INTRODUCTION
INTRODUCTION
A heat exchanger is a device that is used to transfer thermal energy (enthalpy) A heat exchanger is a device that is used to transfer thermal energy (enthalpy) between two or more flu
between two or more fluids, between a solid suids, between a solid surface and a fluid, or between sorface and a fluid, or between solid particulateslid particulates and a fluid, at different temperatures and in thermal contact. [3].he goal of enhancing heat and a fluid, at different temperatures and in thermal contact. [3].he goal of enhancing heat tra
transfnsfer er whwhile ile minminimiimi!in!ing g prepressussure re drodrops ps and and redreduciucingtngthe he si!si!e e and and volvolume ume of of eneenergyrgy conversion"thermal management systems has beenthe sub#ect of intensive research for more conversion"thermal management systems has beenthe sub#ect of intensive research for more than four decades. $ut
than four decades. $ut growing energydemands, the need for increased energy efficiency andgrowing energydemands, the need for increased energy efficiency and materials savings, spacelimitations for device pac%aging, and increased functionality and materials savings, spacelimitations for device pac%aging, and increased functionality and eas
ease e of of ununithithandandlinling g havhave e crecreateated d revrevoluolutiotionary nary chachallellengenges s for for the the devdeveloelopmpment ent of of highperformance, next&generation heat and mass exchangers. 'urrent heat
highperformance, next&generation heat and mass exchangers. 'urrent heat exchanger designsexchanger designs rely heavily on fin&and& tube or plate heat exchanger designs, often constructed using copper rely heavily on fin&and& tube or plate heat exchanger designs, often constructed using copper and aluminum. he strive for heat
and aluminum. he strive for heat exchangers that are more compact exchangers that are more compact and highly efficient hasand highly efficient has led to the development of microchannel heat exchangers.
led to the development of microchannel heat exchangers.
The innovative microchannel heat and mass exchangers appear to be the mostpromising
The innovative microchannel heat and mass exchangers appear to be the mostpromising
way to meet these challenges in thermal management. When properlydesigned and
way to meet these challenges in thermal management. When properlydesigned and
utilized, microchannels can distribute the flow precisely among thechannels, reduce
utilized, microchannels can distribute the flow precisely among thechannels, reduce
flow travel length, and establish laminar flow in the channelswhile achieving high heat
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
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
drops. These are among the major advantages ofmicrochannels for use in a diverse
range of industries
range of industries
..
ecent developments in material sciences, in particular advances in ceramics and ceramic ecent developments in material sciences, in particular advances in ceramics and ceramic matrix composites open opportunities for new heat
matrix composites open opportunities for new heat exchanger designs. 'eramic materialsexchanger designs. 'eramic materials offer potentially significant advantages compared to metal alternatives. A ma#or advantage is offer potentially significant advantages compared to metal alternatives. A ma#or advantage is
the capability to operate at very high temperature. 'eramics
the capability to operate at very high temperature. 'eramics are also much more tolerant toare also much more tolerant to harsh chemical environments than metals. $ecause the oxide ceramics can tolerate strongly harsh chemical environments than metals. $ecause the oxide ceramics can tolerate strongly oxidi!ing environments
oxidi!ing environments, it may be , it may be possible to remove certain fouling deposits bypossible to remove certain fouling deposits by intermittently introducin
intermittently introducing oxygen to g oxygen to burn deposits []. he performance burn deposits []. he performance of counterflow heatof counterflow heat exchangers can be
exchangers can be improved with low thermal conductivitymaterials that impede axial wallimproved with low thermal conductivitymaterials that impede axial wall conduction. *or ceramic microchannel heat
conduction. *or ceramic microchannel heat exchangers thelow value of exchangers thelow value of conductivity hasconductivity has negligible effect on its performance [+].
negligible effect on its performance [+].
1.2 LITERATURE SURVEY 1.2 LITERATURE SURVEY
nly a few researches focus specifically on ceramic microchannel heat exchangers. 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 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& 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 *ischedic% et al. [+] designed and tested -i' plate&and&fin heat exchangers for applications in
in biombiomass ass conveconversionrsion. . heiheir r desigdesignwas nwas also also suppsupported by orted by detadetailed iled threethree&dim&dimensioensionalnal modeling of fluid flow and con#ugate heat transfer. Alm et al.[+] designed and fabricated 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 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 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 counter&flow heat exchangers, it is well %nown that longitudinal conduction within the solid materials can play an important role in performance.
materials can play an important role in performance.
