt spray or road salt laid down in winter. 43
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Rather than use the normal chloride based flux, the NOCOLOK® process uses a potassium fluoro-aluminate flux containing a eutectic mixture of K3AlF6 and KAlF4,
problem of pitting corrosion can be transformed in general corrosion, which is much less damaging.
which melts at approximately 560°C. Fluxless Vacuum Brazing (VBAR)
The key difference between the flux of the FBAR and CAB processes is that while the chloride based flux causes pitting corrosion, the potassium fluoro-aluminate flux residue produces a layer that improves the air-side corrosion resistance.
In the process for vacuum brazed aluminum radiators (VBAR)[19], one side of the materials used to form the tubes, headers and side supports are pre-coated with a layer of brazing compound that contains aluminum, silicon and magnesium.
The CAB process is a continuous process in which the radiators once assembled, are carried via conveyor belt through a degreasing station before being sprayed with flux. The radiators are then dried and passed to the brazing furnace.
The assembly is clamped together in a frame and loaded into a press where the tube ends are expanded to prevent excessive movement during brazing. The core is then thoroughly degreased with a solvent. Batches of heat exchangers are placed into racks which are loaded into the first of three chambers that form the brazing furnace. As heating within the furnace is performed by radiation, all of the heat exchangers within a batch must have identical thermal mass to ensure that all units reach the brazing temperature at the same time.
The furnace is internally divided into three zones; pre-heat, brazing and cooling. To prevent oxidation the atmosphere inside the furnace is carefully controlled to exclude oxygen and water vapor. This is achieved by maintaining an inert nitrogen environment with a dew point less than -40°C inside the furnace. Heat is provided by a combination of radiation and convection.
Once inside the first chamber the external door is closed and the atmosphere evacuated while radiant heaters preheat the assemblies that are to be brazed.
On leaving the furnace, the radiators are allowed to cool slowly. No washing or cleaning is required.
After a period of pre-heating, a door to the second chambers is opened and the radiators are passed into the second chamber where the temperature is raised to approximately 600°C, melting the cladding. In the very low pressure of 10-5 mbar, the magnesium in the cladding evaporates, breaking down the oxide layer, allowing the cladding to flow and form a brazed joint.
Of the three processes discussed, CAB is both the cheapest and most “forgiving”, as it forms the largest fillets. In addition it requires the least amount of energy and has the lowest level of environmental impact. The only significant drawback of the CAB process is that high strength aluminum alloys containing magnesium have significantly reduced brazability[20]. The magnesium reacts with the flux to form a compound that raises the melting point and reduces the fluidity of the molten cladding.
In the final chamber, the heat is removed and the radiators are allowed to cool while the vacuum is maintained. When the temperature reaches to 350°C the outlet door is opened and the radiators are removed. The total process having taken approximately 10-15 minutes.
At a magnesium content of less than 0.4%, increasing the flux loading can offset the effect of magnesium, but at higher levels the braze quality is significantly reduced.
The radiators may then be chromated for enhanced corrosion protection before the header tanks are fitted. (Chromating is not a preferred process today, because of environmental disposal considerations).
This limitation, prevents the use of higher strength, heat treatable, 6000 series aluminum alloys which are used for header plates on FBAR and VBAR, as they generally contain 0.4-0.6% magnesium. The reduced mechanical strength impacts pressure cycling durability on CAB radiators[21].
The brazing process produces relatively small braze fillets and demands very high component tolerances. This coupled with the inherent batch nature of the process and the high capital investment, makes vacuum brazing an expensive process.
Brazed Copper-Brass Radiators The thermal and structural performance of copper/brass radiators has traditionally been limited by the inherent deficiencies of the lead based soldering process used for assembly.
Controlled Atmosphere Flux Brazing (CAB) While vacuum brazing was being deployed in the mid1970’s and early 1980’s, Alcan had been developing a controlled atmosphere brazing (CAB) process, using noncorrosive fluoride based flux known as NOCOLOK®. 44
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c = Cold fluid (air) h = Hot fluid (coolant) i = inlet o = outlet
The development of a flux free brazing alloy CuSnNiP, has enabled copper/brass radiators to be manufactured in both vacuum and controlled atmosphere furnaces[22]. As with the aluminum brazing processes, the size of the fillets generated are significantly smaller than those generated by soft solder, making the process significantly less forgiving of manufacturing variation and tolerance stack-up.
and the Capacity Rate, C is defined as: C c = m· c ⋅ Cp c
C h = m· h ⋅ Cp h
(7)
The smaller of the two values is defined as Cmin and the larger Cmax and C* is the Capacity Ratio: -
Copper-brass automotive radiators manufacture using CAB techniques have yet to demonstrate volume production viability.
C min C∗ = ------------C max
HEAT TRANSFER CONSIDERATIONS
(8)
The number of transfer units, NTU is defined as: -
In order to calculate the size of heat exchanger necessary to meet the required heat dissipation, we require a method defining the performance of a heat exchanger surface in non-dimensional terms.
1 AU NTU = ------------ = ------------ ⋅ U dA C min C min
∫
(9)
A
Where Heat Exchanger Performance Definition
A = Total heat transfer area on Cmin side
Two common methods exist for expressing the heat transfer characteristics of a given heat exchanger surface geometry. These are known as the Log-mean ∆t approach and the ε-NTU approach.
U = Overall heat transfer coefficient (W/m2K) When the inlet temperature and flow rates are specified, the maximum heat transfer rate possible, (for an infinitely sized counterflow heat exchanger), is given by:
Although the Log-mean ∆t approach is useful for quickly rating parallel and counterflow heat exchangers is cumbersome when applied to crossflow heat exchangers of the type used in vehicles. Therefore its use is not recommended.
