Heat transfer introduction

1/18/2014

Conduction and Radiation Heat Transfer ME 301 Course Teacher: Dr. Mohammad Arif Hasan Mamun & Dr. Aloke Kumar Mozumder Professor Department of Mechanical Engineering BUET

Detail Outline of Course:
Basic modes of heat transfer;
General conduction equation for one dimensional and three dimensional situation;
Steady state conduction in different geometrics and composite structures for one dimensional situation;
Effect of variable thermal conductivity;
Analysis of heat conduction of system with heat sources and heat transfer from finned surfaces;
Transient heat conduction in solids with negligible internal resistance and with internal and surface resistance;

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Use of Heisler charts; Analytical and numerical solutions of conduction heat transfer problems.
Heat transfer by the mechanism of radiation; Laws of radiation heat transfer;
Blackbody radiation and radiative properties of surfaces;
Angle factor;
Net radiation interchange between two infinite parallel planes, concentric spheres and long cylinders;
Simple enclosure problems;
Radiation shield;
Solar radiation and its prospects in Bangladesh.

Reference Books: 1. Principle of Heat Transfer -Kreith 2. Heat Transfer - M.N. Ozisik 3. Heat Transfer -Halman 4. Heat Transfer -Chapman 5. Fundamentals of Heat Transfer -F.P. Incorpera & D.P. DeWitt 6. Heat and Mass Transfer -Y. A. Çengel

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Heat Transfer
Heat transfer is a term applied to a study in which transfer of energy in the form of heat takes place as a result of temperature difference.
The temperature is a thermal state of a body which distinguishes a hot body from a cold body.


The temperature of a body is proportional to the stored molecules energy i.e. the average molecules kinetic energy of the molecules in a system.
Thermal energy is related to the temperature of matter.
For a given material and mass, the higher the temperature, the greater its thermal energy.

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Question?
Why study Heat Transfer?
What is Thermodynamics?
What is the difference b/w Thermodynamics and Heat Transfer?

Thermodynamics
It is a science of the relation ship b/w Heat, Work and Properties of the system – it deals with equilibrium states, it does not deal with Rate of Energy (Heat ) Transfer.

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Heat Transfer
It obeys the laws of thermodynamics.
In Heat Transfer, rate (i.e. time) is very important which is absent in thermodynamics.
Mechanism of Heat Transfer is not in Thermodynamics.

Objectives of our study:
Mechanism by which heat is transferred.
Parameters involved
Rate of heat transfer and total quantity of heat transfer over a certain period of time.

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Applications of Heat Transfer:

Heat Transfer From Finned Surfaces

The thin plate fins of a car radiator greatly increase the rate of heat transfer to the air

Some innovative fin designs.

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Modes of Heat Transfer 1. Conduction 2. Convection 3. Radiation

Conduction:
It is a mechanism by which heat flows from a region of higher temperature to a region of lower temperature within a medium or between different medium by kinetic motion of molecules or by direct impact as in fluids or by electron flow as in solid.


Conduction is the transfer of heat through solids or stationery fluids.

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When you touch a hot object, the heat you feel is transferred through your skin by conduction.


Two mechanisms explain how heat is transferred by conduction: lattice vibration and particle collision.


Conduction through solids occurs by a combination of the two mechanisms;


Heat is conducted through stationery fluids primarily by molecular collisions.


In solids, atoms are bound to each other by a series of bonds, analogous to springs as shown in Figure 1.


When there is a temperature difference in the solid, the hot side of the solid experiences more vigorous atomic movements.

Figure 1 Conduction by lattice vibration

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The vibrations are transmitted through the springs to the cooler side of the solid.


Eventually, they reach an equilibrium, where all the atoms are vibrating with the same energy.


Solids, especially metals, have free electrons, which are not bound to any particular atom and can freely move about the solid.


The electrons in the hot side of the solid move faster than those on the cooler side. This scenario is shown in Figure 2.

Figure 2 Conduction by particle collision

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As the electrons undergo a series of collisions, the faster electrons give off some of their energy to the slower electrons.


Eventually, through a series of random collisions, an equilibrium is reached, where the electrons are moving at the same average velocity.


Conduction through electron collision is more effective than through lattice vibration; this is why metals generally are better heat conductors than ceramic materials, which do not have many free electrons.


In fluids, conduction occurs through collisions between freely moving molecules.


The mechanism is identical to the electron collisions in metals.

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Convection: It is a study of the heat transfer process caused by the flow of fluids and is a mode of heat transfer between a solid surface and a liquid or gas.

Figure 1.4 Natural convection

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Convection uses the motion of fluids to transfer heat.


In a typical convective heat transfer, a hot surface heats the surrounding fluid, which is then carried away by fluid movement such as wind.


The warm fluid is replaced by cooler fluid, which can draw more heat away from the surface.


Since the heated fluid is constantly replaced by cooler fluid, the rate of heat transfer is enhanced.

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