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Applied Thermal Engineering 71 (2014) 508e518

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Applied Thermal Engineering

journal homepage: www.elsevier .com/locate/apthermeng

An experimental investigation on flow boiling heat transferenhancement using spray pyrolysed alumina porous coatings

Sujith Kumar C.S. a, Suresh S. a, *, Q. Yang b, Aneesh C.R. a

a Department of Mechanical Engineering, National Institute of Technology, Tiruchirappalli 620015, Indiab Department of Mechanical Engineering, College of Engineering, University of Saskatchewan, Saskatchewan, SK S7N5A9, Canada

h i g h l i g h t s

* Corresponding author. Tel.: þ91 431 2503422; faxE-mail addresses: sujithdeepam@gmail.com (S.K.

qiaoqin.yang@mail.usask.ca (Q. Yang), aneeshcr87@gm

http://dx.doi.org/10.1016/j.applthermaleng.2014.06.061359-4311/© 2014 Elsevier Ltd. All rights reserved.

g r a p h i c a l a b s t r a c t

� Porous alumina coatings were coatedon the copper using spray pyrolysis.

� Effect of coating temperature onporosity was determined.

� Higher enhancement in boiling heattransfer was obtained for 300 �Ccoated sample.

� Local heat transfer coefficient atoutlet increased with increase insubcooling.

a r t i c l e i n f o

Article history:Received 17 April 2014Accepted 28 June 2014Available online 5 July 2014

Keywords:MinichannelFlow boiling heat transferCritical heat fluxSpray pyrolysed alumina coatingSubcoolingWall heat flux

a b s t r a c t

In this work, flow boiling heat transfer experiments were conducted to investigate the effect of a spraypyrolysed porous alumina coatings over a copper substrates. Two different porous alumina coatings wereproduced by varying the deposition temperature of the spray pyrolysis technique. The heat transferexperiments were conducted in a mini-channel of dimensions 30 � 20 � 0.4 mm, with de-mineralizedwater as the working fluid. The coated samples were tested repeatedly for three different mass fluxes andtwo subcooled temperatures, to investigate their effect on the heat flux, surface temperature and averageheat transfer coefficient. An appreciable enhancement in heat flux was observed for the 300 �C spraypyrolysed alumina coated sample, when compared to the bare Cu and 350 �C spray pyrolysed aluminacoated sample. An enhancement of 28.3% in the heat flux was observed for the 300 �C spray pyrolysedalumina coated sample compared to the bare Cu sample, for a lower mass flux of 88 kg/m2s.

© 2014 Elsevier Ltd. All rights reserved.

1. Introduction causes a sudden increase in the surface temperature leading to the

A two phase flow heat transfer under high heat flux conditionsis found to have immense applications in supercomputers, powerdevices, electrical vehicles, nuclear reactors, defence and aero-nautical fields. This can be attributed to the high heat carryingcapacity during the phase change from liquid to vapour, occurringin such flows. But, a major limitation that is hindering furtherdevelopments in this field, is the critical heat flux (CHF), which

: þ91 431 2500133.C.S.), ssuresh@nitt.edu (S. S.),ail.com (A. C.R.).

0

failure of the devices [1,2]. Hence, it is necessary to limit themaximum safe heat flux of the application below the CHF. In thelast decade, extensive research has been carried out to improvethe CHF by means of flow separation, surface modificationsand surface coatings. In the case of the microchannel and microgaps with subcooled flow boiling, the temperature difference be-tween the inlet and outlet of the heater wall is higher, which inturn, reduces the average heat transfer coefficient. The tempera-ture difference can be reduced by passing a part of the fluidthrough a passive microjet, and mixing this fluid at the centre ofthe channel [3].

Nomenclature

Asur top surface area of heating block (m2)Ach cross sectional area of the channel (m2)Cp specific heat capacity of water (J/kg K)h boiling heat transfer coefficient (W/m2K)i enthalpy (J/kg)k thermal conductivity (W/mk)G mass flux (kg/m2s)q heat flux (W/m2)T1, T2 temperature measured from heater section in vertical

direction (�C)

Tfavg average fluid temperature (�C)

Tfout local fluid temperature at outlet (�C)

Tsavg average surface temperature (�C)

Tsout local surface temperature at outlet (�C)

Tsat saturation temperature of the fluidx dryness fraction

Subscriptsavg average

in inletout outlets surfacef fluidsat saturatedlg latenttp two phase

AbbreviationsAFM Atomic Force MicroscopyCHF critical heat flux (W/cm2)CNT carbon nanotubeDLC diamond like carbonPEEK polyether ether ketonePID proportional integral derivativeSEM Scanning Electron MicroscopeXRD X-ray diffraction

Greek symbolsDx vertical distance between thermocouples (m)

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Micro surface modifications primarily increase the surfaceroughness which in turn, enhances the area of heat transfer,thereby increasing the CHF [4]. Surface coatings generally increasethe heat transfer by fin action and porosity effect [5e9].