Q· max = C min ⋅ ( T hi – T ci ) If effectiveness, ε is defined as the ratio of actual dissipation to maximum dissipation we get:
The alternative method using Effectiveness - Number of Transfer Units, ε-NTU is preferred.
·
–T
)
(11)
C c ⋅ ( T co – T ci ) = ------------------------------------------C min ⋅ ( T hi – T ci )
The performance of a heat exchanger can be determined by examining the heat loss and heat gain that takes place between its working fluids: The heat lost by the coolant can be expressed as: -
Effectiveness can be expressed as a function of NTU, Capacity Ratio and heat exchanger configuration
(5)
ε = ε ( NTU , C∗ , Flow Arrangement )
Heat gained by the air can be expressed as: · Q c = m· c ⋅ Cp c ⋅ ( T co – T ci )
C ⋅ (T
Q - = ------------------------------------------h hi ho ε = ------------C min ⋅ ( T hi – T ci ) Q· max
ε-NTU Approach
· Q h = m· h ⋅ Cp h ⋅ ( T hi – T ho )
(10)
(12)
A typical graph of heat exchanger effectiveness is shown in figure 45.
(6)
Where Q· = Overall heat transfer (kW) m· = Mass flow rate (kg/s) Cp = Specific heat capacity (kJ/kgK) T = Temperature (°C) 45
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–B 1 - ⋅ (1 – e ) ε = ------
1.0 0.9
For number of rows, N =2
0.8 0.7 Effectiveness
(13)
C∗
– B ε e –B 1 - ⋅ 1 – e – ------ ⋅ e ε = ----- 2 C∗
0.6 0.5
(14)
Where
0.4 0.3
1.0 kg/s
0.2
2.0 kg/s 3.0 kg/s
0.1
B = N ⋅ ε e ⋅ C∗
(15)
and elemental effectiveness, εe is given by: -
3.5 kg/s
0.0 0.0
5.0
10.0
15.0
Airflow Velocity (m/s) @ NTP
εe = 1 – e
NTU C min –-------------- ⋅ --------- N Ca
(16)
For the general case: -
Figure 45, Heat exchanger effectiveness
–B 1 -⋅1–e ⋅1 ε = ------
(17)
C∗
Flow Arrangement
N
It has been traditional to consider automotive heat exchangers as having a flow arrangement where the fluids on both sides of the heat exchanger are “unmixed”. The term unmixed, refers to the fact that flow through the heat exchanger behaves as if it was divided into a large number of separate passages with no lateral mixing.
1 + ---- ⋅ N
n–1
n–k–1
∑ ∑ βk ⋅ ∑
( j + k – 1 )! j -------------------------- ⋅ a j! ⋅ ( k – 1 )!
where ( ε e ⋅ B )! β i = --------------------i!
While this may be accurate for multi row heat exchangers with many rows of tubes in the airflow direction, in one row heat exchangers this assumption is invalid. Mixing of the coolant will take place within each tube.
a = e
Similarly, for multi row heat exchangers with only 2 or 3 rows of tubes in the airflow direction, it would be more appropriate to consider the flow to be mixed within each tube, but unmixed between tube rows.
(18)
NTU C min –-------------- ⋅ --------- N Ca
(19)
Overall Heat Transfer Coefficient. Plain surface heat transfer– Consider figure 46, which shows a plain wall separating two flowing fluids; coolant on one side, air on the other.
Webb[23], also suggests that difficulties exist on the airside, as some lateral mixing will also takes place through the louvers of the fins. To compensate, he proposes a “mixing fraction” that can be used to reduce the overprediction. While this argument has merit, the effect is small compared to the coolant side error. Provided that the geometries being compared are similar and the same algorithm is used to “wind-up” and “unwind” the problem, the errors are largely self-cancelling.
Th hh
t Air
Tw1 Tw2
Tc hc
Coolant k
Glober[24], proposes that each row be treated as a separate heat exchanger placed in series, with the air outlet temperature of the former row, being the air inlet temperature of the later row, giving the following relationships: -
Q A
For number of rows, N = 1
Tc
1 hh
Tw1
t k
Tw2
1 hc
Ta
Figure 46, Heat flow through wall of a plain (unfinned) flat tube
46
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If we assume the fin has efficiency ηf, then equation 21 can be rewritten to give:
The flow of heat from the hot coolant to the cold air, can be represented by Newton’s Law of Cooling as shown in Equation 20. Q = U ⋅ A ⋅ ( Th – Tc )
1 1 1 t 1 -------------- = ------------- = ------------ + -- + ------------------Uh Ah Uc Ac A h hh k η0 A c h c
(20)
Where the overall heat transfer coefficient, U comprises of three separate resistances as shown in equation 21. t 1 1 1 -------- = --------- + -- + --------UA Ah c k Ah h
(22)
and total surface efficiency, η0 Af η 0 = 1 – ----- ( 1 – η f ) A
(21)
(23)
where
where
A = surface area on which U is based
Q = heat transfer rate (W) A = area (m²) T = temperature (°K) h = heat transfer coefficient (W/m²K) t = wall thickness (m) k = thermal conductivity of wall (W/mK)
and fin efficiency, ηf T f ( mean ) – T a Fin Efficiency ,η = ---------------------------------- f T w2 – Ta
subscripts c = air (cold fluid) h = coolant (hot fluid) w = wall
where Tf(mean) = Mean effective fin temperature The fin efficiency is a function of cross sectional area, its length, the thermal conductivity of the fin and the surface heat transfer coefficient.
Extended surface heat transfer– As with most liquid-to-air heat exchangers, the thermal performance of the radiator is constrained by the poor thermal conductivity of air, a medium normally associated with insulation rather than conduction. To compensate for air having a thermal conductivity of less than 5% of the coolant, it is necessary to substantially increase the surface area on the air-side of the heat exchanger by the addition of fins.
