Ceramic Microchannel Heat Exchangers

INTRODUCTION

A heat exchanger is a device that is used to transfer thermal energy (enthalpy)  between two or more fluids, fluids, between a solid surface surface and a fluid, or between solid solid particulates and a fluid, at different temperatures and in thermal contact. [3].he goal of enhancing heat transf transfer er while while minimi minimi!in !ing g pressu pressure re drops drops and reduci reducingt ngthe he si!e si!e and volume volume of energy energy conversion"thermal management systems has beenthe sub#ect of intensive research for more than four decades. $ut growing energydemands, the need for increased energy efficiency and materials savings, spacelimitations for device pac%aging, and increased functionality and ease ease of unith unithand andlin ling g have have create created d revolu revolutio tionary nary challe challenge ngess for the develo developm pment ent of  highperformance, next&generation heat and mass exchangers. 'urrent heat exchanger designs rely heavily on fin&and& tube or plate heat exchanger designs, often constructed using copper  and aluminum. he strive for heat exchangers that are more compact and highly efficient has led to the development of microchannel heat exchangers.

The innovative microchannel heat and mass exchangers appear to be the mostpromising way to meet these challenges in thermal management. When properlydesigned and utilized, microchannels can distribute the flow precisely among thechannels, reduce flow travel length, and establish laminar flow in the channelswhile achieving high heat transfer coefficients, high surface area-to-volume ratios,and reduced overall pressure drops. These are among the major advantages ofmicrochannels for use in a diverse range of industries .

ecent developments in material sciences, in particular advances in ceramics and ceramic matrix composites open opportunities for new heat exchanger designs. 'eramic materials offer potentially significant advantages compared to metal alternatives. A ma#or advantage is

the capability to operate at very high temperature. 'eramics are also much more tolerant to harsh chemical environments than metals. $ecause the oxide ceramics can tolerate strongly oxidi!ing environments, environments, it may be possible to remove certain fouling deposits by intermittently introducing introducing oxygen to burn deposits []. he performance of counterflow heat exchangers can be improved with low thermal conductivitymaterials that impede axial wall conduction. *or ceramic microchannel heat exchangers thelow value of conductivity has negligible effect on its performance [+].

1.2 LITERATURE SURVEY

nly a few researches focus specifically on ceramic microchannel heat exchangers. a%euchi et al. [+] developed and applied three dimensional models to assist the design of a silicon&carbide (-i') heat exchanger for application to / nuclear reactors. -chulte& *ischedic% et al. [+] designed and tested -i' plate&and&fin heat exchangers for applications in biomass biomass conversion conversion.. heir heir designwas designwas also supported supported by detailed detailed three&dim three&dimensio ensional nal modeling of fluid flow and con#ugate heat transfer. Alm et al.[+] designed and fabricated small alumina microchannel counter& flow heat exchangers, and evaluated performance at low temperature using water as the wor%ing fluid. 0specially in small high&performance counter&flow heat exchangers, it is well %nown that longitudinal conduction within the solid materials can play an important role in performance.

Although the general formulation of longitudinal conduction isavailable in textboos, more comprehensive analyses can beachieved via three-dimensional simulation of the fluid flow andconjugate heat transfer. Although correlations for fully developed, steadystate, laminarflow, heat-transfer coefficients are readily available, details of theflow and heat transfer within microchannels depend upon channeland manifold geometry. !randner et al."#$ exploredopportunities for enhancing heat transfer within channels by usingsmall obstructions that alter the flow patterns. %iofalo "#$ explored theeffects of spatial variations in the local heat-transfer coefficient onthe longitudinal conduction. &n the study conducted by A.'ommers et al "#$ assessment of potential bene fts of ceramic materials (both monolithic and composite) for usein heat exchangers, and the and feasibility for application in heat transfer systems was done. A detailed comparative study of application of metals and ceramics was done and besides the concept of application of ceramic matrix composites in heat exchangers was also discussed. A detailed experimental study and performance assessment of a counter flow ceramic microchannel heat exchanger was conducted by *.+ ee et al."$. This study highlights

the performance of microchannel heat exchangers besides emphasizing the advantages of ceramics over metals. The textboo undamentals /f 0eat 1xchanger 2esign by *. 'hah "3$ gives the basic definition, functioning of a typical microchannel heat exchangers. 4ext 5eneration 6icrochannel 0eat 1xchangers by /hadi et al "7$ clearly describes the advantages, applicatins and woring of various microchannel heat exchangers.0eat Transfer And luid low &n 6inichannel And6icrochannles written by'atish 5 andliar et al "8$ gives the detailed study and analysis of various flow regimes that can happen in a microchannel heat exchangers .