Although the general formulation of longitudinal conduction isavailable in textboos,
Although the general formulation of longitudinal conduction isavailable in textboos,
more comprehensive analyses can beachieved via three-dimensional simulation of the
more comprehensive analyses can beachieved via three-dimensional simulation of the
fluid flow andconjugate heat transfer. Although correlations for fully developed,
fluid flow andconjugate heat transfer. Although correlations for fully developed,
steady-state, laminarflow, heat-transfer coefficients are readily available, details of theflow and
state, laminarflow, heat-transfer coefficients are readily available, details of theflow and
heat transfer within microchannels depend upon channeland manifold geometry.
heat transfer within microchannels depend upon channeland manifold geometry.
!randner et al."#$ exploredopportunities for enhancing heat transfer within channels by
!randner et al."#$ exploredopportunities for enhancing heat transfer within channels by
usingsmall obstructions that alter the flow patterns. %iofalo "#$ explored theeffects of
usingsmall obstructions that alter the flow patterns. %iofalo "#$ explored theeffects of
spatial variations in the local heat-transfer coefficient onthe longitudinal conduction. &n
spatial variations in the local heat-transfer coefficient onthe longitudinal conduction. &n
the study conducted by A.'ommers et al "#$ assessment of potential bene
the study conducted by A.'ommers et al "#$ assessment of potential bene
ffts of ceramic
ts of ceramic
materials (both monolithic and composite) for usein heat exchangers, and the and
materials (both monolithic and composite) for usein heat exchangers, and the and
feasibility for application in heat transfer systems was done. A detailed comparative
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
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
application of ceramic matrix composites in heat exchangers was also discussed. A
detailed experimental study and performance assessment of a counter flow ceramic
detailed experimental study and performance assessment of a counter flow ceramic
microchannel heat exchanger was conducted by *.+ ee et al."$. This study highlights
microchannel heat exchanger was conducted by *.+ ee et al."$. 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 woring of various microchannel heat
exchangers.0eat Transfer And luid low &n 6inichannel And6icrochannles written
by'atish 5 andliar 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 uniue 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 reuirements 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 reuires 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.+6777') 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 687'. -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 techniue is air dilution, where ambient air is added to the hot upstream exhaust gases upstream of the heat exchanger. his techniue 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 reuirements 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 rocet and jet engines, gas turbines for power plants, heat shields for space vehicles,
fusion reactor walls, aircraft braes, 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, 1h9mm for operating in a gas stream and 677 ; for
operating in a liuid 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 1h +
mm. he term micro heat exchanger is used if the surface area density is greater than about +8,777 ; (or + m 1h +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
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 .<mmwide. he channel floors that separate the hot and cold streams are approximately 977 microns thic%. he axial gaps in channel ribs serve three purposes. he first is to enable pressure euali!ation between channels. he axial pressure drop is approximately inversely proportional to the cube of the channel height, which means that small fabrication variations have a large impact upon flow distribution between channels. hus, the gaps tend to improve flow distribution. he gaps serve a
secondary purpose by tending to reduce potentially deleterious longitude al wall conduction. 'onduction along the channel floors and ribs (i.e., parallel to the flow direction) is %nown to degrade counter&flowperformance toward the lower performance of coflow heat
exchangers, especially for high&effectiveness
Fig 4.1. Ceramic microchannel heat exchanger Fig 4.2.Exploded iew showing internal [+] channel structure [+]
A third beneficial effect of the rib gaps is to cause local entry&length boundary&layer behavior as the flow enters the microchannel sections, which serves to increase local heat& transfer coefficients.As illustrated in *ig. 6. the hot and cold fluids enter through central holes from the bottom and exit via outboard holes at the top. 0ach of the internal layers is identical, but with alternating layers rotated +<77 relative to the underlying layer. he feed
and exhaust manifolds at the ends of the layers are designed such that there is exact alignment upon layer rotation. =sing identical layers reduces the manufacturing cost. ne of flow directions can be reversed (i.e., inlet flow through the outboard holes), producing a coflow configuration. he heat exchangers to date have been fabricated with four flow layers (two hot and two cold), but are designed to accommodate more flow layers. he relatively large diameters of the central inlets and the outboard exhausts enable the use of many heat& exchanger layers. he large diameters also enable the staging of several heat exchangers without significant pressure drops in the connecting tubing.