For a flat tube heat exchanger[25], it can be shown that: tanh ( mL ) η f = -----------------------mL
(25)
where
Figure 47, shows a wall with a fin on the air side. Heat transfer now takes place from the fin surface as well as the unfinned surface.
m =
Air
2h ------ k δ
(26)
Increasing the secondary surface area by itself is an inefficient method for improving the overall heat transfer rate as it increases the cost of the heat exchanger.
L
k
(24)
δ
Coolant
As can be seen in equation 26, fin efficient can be increased without adding material if the heat transfer coefficient, h can be increased.
Tc
Coolant-side Heat Transfer Coefficient. Tw1
Tw2 h
The primary purpose of the radiator tubes is to permit the efficient transfer of heat from coolant to air. A convenient method of quantifying this heat transfer is through the dimensionless parameter Nusselt Number, Nu.
Tf Ta
(x ⁄ k) Fluid layer resistance Nu = ---------------------------------------------------- = -------------(1 ⁄ h ) Convective resistance
Figure 47, Heat flow through the wall of a finned flat tub.
47
(27)
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For flow through a tube the characteristic length x, is defined as the hydraulic diameter Dh 4 × Flow Area D h = -----------------------------------------Wetted Perimeter
Nu = 0.023 ⋅ Re
(28)
Nu = 0.027 ⋅ Re
Heat transfer coefficient on the coolant-side of the heat exchanger hh, is given by equation 29. k h h = Nu ⋅ ------Dh
A number of forced convection heat transfer correlations have been developed for laminar, transitional and turbulent (A comprehensive survey of these correlations and their applicability can be found in Karaç and Liu[36])
(35)
1 ⁄ 3 µb
0.14
⋅ ------- µ w
(36)
(37)
where Fanning Friction Factor, f is given by: f = ( 1.58 ⋅ Ln Re b – 3.28 )
–2
(38)
Modifications to the Petukhov model, by Gnielinski using experimental data has extended the correlation to include the transitional range (2300 < Re < 104) where most automotive radiators operate.
(30)
w
Reynolds Number, Re is defined as
( f ⁄ 2 ) ⋅ ( Re b – 1000 ) ⋅ Pr b Nu = -------------------------------------------------------------------------------1⁄2 2⁄3 1 + 12.7 ⋅ ( f ⁄ 2 ) ⋅ ( Pr – 1)
2
ρ⋅u⋅D Re = ---------------------hµ
⋅ Pr
0.3
( f ⁄ 2 ) ⋅ Re b ⋅ Pr b Nu = --------------------------------------------------------------------------------------1⁄2 2⁄3 1.07 + 12.7 ⋅ ( f ⁄ 2 ) ⋅ ( Pr – 1)
The most commonly used relationship for laminar flow (Re < 2300) is the correlation proposed by Seider and Tate in 1936.
ρ⋅u Momentum forces Re = -------------------------------------------- = ---------------------µ ⋅ (u ⁄ x) Viscous forces
0.8
⋅ Pr
Webb[23] identifies the three-layer, turbulence boundary layer model of Petukhov[37] (Re > 104) as more accurate than the Dittus-Boelter relationship, which is good for “quick approximate calculations”.
(29)
D h 1 ⁄ 3 µ b 0.14 ⋅ ------- Nu = 1.86 ⋅ Re ⋅ Pr ⋅ ------- L µ
0.8
(31)
(39)
Figure 48, shows a comparison of Nusselt Number verses Reynolds Number for the correlation equations discussed above.
(32)
Prandtl Number, Pr is defined as
80
(µ ⁄ ρ) Momentum diffusivity Pr = ------------------------------------------------------ = ------------------------------( k ⁄ ( µ ⋅ Cp ) ) Thermal diffusivity
(33)
Cp ⋅ µ Pr = -------------k
(34)
70
Nusselt Number
60
and L = Length of tube (m) µ = Viscosity (Ns/m²) Cp = Specific heat capacity (J/kgK) ρ = Density (kg/m³) u = Velocity (m/s) subscript b = bulk w = wall
50 40 30
Seider-Tate (Laminar) Seider-Tate (Turbulent) Dittus-Boelter Petukhof Gnielinski
20 10 0 0
2000 4000 6000 8000 Coolant Side Reynolds Number
10000
Figure 48, Nusselt Number vs. Reynolds Number for brazed aluminum radiator tube, Dh=2.2mm L=770mm @ 50% Glycol
104)
For turbulent flow (Re > the correlation proposed by Dittus-Boelter (equation 35) or the modification by SeiderTate (equation 36) are frequently used: -
48
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Rather than use dimples in the tube walls, some manufacturers have developed “ladder” inserts, which are placed inside the radiator tubes[38].
As can be seen in figure 48, at a Re = 1000, the Nusselt Number predicted using the Petukhov turbulent model of equation 37, is 3 times that predicted by the laminar model of equation 30. Similarly at Re = 2000, Nuturb = 4 x Nulam. These differences represent the potential gain to be made by the use of coolant side augmentation devices.
In general these inserts provide a greater performance improvement than can be achieved through the use of dimples, as the presence of the “ladder” inside the tube also increase Reynolds Number due to the reduction of cross sectional area.
Heat transfer augmentation– To improve the coolant-side heat transfer coefficient, augmentation devices are sometimes added to the coolant tubes to breakup the boundary layer and induce turbulence at low Reynold Number. These devices may be modifications to the tube itself, or inserts placed into the tube.