CERAMICS AND CERAMIC MATRIX COMPOSITES

-olis materials used in heat exchangers can be divided into four categories&  polymers, metals, ceramics and carbonaceous materials. 1oubtlessly the most widely adopted material is metal due to its high thermal conductivity. 2nstead of depending upon monolithic materials composite materials can also be employed. 'omposite materials offer  engineers an ability to create a limitless number of new material systems having uniue  properties that cannot be obtained using a single monolithic material. his approach to construction holds tremendous promise for future heat exchanger designs rather than selecting a single material, multiple materials may be selected and then tailored to meet the specific reuirements of the application. 'omposite materials are constructed of two or more materials, commonly referred to as constituents, and have characteristics derived from the individual constituents. he constituent that is continuous and which is often, but not always, present in the greater uantity in the composite istermed the matrix. he second constituent is referred to as the reinforcing phase, or reinforcement, as it enhances or  reinforces the properties of the matrix.

2.1 CERAMICS

he American -ociety for esting and 4aterials (A-4) definesa ceramic material as "an article [whose] body is produced fromessentially inorganic, non- metallic substances and either is formedfrom a molten mass which solidifies on cooling, or is formed  andsimultaneously or subsequently matured by the action of the heat. 54ost ceramic materials are hard, porous and brittle so the use ofceramics in application often reuires methods for mitigating theproblems associated with these characteristics. 'eramic materialsare usually ionic or covalently bonded and may be

crystalline oramorphous in structure. $ecause of this type of electronic bonding,ceramics tend to fracture before undergoing plastic deformationoften resulting in fairly low tensile strength and generally poormaterial toughness. 4oreover, because these materials tend to beporous, the microscopic pores can act as stress concentratorsfurther decreasing the toughness and strength of ceramics. hesefactors can combine, leading to a catastrophic failure of the materialinstead of the normally more gentle modes of failure associatedwith metals.

2.2 ADVANTAGES OF CERAMICS

he two main advantages for using ceramic materials in heat exchanger construction over more traditional metallic materials are their temperature resistance and corrosion

resistance. *irst, ceramic materials can withstand operating temperatures (i.e.+677 7') that far exceed those of conventional metallic alloys. *or example, the bul% material temperature of a heat exchanger made of carbon steel should not exceed 68 7'. -imilarly, the bul% material temperature of a heat exchanger manufactured from stainless steel t ypically should not exceed 9877'. As a result, the heat exchanger must be protected in some applications. hermal  protection can be accomplished by means of an environmental barrier coating that overlays the metal which has the effect ofadding a thermal resistance to the transfer of heat thereby reducing the overall performance of the unit. 2n other cases, the unit is operated in the parallel flow mode rather than the counterflow mode to maintain a lower overall material temperature. his mode of operation has the effect of increasing the lifetime of the heat exchanger at the expense of lowering the overall thermal efficiencyof the unit. Another commonly employed techniue is air dilution, where ambient air is added to the hot upstream exhaust gases upstream of the heat exchanger. his techniue also has the effect of lowering the overall efficiency of the heat exchanger.

he second ma#or advantage of ceramic&based heat exchangers is their resistance to corrosion and chemical erosion. 'orrosion which occurs under normal conditions is exacerbated by elevated operating temperatures. 4oreover, corrosion can occur in many different forms in an exhaust gas stream. *or example, an exhaust stream rich in oxygen can actually attac% a metallic surface. 2n this case, the diffusion of oxygen into the material causes scaling.Although this scaling initially forms a protective layer, the intermittentuse of  the heat exchanger and the resulting thermal cycling can cause the scale to fla%e off, exposing the underlying material to further attac%. ther possible gaseous constituents include sulfurand carbon which can also diffuse into the grain boundaries. hemigration of  sulfur into the grain boundaries forms eutectics thatmelt at temperatures significantly lower  than the material meltingtemperature. he diffusion of carbon into the metallic surfaceresults in the formation of carbides which can cause residualstresses and embrittlement to occur.