&n order to understand the response and evaluate the performance of a ceramic
microchannel heat exchanger experimental analysis and simulation was carried out. *.+
ee et al."$ The results of experimental testing and simulations of the same are
discussed in the following sections
.
THREE-DIMENSIONAL SIMULATION
he results of three&dimensional simulationsobtained by .> ?ee et al. [6]that predict both fluid and solid&body temperatures are illustrated in *ig. 8.+ and 8.. hesesimulations are used to guide tradeoffs between heat transferperformance and reuirements of the fabrication process. 2n additionto fluid flow, the models predict the solid material thermal fields.@ocal thermal gradients cause local strain through thermal expansion,which in turn can lead to failure of the ceramics. he currentdesign is in its fifth generation, with each
generation
improvingperformance and manufacturability. he design is based uponsimulation using three&dimensional computational fluid
dynamics('*1), including the con#ugate heat transfer between fluids and solidmaterials. he '*1 models are implemented in *@=0. emperature&dependent properties are used for
Fig 5.1 !ertical cut through a three-dimensional simulation, showing gas and solid temperature fields. [+]
air and for alumina.he results in *igs. 8.+ and 8. are for balanced air flow (i.e., thesame mass&flowrates through both the hot and cold inlets). he hotair enters at 877 7' and the cold
flow enters at 37 7'. he simulationsare based upon assuming that all external surfaces
areperfectly insulated. he results in *ig. 8.+ are for flow rates of+.8< x+7&3 %g s&+ (<7
standard liters per minute of air, slm) and*ig. 8. is for flow rates of 6.B3 x +7&6 %g s& + (8
slm). $oth airstreams enter from below, flow in opposite directions through themicrochannels, and then exhaust through tubes at the top. *ig. 8.+shows predicted
temperature fields on a vertical cut near theheat&exchanger centerline, in the middle of the first flow channelaway from the heat&exchanger centerline. *ig. 8.shows
temperaturefields at the mid&planes of a hot and a cold layer.he design process is assisted greatly by three&
dimensionalsimulations that are used to predict uantitatively the influence ofmanifold and channel geometry on heat exchanger performance.
*or example, there are practical tradeoffs between increased heat transfer performance and increased pressure drop. 4anufacturing processes also introduce practical tradeoffs that must be considered.
&t is evident from both igs. 8.# and 8.7 that the axial temperaturevariations are
essentially one-dimensional (i.e., varying primarily inthe axial direction with little
variation across the short dimension).This is a desired result, leading to similar heat
transfer performancewithin all channels. ig. 8.7 shows that the floor layer (i.e.,
between hotand cold flows) is nearly uniform in the vertical direction and essentiallyat
the average temperature between the local mean temperaturesof the hot and cold air.
Again, this is a desired result, indicatingrelatively little heat-transfer resistance through
the layer floors
.
ANALYSIS
0ven with comprehensive three&dimensional model in hand,analytic models provide great value in understanding performancetradeoffs and interpreting results. f course, the relatively simplermodels rely on significant approximations.2n compact primary&surface heat exchangers, the dominant heattransfer is between the hot and cold flow streams directly acrossthe separating wall, without the influence of extended surfacessuch as fins. he overall conductance =A between hot and cold streams can be represented in terms of contributing thermalresistances as
where h is the heat&transfer coefficient between fluid and wall, A isthe primary surface area, % is the thermal conductivity of the wall,and t is the wall thic%ness. 2n optimi!ed primary&surface heatexchangers the conductive heat flux across the walls is relativelylow and the conduction distance is small (i.e., the wall thic%ness).'onseuently the thermal resistance of the wall t"%A s
can be smallrelative to the convective resistance even when low&conductivitywall materials are used. 2n contrast to extended&surface heatexchangers designs, which are often fabricated with high&conductivitymaterials (e.g., copper or aluminum), the following analysisshows that compact primary&surface heat exchangers do notnecessarily benefit from the use of high& conductivity materials.As a limiting case, assume steady, fully developed, laminar flowin high& aspect&ratio channels the usselt number is constant (i.e.,independent of eynolds number).