The use of tube inserts does require some additional design considerations, including the following: -
The most common method of tube modification, is to roll dimples into the surface of tube’s major axis. Figure 49, shows a comparison of two heat exchangers with 26mm tubes that are identical except for dimples. Since the air-side heat transfer coefficients are constant and the effect of intermittent tube to fin contact with the fin is very small, we get a direct comparison of the change in coolant-side heat transfer coefficient.
•
Galvanic interaction between the turbulator and the tube.
•
Fretting due to relative movement between the insert and the tube, caused by turbulence induced vibration.
•
Corrosion resistance of the insert.
•
Increased sensitivity to tube blockage.
Considerable care should be taken when using turbulation devices as the increased turbulence, has been shown to increase the corrosion sensitivity of the tubes[39].
Performance Improvement
70%
Coolant-side heat transfer augmentation devices shall not cause a reduction in the heat exchanger useful life
Air Velocity 1 m/s 5 m/s 10 m/s 15 m/s
60% 50% 40%
Effect of Ethylene Glycol– The addition of ethylene glycol to the coolant reduces the temperature at which the coolant freezes and increases the temperature at which it boils. It also significantly changes the physical properties of the coolant, as shown in table 4
30% 20% 10% 0% 0
1000 2000 3000 4000 5000 Coolant Side Reynolds Number
.
6000
Figure 49, Typical improvement of heat exchanger performance due to use of “dimpled” tubes.
•
•
•
At low airflow rates, the relative contribution to the overall heat transfer coefficient U, made by the coolantside heat transfer coefficient hh, is small. As airflow rate and air-side heat transfer coefficient hc increases, the effect of improved hh due to dimpling becomes greater. As the Reynolds Number on the coolant-side increases, the flow in general becomes more turbulent and the improvement diminishes.
Water
50% Glycol
Specific Heat Capacity (kJ/kgK)
4.219
3.625
Density (kg/m3)
957.9
1009.5
Dynamic Viscosity (N.s/m2)
2.79E-4
6.30E-4
Thermal Conductivity (kW/mK)
6.81E-4
4.20E-4
Prandtl Number
1.73
5.44
Freezing point
0°C
-37°C
Boiling point
100°C
108°C
Table 4: Comparison of physical properties for water and 50% ethylene glycol by volume @ 100°C.
49
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Air-side Heat Transfer Coefficient
It is a common mistake to think that the 8°C increase in boiling point of the coolant due to the addition of 50% glycol will improve heat exchanger performance. This apparent benefit is more than offset, by the coolant’s loss of heat transfer performance.
Heat transfer to a gas of low thermal conductivity is essentially a boundary-layer problem. For flow over a flat plate, the heat transfer coefficient is proportional to the surface shear stress. If the thickness of the viscous boundary layer can be reduced then the thermal boundary layer will also be reduced.
The increased viscosity, reduced heat capacity and thermal conductivity combine to reduce the coolant-side heat transfer characteristic by over 50%, as shown in figure 50.
Figure 52 shows the growth of a boundary layer from the leading edge of a flat plate.
Heat transfer coefficient (kW/m²K)
15
y ua, Ta 10
Tw X
5
Water (S-T Laminar) Water (Gnielinski) 50% Glycol (S-T laminar) 50% Glycol (Gnielinski)
Figure 52, Boundary layer growth due to flow over a flat plate
0 0.0
1.0 2.0 Coolant flow rate (m/s)
3.0
The wall shear stress, τ can be expressed as follows: du τ = – µ ⋅ ------ dy y = 0
Figure 50, Effect of Ethylene Glycol on coolant-side Heat Transfer Coefficient for radiator tube, Dh=2.2mm
(40)
Similarly heat flux is defined as: dT Q ---- = – k ⋅ ------ dy y = 0 A
At higher glycol concentrations, the loss of heat transfer performance is even worse. This can be a problem for vehicle that are required to operate in severe cold weather, where freeze protection needs to be maximized.
(41)
Where m = viscosity (kg/ms) u = velocity (m/s) y = distance perpendicular to surface (m)
Multiple row configurations– Due to the manufacturing methods traditionally used to make tubes it has not been possible to produce tubes of wide enough to span deep heat exchanger cores. Manufacturers have therefore used multiple tube rows to solve this problem.
For a flat plate, heat transfer coefficient can be shown to be a function of distance along the plate in the direction of flow, as shown in equation 42 and figure 53.
As tube contact with the fin is not continuous, between the tubes exists a zone of poor heat conduction, as shown in figure 51. This zone of poor conduction reduces the efficiency of the fin and should be avoided if possible.
1 h ∝ --- x
Heat transfer coefficient
(42)
Zone of poor Conduction
Distance along plate, x
Figure 53, Heat transfer coefficient along a flat plate Figure 51, Poor conduction zone on multiple tube row radiators
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Two main methods of reducing the thickness of the boundary layer are commonly used. In the first, (more easily manufactured method), surface discontinues such as dimples or corrugations are added to disrupt the flow. Turbulence within the boundary layer causes the airflow to periodically separate and re-attach, limiting any growth in boundary layer thickness.
convective heat flux h St = ------------------------------------------------------ = ---------------------ρ ⋅ u ⋅ Cp fluid heat capacity rate
(43)
Colburn Factor is defined as: j = St ⋅ Pr
In the second method, holes, slits or louvers are added to the surface in order to cause the formation of short and thin boundary layers, rather than a single long and thick boundary layer, as shown in figure 54.