2.3 CERAMIC

MATRIX COMPOSITES (CMCs)

Although the impetus behind the use of cera mics in the manufacturing and design of  heat exchangers arises from their excellent corrosive properties, their ability to withstand extremely high operating temperatures, and the economics of their use in heat recovery systems, radiant heating applications, and microreactors, ma#or obstacles facing the incorporation and use ofceramics in these systems remain. hese obstacles include ceramic metallicmechanical sealing, manufacturing costs and methods, and their brittleness in tension. herefore, to help meet the specific reuirements of the application, ceramic matrix composites ('4's) were developed to overcome the intrinsic brittleness and lac% of  reliability of monolithic ceramics.

%eramic matrix composites (%6%s) combine reinforcing ceramic phases within a ceramic matrix to create materials with improved properties. The desirable characteristics of %6%s include high-temperature stability, high thermal shoc resistance, high hardness, high corrosion resistance, non-magnetic and nonconductive properties, and greater versatility in providing uni9ue engineering solutions. The most commonly used %6%s are nonoxide %6%s-namely carbon:carbon (%:%), carbon:silicon carbide (%:'i%), and silicon carbide:silicon carbide ('i%:'i%). or their merits, ceramics and %6%s are promising thermo structural materials for heat exchangers (i.e. li9uid li9uid, li9uid gas, gas gas, etc.) used in severe environments such as rocet and jet engines, gas turbines for power plants, heat shields for space vehicles, fusion reactor walls, aircraft braes, heat treatmentfurnaces, etc. &n the following sections, the properties of the most promising ceramic and %6% materials will be presented along with identified industrial applications and recently improved manufacturing methods .

MICRO HEAT EXCHANGERS

/eat exchangers are classified the basis of various parameters such as flow arrangement, compactness, construction etc. $ased upon the degree of compactness it is  broadly classified into compact and non&compact heat exchangers. but a more accurate classification differentiates compact heat exchangers as compact, meso and micro heat exchangers based upon the their surface area density or hydraulic diameter [3]. he surface area density factor, is defined as A gas&to&fluid exchanger is referred to as a compact heat exchanger if it incorporates a heat transfer surface having a surface area density greater than about :77 ; or a hydraulic diameter, 1 h 9mm for operating in a gas stream and 677

;

for 

operating in a liuid or phase&change stream. A laminar flow heat exchanger (also referred to as a meso heat exchanger) has a surface area density greater than about 3777 ; or +77 m 1

h

+

mm. he term micro heat exchanger is used if the surface area density is greater than about +8,777 ; (or + m 1 h +77 m) [3].

A similar classification proposed by 4ehendale et al.[6] classifies flow passages in the dimension ranging from + to +77 as mmicrochannels, +77 to + mm as meso&channels, + to 9 mm as compact passages and above 8mm as conventional passages [6].

%ompared to shell-and-tube exchangers, compact heat exchangers are characterized by a large heat transfer surface area per unit volume of the exchanger, resulting in reduced space, weight, support structure and footprint, energy re9uirements and cost, as well as improved process design and plant layout and processing conditions, together with low fluid inventory. /f these micro heat exchangers are the most compact. !esides they weigh less and provide more effective heat transfer. 6icrochannel is a suitable techni9ue that can be employed to fabricate micro heat exchanger s

MICROCHANNEL HEAT EXCHANGERS

A typical ceramiccounter&flow microchannel heat&exchanger is shown in *ig +. he design has an overall footprint of 87 mm by +77 mm. 0ach flow layer contains +7 microchannels that are approximately 887 microns high and .erry @. 4artin /0 10-2L, *A$2'A2, A1 0A@=A2 * A '0A42' '=0& *@E 42'&'/A0@ /0A 0M'/AL0 Applied hermal 0ngineering 3+ (7++) 776e7+

"8$

-atish

L ?andli%ar, -rinivasLarimella, 1onging @i, -tJphane 'olin,

4icheal . ?ing

/0A A-*0 A1 *@=21 *@E 2 422'/A0@ A1

42''/A@0-

[0@-020 F=$@2-/0-]