where 1h is the channel hydraulic diameter and % f is the thermalconductivity of the fluid. 2t is
interesting to compare the relativethermal resistances of the convection and conduction. Assuming airas the fluid and a hydraulic diameter of 1hC 877 Dm, the convectiveheat&transfer
coefficient is h C 887 E m& ? &+ Assuming anaverage thermal conductivity of +7 E m&+ ? &+
(alumina at around 87 o') and a wall thic%ness of t C 877 Dm, the ratio of conductivethermal resistance offered by the wall and the total thermal resistance(i.e., conduction and convection) is
&n other words, even with low-conductivity ceramics, the influenceof conduction across
the wall is very small.Although conduction across the wall (i.e., normal to the
flowdirections) does not contribute significantly to performance, longitudinalheat
conduction (i.e., parallel to the flow directions) can significantlydegrade counter-flow
heat exchanger performance. The effect of longitudinal conduction can be particularly
important in compactheat exchangerswith short distances ; between inlet and outlet
ports.The dimensionless longitudinal conduction parameter can be used to estimate the
effect of longitudinal conduction onperformance. &n this e9uation, A
cis the longitudinal
cross-sectionalarea of the wall and %
minC< %
pis the minimumcapacity rate ( < isthe
mass-flow rate and %
pis the heat capacity of the fluid)
PERFORMANCE EVALUATION
.>.?ee et al[6] developed a ceramic microchannel heat exchanger and conducted experimental tests for performance evaluation. he test stand has been developed to measure heat exchanger performance. 4ass flow controllers areused to specify hot and cold inlet air flow rates. he mass&flow controllers (Alicat -cientific 4'& 877-@F4) have specified accuracy of 7.<G of reading plus 7.G of full scale, corresponding to approximately . standard liters per minute at +87 standard liters per minute(slm) . ype&? thermocouples are used to measuretemperatures of the inlet and outlet air. he thermocouples (7mega?4H--& 79=&9) have specified accuracy to within .o' or 7.:8Gof measured temperature,
corresponding to about 8.9o' at :87o'./ot&side air is heated by passing room&temperature
air throughmultiple small tubes that are positioned within a large hightemperaturefurnace. he hot inlet temperature is varied bychanging the electrical input to the tube furnace. he settling timeassociated with changing inlet temperature by 87o' is approximately+8 minI
data is ta%en after waiting 37 min. nce the steadyinlet temperatures are achieved, the standard deviation temperaturevariation is less than 7.8o'. he mass&flow
controllers andthe furnace, as well as data collection, are
ceramic heat exchangerbody. he graphite is compressed using molybdenum bolts.4olybdenum is chosen because it has low thermal expansion, thustending to retain compression at high temperature. hermocouplesare positioned at the centerline of the inlet and outlet tubes, #ustupstream and downstream of the heat exchanger itself. Frior totesting, thic% fiberglass insulation is pac%ed around the assembly(heat exchanger and manifold). Although heat loss through theinsulation is not directly measured, the heat transfer from the hotflow is always measured to be within a few percent of the heattransferred to the cold stream.2n addition to reporting inlet and outlet temperatures, the netheat transferred and heat& exchanger effectiveness are also evaluated.he maximum heat that can possibly be transferred is given as where is the minimum capacity rate and and arethe hot and cold inlet temperatures, respectively. he capacity ratesare evaluated as ' C J ' p, where J is the mass&flow rate and ' p
is the heat capacity of the fluid. he heat exchanger effectivenessis defined as where H is the actual heat transferred. he actual heat transferred isevaluated in terms of the enthalpy change between inlet and outlet flows.
1. Inlet temperature
*ig. illustrates measured heat exchanger performance asa function of hot& side inlet
temperature with hot& and cold&sideflow rates fixed at +.B: +7&3%gs&+ (+77 slm). 2n all cases the
coldsideinlet temperature is approximately 67 7'.