( 2 ⁄ 3)
(44)
Similarly, the friction factor, f for the air-side can be calculated from the core pressure drop using the following relationship:Equivalent shear stress f = --------------------------------------------------------------------------------------Flow kinetic energy per unit volume
Fin depth Louver pitch
(45)
or τw Dh ∆P f = ---------------------------------------- ≈ --------------------------------- ⋅ ------2 2 4L ρ ⋅ u m ⁄ ( 2 ⋅ g a ) ρ ⋅ u m ⁄ ( 2g )
α Air Flow
Louver angle
(46)
Figure 54, Section through louvered fin
Following Kays and London[31], equation 46 can be rewritten to give: 2
Louvered fins– The addition of louvers to the extended airside surface can increase the heat transfer coefficient by a factor of 3 to 4 times over that of a plain fin, depending on the detailed surface geometry.
G 1 ∆ P = ------- ----2g ρ i
entrance flow core exit + + – effect acceleration friction effect
(47)
or 2 ρ 2 G 1 ∆ P = ------- ----- ( K c + 1 – σ ) + 2 -----i- – 1 2g ρ i ρ
The difference between a surface with an enhancement factor of 4 rather than 3, represents a significant competitive cost advantage. It should be no surprise that heat exchanger manufacturers jealously guard the details of their louver geometries and detailed information on the performance of current fin geometry is seldom published. Notable exceptions to this are the works of Davenport[26][27], Achaichia[28] and Webb[29].
(48)
o
ρi 2 A ρ1 + f ------ ------- – ( 1 – ρ – K e ) -----Ac ρm ρo
Where G = mass velocity (kg/s) g = gravitational constant (m/s²) σ = ratio of free flow area to frontal area Kc = entrance loss coefficient Ke = exit loss coefficient A = total surface area air-side (wetted perimeter) Ac = minimum free flow area (cross-sectional area)
The use of louvering, not only increases the heat transfer coefficient of the surface, but also the air-side pressure drop. As the number of louvers increase, the friction factor will increase at a greater rate, since the form drag due to the blunt edge on each louver, is of the same order as skin friction[30].
subscripts i = inlet o = outlet m = mean
The evaluation of different louver geometries, requires that the both the heat transfer and friction performance be considered simultaneously. To facilitate the comparison of different heat exchanger geometries, the heat transfer coefficient is normally made into a non-dimensional number using either Stanton Number, St or Colburn Factor, j and compared at constant Reynolds Number, Re
A 4L note: by definition ------ = ------Ac Dh The coefficients Kc and Ke represent the sudden contraction and expansion losses at heat exchanger inlet and exit. For common geometries, these coefficients can be obtained from Kays and London[31].
Stanton Number is defined as: -
51
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Performance verification methods– If correlation equations for Friction Factor and Colburn Factor are not available, then the performance of a heat exchanger must be established by testing in a calorimeter windtunnel.
t 1 K = ------------ + -Ah hh k •
Substituting equation 23 into equation 51, we get equation 53,
Dependent upon how the data is to be used two basic types of performance test may be conducted: an air fractional test and a “water” fractional test.
Af UA – K = h c ⋅ A c ⋅ 1 – ----- ( 1 – η f ) A
Although called a water fractional test, the test could be performed using a glycol based coolant mixture. Water is preferred for two reasons, firstly a higher coolant-side Reynolds Number can be achieved for any given pump, and secondly, glycol base coolants are toxic and unnecessary handling should be avoided. These tests are performed by holding either the coolant flow or the airflow constant while the other flow is adjusted in discrete intervals. The heat transfer on each side of the heat exchanger is measured using the relationship: Heat dissipation, Q· = m· ⋅ Cp ( mean ) ⋅ ∆ T
(52)
(53)
•
Using the fin efficiency relationships equations of 25 and 26 we can solve for fin efficiency, η and air side heat transfer coefficient hc. As fin efficiency is a function of heat transfer coefficient, the solution will be iterative.
•
Stanton Number can then be determined using equation 43 and Colburn Factor using equation 44.
Friction Factor, f can be determined from the air-side pressure drop data using the relationship of equation 48.
(49) 1 Friction Factor - Stanton Number
At each condition, the heat transfer is allowed to stabilize, to within the require degree of accuracy, typically ± 3% on heat balance. Using the following procedure Colburn Factor, j can be determined: •
For best accuracy select data for a coolant flow rate where Re > 4000
•
As the inlet and outlet temperatures and flow rates are known on both sides of the heat exchanger calculate effectiveness, ε using equation 11
•
0.01 100
Using the appropriate ε-NTU relationship, calculate NTU. (For 1 row use equation 13, for 2 row use equation 14 or equation 17 for other configurations).
•
Using equation 9 calculate UA.
•
Use the Gnielinski correlation of equations 38 and 39, obtain the coolant side Nusselt Number.
•
Then use equation 29, to convert Nusselt Number to heat transfer coefficient, hh
•
Substitute for hh, Ah, t, and k into equation 50 1 1 t 1 -------- = ------------ + -- + ------------------UA A h hh k η0 Ac hc
1000
10000
Reynolds Number Coburn Factor, j
Friction Factor, f
Figure 55, Heat exchanger performance as Friction Factor and Stanton Number Vs. Reynolds Number
More recently, computational fluid dynamics CFD techniques have be used to analytically determine f and j factors[32]. For fins of the type shown in figure 56a, two dimensional modeling is adequate, but for fins of the type shown in figure 56b modeling must be three dimensional.
(50)
To give 1 1 -------- = K + ------------------UA η0 hc Ac
0.1
(51)
Where constant K is given by
52
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a) Parallel fins
Air-side fouling– An automotive heat exchanger is subject to a variety of operating environments throughout its life. During normal operation it is likely to encounter dust, oil, muddy water, rubber, sand, small stones, vegetable matter and insect. For vehicles operating off-road, the situation is even more severe, as it may involve construction sites and quarries.
b) Triangular Fins
Cowell and Cross[34] developed a “fouling windtunnel” in which they investigated 17 different fin geometries, with hydraulic diameter ranging from 5.5mm down to 1.5mm.