As the hot& side inlettemperature increases, heat transfer within the metal manifoldscan increase the cold&side inlet temperature by a few degrees. Atthe highest inlet temperature of :877', the net
heat transferred isapproximately :87 E and the effectiveness is nearly constant atapproximately 88G. he outlet temperatures and net heat transferred are nearly linear functions of the hot inlet temperature.
2. FlowRates
ig. =.3illustrates measured performance as a function ofbalanced inlet flow rates, with
the hot- and cold- side inlettemperatures fixed at 8>>
7%
- 42 7 4 l c 3 9 7 3 33+ 2& 2 2 f $alanced now, +.B:x+7J s+f'old inlet, :C67 K' C!"#$n"%&-Fig. easured heat exchanger performance as a function of inlet flow rates [6]
and 677', respectively. At low flowrates, the
differences between inlet and outlet temperatures arehigh. As the flow rates increase, the hot&side outlet temperatureincreases, the cold&side outlet temperature decreases, and there isa net increase in heat transferred. he effectiveness decreases
fromapproximately :7G at low flow rates of 6.B3 x +7&6 %g s&+ (8 slm)to approximately 87G at the
highest flow rates of +87 slm.
3. Unbalanced Flow
*ig. :.6illustrates performance with unbalanced flow. 2n thiscase, the hot inlet flow is fixed at :77o' and +.8< x +7&3 %g s&+ (<7 slm), while the cold inlet flow rate varies from+.+<
x+7&3 %g s&+ to +.B: x +7&3 %g s&+ (97&+77 slm). $oth the hotandcold& side outlet temperatures
decrease as the cold&side flowrate increases. According to heat exchanger theory,
theeffectiveness is minimum when the ratio of inlet capacity rates is unity. 2nthe experiments shown in *ig. <, the mass&flow rates arecontrolled by mass&flow controllers. /owever, because the specificheat of air varies with temperature (for air at :77 7','
pC++67 > %g&+ ? &+I at 877',
' pC+77< > %g&+ ? &+) and the averagetemperatures are different on the two sides, the capacity
rates arenot eual when the mass&flow rates are eual.
Mas* flow rale xlO-' (kg s ')
Fig. 7.3. easured performance as a function of cold-side inlet flow rate with hot-side inlet temperature and flow rate fixed.
4. Pressure Drop
*ig. :.8shows measured pressure drop under room&temperatureconditions and pressure drop predicted by the three&dimensional'*1 model. =p to about x+7&3%g s& + (+77 slm), the
model andmeasurements are in excellent agreement. At higher flow ratesthere may be transition to turbulent flow, which is possibly thecause of some disagreement between model and measurement.he current experiment is not configured to measure pressuredrop during high& temperature operation. /owever, the
pressuredrop scales with fluid viscosity, which, according to %inetic theory,scales approximately as 7.98. Liven good agreement for roomtemperatureoperation, the '*1
model, which considers temperature&dependent properties, is expected to provide accurate pressurepredictions for high&temperature operation.Lenerally spea%ing, all other performance measures beingeual, pressure drop should be as low as possible.
2mportantly,however, 5high5 and 5low5 pressure drop must be understood ina relative context. he ratio of heat&transfer&to&friction loss is oftenan important performance parameter. 4icrochannel heatexchangers are designed to achieve excellent heat transfer performancerelative to pressure drop. Fressure drop can be reduced, but only at theexpense
of ma%ing the heat exchanger significantly larger. he design by .> ?ee et al., which is ideally si!ed for mass&flow rates below3 x+7&6 %g s&+ (7 slm),
incorporates a relatively large heattransfersurface area into a small volume. At these flow rates, theeffectiveness is high and the pressure drop is uite low for the =sdelivered. 2ndeed, achieving these attributes is a primary ob#ectiveof compact heat exchangers.