Figure 56, Variation in fin profile for two fin geometries with identical fin pitch
During the testing they observed that “fouling was confined almost totally to within a few millimeters of the front face”, and that a strong correlation to hydraulic diameter existed. Louver flow structure– In a two dimensional louver array, as the airflow passes from front to back, the flow direction becomes aligned with the louvers.
The effect on heat transfer, for a given flow rate was found to be of secondary importance, compared to the effect on pressure drop. They found that in the installed condition, a 25% reduction was typical, of which only 6-7% was due to changes in specific dissipation.
In an ideal heat exchanger, the airflow would become parallel to the louvers, as shown in figure 57. In practice however, the flow only partially aligns with the louvers.
As the fouling build-up occurred immediately at the entry to the heat exchanger, they concluded that it was inappropriate to build a fouling margin into the heat exchanger surface design. Any fouling margin should be added to the airflow system requirements. Heat exchanger fins shall be protected from road hazards, splash, stones, sand/gravel or any dirt build-up that would affect functional performance
Ideal flow path Typical flow path
HEAT EXCHANGER DESIGN PROCEDURE
Figure 57, Flow direction through a louver array
The general procedure for the design of compact exchangers is both well established and documented. The approach used, however, varies depending on whether the task is to rate a heat exchanger of a particular size, or size a heat exchange to meet a specified performance.
The degree of louver alignment was found to be a function Fin Pitch of Reynolds Number and the ratio of ------------------------------- [33] Louver Pitch
Heat Exchanger Performance Rating In the rating problem, the objective is to verify that the performance of an existing heat exchanger design is adequate to meet the required vehicle specification. To perform this analysis the following data is required:-
At low Reynold Numbers and high fin pitch/louver pitch ratios flow was found to completely bypass the louvers, with the fins behaving as the sides of a duct. For optimal performance of heat exchangers at low Reynolds Numbers, where fin density must be kept low to prevent fouling, fine louver pitch may reduce air-side heat transfer coefficients. In triangular fin arrays the flow pattern is more complex, as the distance between adjacent fins varies along the louver length. At low speed, duct flow is more pronounced, as the hydraulic diameter is greater than for parallel fins of the same pitch.
53
•
Overall dimensions
•
Air-side surface geometry and the associated f- and jfactors
•
Details of the tube geometry
•
Fluid flow rates
•
Fluid properties
•
Inlet air and coolant temperatures
•
Any allowances to be made for fouling.
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From this data it is possible to calculate the fluid outlet temperatures, pressure drops and the overall heat transfer rate.
•
The calculation procedure is simple and can be easily mechanized by computer as no judgemental decisions need be made if all of the data is present. If some data is missing, the process can still be automated and a series of solutions generated. The basic calculation procedure is outlined below
Use ε - NTU relationship for unmixed on air-side and mixed within each tube but unmixed between rows
9. Calculate outlet temperature. 10. Calculate pressure drops. 11. Compare old and new outlet temperature values if they are different, assign new to old and loop back to step 3.
1. Calculate surface geometric properties. • Primary and secondary surface area.
12. Calculate overall heat transfer rate.
•
Minimum free flow area.
•
Frontal area.
Heat Exchanger Performance Sizing
•
Hydraulic diameter.
•
Flow length.
•
Ratio of free flow area to frontal area.
•
Fin length.
In the problem of heat exchanger sizing, the task of the designer is to select a construction type, flow arrangement, surface geometry, tube configuration and size, and the overall core dimensions necessary to meet the specified heat transfer rate and pressure drops. The problem is appreciably more difficult than the problem of rating, as the number of variables involved is very much greater.
2. Approximate bulk mean temperatures • Use an effectiveness of 75% to approximate outlet temperatures. •
Mean air temperature.
•
Mean coolant temperatures.
In practice, the heat exchanger design engineer is seldom given a completely free hand in the selection of all of these parameters. In particular, the choice of construction type is frequently controlled by manufacture and assembly requirements and the choice of core depth is constrained by investment, as discussed earlier.
3. Calculate fluid properties. • Viscosity. •
Thermal conductivity.
•
Density.
•
Heat capacity
•
Prandtl number
However, if we are to calculate meaningful loss functions for the heat exchanger we must be able to calculate the characteristics of an ideal heat exchanger. In performing a sizing calculation, the first question to be resolved is which surface configurations should be considered. Traditionally, the number of possible designs was limited by the availability of f and j-factor data for different surfaces, however, the use of correlation equations or CFD, can make f and j data more readily available.
4. Calculate mass velocity and Reynolds Number. 5. Correct friction factor for temperature if measured isothermally. 6. Calculate heat transfer coefficients & fin effectiveness. • Check coolant-side flow regime (laminar, transitional or turbulent). •
Coolant-side Nusselt number.
•
Coolant-side heat transfer coefficient.
•
Thermal resistance of tube-to-fin joint.
•
Air-side heat transfer coefficient.
•
Overall thermal conductance.
•
Fin effectiveness.
The major difficulty with sizing a heat exchanger is that although the fluid flow rates on both sides of the unit are generally known, the mass velocities which are required to calculate the performance are not known. Mass velocities are a function of flow cross-sectional areas, which are the parameters we seek to determine. To resolve this problem Kays and London proposed a scheme for using the ratio j/f to couple the heat transfer and pressure drop characteristics and enable an approximation of core mass velocities to be made.
7. Calculate flow stream capacities. • Assign Cmin and Cmax
In this approximation certain assumptions are made:
8. Calculate NTU and effectiveness.
54
•
Core inlet and outlet loss coefficients are negligible.
•
Flow acceleration within the core is less than 10% of the core friction term and may therefore be neglected at this stage.