: ollowing an analysis introduced by ays and ;ondon "#$ the ?friction power per unit
of surface area? can be represented a
s
Fig. easured and predicted pressure drop as a function of room-temperature air flow rate. [6]
Ehere is the pressure drop, is the heat transfer area, is the mean fluid velocity, and is the channel cross§ional area. he pumping power is
J ,where J is the volumetric flow rate. *or a rectangular channel with height / and widthE, the heat&transfer surface area is C E@ (i.e., the floor area between the
hot and cold fluids). he heat&transfer coefficient h, which can be interpreted as the 5heat transfer power per unit of surface area for onedegree of temperature difference,5 can be evaluated from the usselt number as
Assuming a channel width of E C
8 mm and properties of air at C 977 ?, *ig. :.9 shows the ratio 0"h as a function of Fig. 7. #atio of friction loss E and heat transfer h as a function of #eynolds number for three rectangular channel
eynolds number for three channel heights. $oth the heat transfer and the pumping losses increase greatly as functions of increasing eynolds number. At low eynolds number, the
heat transfer dominates (i.e., 1:h #). 0owever, at high *eynolds number the pumping
losses can greatly exceed the heat transfer (i.e., 1:h #)
.
FA'RICATION METHODS
arious methods used for the fabrication of microchannels include 4icro& 4achining, 1iffusion $onding, 'hemical 0tching, -tereolithography, @2LA and F@2-. esearchers have found the best method for fabrication of ceramic microchannels as F@2-.
he Fressure @aminated 2ntegrated -tructures (F@2-) fabrication begins by mixing ceramic powders with appropriate binders. 2n production, the green (i.e., unfired) layersthat incorporate manifold and channel geometry are formed usingcustom dies and high&pressure hydraulic presses. /owever, toavoid the expense of fabricating production dies, prototype designsare machined from pressed green& state bloc%s. 2n preparation forlaminating together in a hydraulic press, green layers are orientedand assembled as shown in *ig. <.+. After laminating the layers ina press, the assembled part is fired. As
Fig !.1 $canning electron microscope images showing channel structures, with high
illustrated by the scanningelectron microscope (-04) images shown aside the fired part becomesa single polycrystalline ceramic piece with essentially no evidenceof the bond lines between initially laminated layers.
he F@2- manufacturing process depends upon relativelycomplex interactions among ceramic powders and binders, layerformation pressures, and layer
"
$
lamination pressures. or example,excessive pressures during lamination can damage or
crushchannel ribs, while insufficient pressure can cause poor bondingbetween layers.
2uring development, a careful design of experimentsprocess was used to determine best
material combinationsand processing conditions. The above fig.is microscopic image
that shows essentially perfectpolycrystalline joining between layers. &n fact, the layers
are not?bonded? together. *ather, after sintering the resulting part isa single
polycrystalline ceramic. @rior to heat-exchanger testing,the finished parts are lea tested
to an internal pressure of 3 atm,assuring no external leas between layers or to the
.
CONCLUSION
'urrent research on microchannels may represent only the tip of the iceberg of future possibilities for their expanded use. apid advancements in micro&machining and micro&
deformation techniues are reducing the cost of fabrication while improving the reliability of microchannel systems, thus minimi!ing one of the main limitations of microchannels.
'eramic materials enable applications at hightemperature and in harsh chemical environments that may be difficult to achieve with metal heat exchangers. 2n addition to expanding operating space with ceramics, the
F@2- process has the potential to reduce manufacturing costs relative to a comparable metal heat exchanger. 'ompared to approaches in which layers are fabricated by diffusion bonding multiple thin metal shims, the F@2- approach reuires fabricating and handling far fewer parts. -caling from the current %ilowatt&scale systems to much larger capacities presents some significant challenges. he manifold design enables layering up with
negligible effect on pressure drop. /owever, further scaling probably reuires serial staging of multiple units. -erial staging of thermally isolated small units in a counter&flow configuration can improve overall effectiveness. /owever, because the microchannel flow length is increased, serial staging also increases pressure drop.
4icro heat exchangers are becoming an important area of interest in many fields of developing technology that reuire compact high heat energy removal solutions. *ields such as 4icro 0lectro 4echanical -ystems (404-), microelectronics, biomedical, fuel processing, and aerospace are all pushing the limits of thermal control and are finding ways to ma%e smaller devices with higher heat flux potential & reuiring more efficient smaller heat exchangers tocool their %ey wor%ing components.
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