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•
Fin efficiency is assumed to be 80% as starting point for approximation of mass velocity.
The advantage of this approach is that the ratio of j/f is a relatively flat function of Reynolds number, thus minimizing any errors introduced when making a “ball park” estimation of Reynolds number.
Mean air temperature. Mean coolant temperatures.
•
Fin effectiveness.
•
Overall effectiveness
•
Overall thermal conductance.
13. Calculate minimum free flow areas from total flow rate and mass velocity rate.
1. Calculate fluid outlet and mean temperatures • Use an effectiveness of 75% to approximate outlet temperatures. •
Air-side heat transfer coefficient.
12. Calculate ratio of heat transfer surface areas for selected geometry.
The basic calculation procedure is as follows:
•
•
14. Calculate length in direction of flow from hydraulic diameter, minimum free flow area and total heat transfer area. 15. Calculate frontal area, and core face dimensions.
2. Calculate fluid properties. • Viscosity. •
Thermal conductivity.
•
Density.
•
Heat capacity
•
Prandtl number
16. Calculate pressure drop characteristics. 17. Correct f-factor for thermal conditions if originally deter∆P mined isothermally, and recalculate ------p ∆P 18. Compare old value of ------- with new value, if different p recalculate mass velocities using the required values and the new values for f-factor and overall core dimensions, and loop back to step 9.
3. Calculate flow stream capacities. • Assign Cmin and Cmax 4. Calculate effectiveness and determine overall NTU. • Use ε - NTU relationship for unmixed on air-side and mixed within each tube but unmixed between rows
Using this approach the overall heat exchanger dimensions for any given fin and tube geometry can be determined for a required heat transfer rate and pressure drop. If the optimization criterion is weight or cost, overall heat exchanger size can be easily translated into the required criterion.
5. Estimate the NTU for each side of the heat exchanger by assuming 80% of overall thermal resistance is on the air side.
Flow and Temperature Maldistribution
6. Select fin and tube geometry and calculate hydraulic diameters
In calculating heat exchanger performance it is normally assumed that flow is uniform in both rate and temperature distribution. In practice neither is likely to be true.
7. Make a “ball park” estimation of Reynolds number and obtain a value for j/f. 8. Calculate the mass velocities for both sides of the heat exchanger using the Kays and London approximation.
While the effects might be relatively small on the coolant side, on the airside the effects can be very large. The causes of this maldistribution include: -
9. Calculate Reynolds number values for mass velocities.
approximated
•
10. Determine actual values of f- and j-factors for calculated Reynolds number
Heat load and flow resistance from air-conditioning and auxiliary heat exchangers placed in front of the “radiator”.
•
Non-uniform inlet conditions caused by the radiator grill.
11. Calculate heat transfer coefficients and effectiveness. • Check coolant-side flow regime (laminar, transitional or turbulent).
•
Inherent mismatch between the annular flow of the fan and the rectangular heat exchanger. (The fan shroud will never be deep enough to allow full transition).
•
The proximity of objects placed downstream of the fan, cause asymmetric fan airflow.
•
Hot air recirculation caused by low pressure between the grill and the heat exchangers.
using
•
Coolant-side Nusselt number.
•
Coolant-side heat transfer coefficient.
•
Thermal resistance of tube-to-fin joint. 55
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area, are better than devices that measure a point source. Point source measurement may record the effect of damage to an individual fin rather than the average flow through an entire element.
Figure 58 and 59, show histograms of radiator airflow maldistribution taken from CFD analysis of a Ford F350 vehicle, with ducting and seals removed. 25%
Frequency
20%
Tco Thi
15%
Tho
.
Tci, mc
10% 5%
Figure 60, Elemental approach to performance calculation for maldistributed air inlet conditions.
0% 46
48
50
52
54 56 58 60 62 Air Temperature (°C)
64
66
68
Figure 58, Radiator inlet temperature variation, Ford F350 at 72kph ambient temperature 38°C
Rather than use heat exchanger effectiveness based on the entire heat exchanger, the effectiveness used to calculate performance must be the elemental effectiveness. The calculation procedure is slightly modified from that used for a heat exchanger with uniform airflow. The elemental effectiveness, rather than the overall effectiveness is used to determine the temperature of the coolant at the cell ouutlet, as follows:
20%
Frequency
15%
10%
5%
0% 1
2
3
4
5
6 7 8 9 10 11 12 13 14 15 Air Velocity (m/s)
Figure 59, Radiator inlet velocity variation, Ford F350 at 72hph
•
Calculate NTU for the overall heat exchanger, using Colburn factor or overall effectiveness.
•
Use substitute NTU in to equation 16 to determine the elemental effectiveness.
•
Use the elemental effectiveness in equation 11 to calculate the air outlet temperature.
•
Calculate the elemental heat rejection using equation 6
•
Use equation 5, calulate the element’s coolant outlet temperature.
•
Integrate the elemental heat rejection to give total heat transfer.
Performance Variability
To account for this maldistribution, an elemental approach to heat transfer coefficient calculation, as shown in figure 60, is required. Each element is treated as a separate heat exchanger.
In conducting verification testing or analytical sign-off, the engineer is require to place confidence limits on system performance. Traditionally, large safety margins have been built into the sign-off specification to account for component variability.
On the coolant-side, flow is passed from one element to the next along the flow direction, with the outlet temperature of the upstream element being the inlet temperature of the downstream element.
With the move from copper/brass to aluminum, the variability associated with the heat exchanger has been significantly reduced. This reduction is attributable to two factors: -
On the air-side, flow distribution can be determined analytically using CFD, or experimentally, by directly measuring the velocity, or indirectly by measuring the difference is pressure across the heat exchanger. In general, devices that integrate flow or pressure over an
•
56
The unforgiving nature of the brazing process demands better quality control.
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•
Thermal performance– The improved tube to fin joint quality and the more accurately formed louvers combine to significantly reduce performance variation.
Quality of fin shape.
Brazing Process– No matter which brazing process is utilized, they all have one thing in common; they are relatively unforgiving and demand quality control standards, much higher than those traditionally used for copper/brass designs.
Figure 62 shows a comparison of heat dissipation, at a fixed airflow rate, for a copper/brass heat exchanger and the aluminum model that replaced it. (The installed performance of copper/brass heat exchangers had even greater variability due to changes in air-side pressure drop).
This unforgiving nature stems from the size of the fillets created by the brazing process, which are significantly smaller than those created by solder.
The reduced variation enables the cooling system design engineer to work to much smaller robustness margins, which can result in significantly lower heat exchanger costs.
In copper /brass construction any gap in the tube to fin joint was easily bridged with a soft solder fillet. Visually the heat exchanger looked perfect, so manufacturing defects would go uncorrected. Unfortunately, solder has only 10% of the thermal conductivity of copper and heat transfer was often significantly reduced. With aluminum, however, fin profile defects must be corrected or the joint may not be created. Furthermore, since the braze fillet shares the same physical properties as the parent metal, no loss of performance occurs
Number of samples
50
Fin Shape Quality– In corrugated fin heat exchangers the shape of the fin is formed by sandwiching the fin material between two meshing gear shaped rolls. The rolls are made from a stack of parallel plates whose edges are ground at an angle. The edges of one fin roll interlock with those of the other to form a shear that cuts and bends the louvers of the fins.
45
Aluminum
40
Copper/Brass
35 30 25 20 15 10 5 0
If the tools are not set up perfectly, or are worn, rather than cut the louvers, they are torn, producing a hooked or “hockey sticked” edge, as shown in figure 61.
0
Somewhat ironically, because copper is easier to work than aluminum, the quality of the louvers are typically worse: As aluminum is highly abrasive and produces greater tool wear, it is necessary to maintain tighter tool setup tolerances and keep the tools very sharp.
5
10 15 20 % Better than specification
25
Figure 62, Comparison of heat dissipation variation for a sample of 155 copper/brass radiators tested over a 3 year period and its direct replacement manufactured in aluminum (92 samples over 2 years)
MECHANICAL/STRUCTURAL CONSIDERATIONS The coolant tubes of a radiator are subject to a number of environmental conditions and forces: •
Internal pressure cycling
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Internal erosion/corrosion
•
External corrosion
•
Axial stress due to thermal expansion
•
Stone impact
Tube Mechanical/Structural Consideration.
Figure 61, Section through fin louvers showing “hockey stick” burrs
In almost all cooper/brass radiators, the tubes represent the weakest point of the radiator and are the most common 57
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the pressure cycle durability of conventional tubes and the ability to withstand 6.5bar without permanent distortion[35].
source of coolant leaks. The majority of failures occur in the tube seam or tube to header joint, due to the very poor fatigue strength of solder.
The improved dimensional stability and enhanced structural strength of the folded tube design has enabled tubes up to 50mm in depth to be manufactured.
In copper/brass designs, the length of the major axis of the tube is typically kept below 20mm to prevent curvature of the tube walls. Any gap between the fin and the tube produced by curvature will be filled with solder during assembly. As solder has less than 10% of the thermal conductivity of copper, any bridging fillet will significantly increase the thermal resistance of the fin to tube joint, causing a loss of performance. If the solder fillet fails to bridge the gap then the resistance will be increased still further.
Heat exchanger tubes shall be capable of withstanding an internal pressure of TBD kPa without distortion or leaking.
Internal Erosion/Corrosion. The corrosion and erosion sensitivity of the coolant tubes in an aluminum radiator is depend on a number of factors including: -
In aluminum radiators, vibration and pressure induced fatigue failures are greatly reduced as the lock seam joint of the type shown in figure 63 is replaced by a butt welded or brazed seam. Similarly, the tube to header joint is also improved as it is brazed, rather than soldered. Folded Tube Brazed Seam
•
Material properties.
•
Coolant corrosion inhibitor formulation.
•
Hydrodynamic forces due to coolant flow.
A detailed discussion of material selection is outside the scope of this paper, but numerous papers have been published on the subject [21][40][41][42]. Although the formulation of the corrosion inhibitor is discussed elsewhere in this paper, some discussion is required here to understand the interaction with the hydrodynamic aspects of coolant tube durability.
Figure 63, Folded tube with center joint
Ever since aluminum heat exchangers were first introduced, considerable effort has been placed in developing a coolant that will provide internal corrosion protection.
Tube leaks however, remain a concern due to corrosion sensitivity and vulnerability to stone impact. In radiators with horizontal tubes thermal fatigue may also be an issue.
If the wall shear stress created by the flowing coolant become too great, the protecting silicate and the oxide layer can be eroded, leaving unprotected metal ions that can react with the coolant.
To compensate for the lower corrosion resistance of aluminum, tube thickness is typically increased from 0.1mm in brass tubes to 0.32-0.38mm in aluminum designs. This increased material gauge has the benefit of producing a tube of greater dimensional consistency along the major axis.
At Reynolds Numbers greater than 104, the flow will be fully developed turbulent flow. Momentum from the free stream will be transferred into the boundary layer increasing its velocity and the wall shear stress. Flow in this regime should be avoided.
The increased tube dimensional stability, coupled with the significantly stronger tube to fin bond created by brazing, has allowed conventional aluminum tubes with a major axis of over 40mm to be manufactured.
At transitional flow rates 2300 < Re
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