Nowadays, surface coatings have emerged as a more competentapproach for CHF enhancement in Micro and Mini scale devices,since larger enhancement in heat transfer is achieved with thinnercoatings, resulting in a minimized pressure drop. Among thesemethods, Nano fins obtained using CNT coatings have emerged aspotential candidates for improving the CHF, because of their highthermal conductivity and fin action. Previous studies have observedamaximum enhancement in CHF in a CNTcoated surface comparedto the uncoated surfaces, under lower mass flux conditions. But asthe mass flux increases, the enhancement in the CHF is found todecrease in the CNT coated surface. Hence, further studies werecarried out to correlate the stability of the CNT coatings withrespect to time. The results indicate that the CNTcoatings are stableonly for lower mass fluxes due to the bending of the CNTs towardsthe surface, as a result of their high aspect ratios [7,8]. Anotherpromising candidate for flow boiling heat transfer enhancement ina microchannel, is Metal nano wires and silicon wires. Morshedet al. [9] investigated the effect of copper nanowires on flow boilingheat transfer. They found an approximately 56% enhancement inthe heat transfer coefficient in the two phase flow usingmetal nanowires compared to the bare copper surface. D. Li et al. [10] con-ducted a flow boiling heat transfer enhancement study in micro-channels, using monolithically integrated silicon nanowires(SiNWs). They experimentally found that the integration of nano-wires produced an early onset of nucleate boiling (ONB), a delayedonset of flow oscillation (OFO), suppressed oscillating amplitude oftemperature and pressure drop, and an augmented heat transfercoefficient (HTC). A stable flow boiling trend was observed for thenanowire coatedmicrochannel over a range of heat fluxes, where asthe uncoated microchannel produced rapid growth of bubbleswhich led to an unstable boiling at a higher heat flux. F. Yang et al.[11e13] observed flow boiling regimes and conducted heat transferstudies on a silicon nanowires coated microchannel. They visual-ized the flow regimes for various high heat flux conditions on asilicon nanowires coated microchannel and obtained a singleannular flow regime instead of multiple flow regimes, as seen in the

microchannel without coating. They also obtained 300% enhance-ment in the CHF for the coated microchannel, compared with thatof the uncoated microchannel. They concluded that such aremarkable enhancement in CHF was due to the unified flowregime, which is highly stabilized. This phenomenon also leads to48% reduction in the pressure drop.

Porous coatings usingmetals and ceramics are also found to be acompetent method for improving the, CHF where parameters likethe particle size, particle shape, coating thickness and porositydetermine the CHF. Sarwar et al. [5] conducted a subcooled flowboiling CHF enhancement with porous alumina and titanium oxidecoatings. They coated varied sizes of nano particles using omegabond epoxy. The experimental results show that the aluminamicroporous coatingwith <10 mmparticle size and thickness 50 mmhas the maximum CHF enhancement. They calculated the effect ofsubcooling on CHF, and obtained greater enhancements at higherinlet subcooling temperatures. Bai et al. [14] conducted flow boilingheat transfer studies in parallel microchannels withmetallic porouscoatings. They used the solid state sinteringmethod for coating andcompared the boiling heat transfer of the porous coated micro-channel with that of a bare microchannel, where anhydrousethanol was used as the working fluid. The experimental resultshows the dramatic enhancement of boiling heat transfer in theporous coated microchannel, compared to a bare microchannel.They also found that the enhancement of CHF diminishes with theincrease in vapour quality. The enhancement of heat transfer inlower vapour quality is mainly due to the higher nucleation density,while in a higher the vapour quality heat transfer enhancementreduces due to the deterioration of nucleation density. Wang et al.[15] studied the flow boiling heat transfer enhancement in verticalnarrow channels coated with sintered aluminium powder. Theyobtained an enhancement of 2e5 times the boiling heat transfercoefficient with porous aluminium coating. Y. Sun et al. [16]investigated the subcooled flow boiling heat transfer from microporous surface in a small channel. They found that a small channeleven without the coating, showed enhancement in heat transfer atlower vapour quality due to the size effect of the channel. Furtherenhancement in heat transferwas also found using sixmicroporouscoatings. They also found that under the optimum condition, themicroporous coating produced three times the enhancement in the

S.K. C.S. et al. / Applied Thermal Engineering 71 (2014) 508e518510

heat transfer coefficient at saturated flow boiling, compared to asmooth surface. Their experimental results showed that higherinlet subcooling produced a higher heat transfer coefficient in flowboiling, due to an adequate increment in heat flux with an increasein subcooling.

Flow boiling heat transfer has a significant relation to thewettability of the surface. The onset of boiling was found to occurfaster for hydrophobic surfaces when compared to hydrophilicsurfaces, due to the lower free energy requirement for the nucle-ation. But for a higher heat flux, hydrophobic surfaces produce alesser heat transfer compared to hydrophilic surfaces, due to thesudden growth of bubbles, which will hinder the transfer of heatacross the surface [17e21]. Sarwar et al. [5] investigated the hy-drophilicity of coated surfaces, and its dependence on the surfaceroughness. They found an increase in the wettability of the coatedsurface as compared to an uncoated surface with respect to time.This has been attributed to the entrapment of water inside themicropores. Phan et al. [21] conducted flow boiling heat transferstudies on diamond like carbon (DLC), and hydrophilic titaniumoxide coatings, and found about 10% reduction in the CHF on DLC,as compared to the titanium oxide coating. B. J. Zhang et al. [22]conducted studies on the nucleate pool boiling heat transfer per-formance of alumina sponge like nano porous structures, by con-trolling their wettability. They produced nano-porous structures onaluminium substrates using the anodization technique. They ob-tained three dimensional porous networks, which could increasethe heat transfer surface area as well as the active nucleationdensity. Further improvement in nucleate pool boiling heat transferwas obtained by coating the hydrophobic self-assembled mono-layer (SAM), which in turn, increases the active pores fornucleation.

Various techniques such as solegel synthesis, thermal spraying,sintering and adhesives, are available for coating porous metaloxide on a metal substrate. Hsu et al. [23] used the solegel methodto prepare Nano-silica particle coatings on a copper surface. Thishelps in varying the wettability of the copper surface, from superhydrophilic to super hydrophobic, by modifying the surfacetopography and chemistry, and was used in pool boiling heattransfer studies. They found that the CHF increases with a decreasein the surface contact angle when the surface is relatively wetted.Yang et al. [24] mixed aluminium particles of diameters rangingfrom 11 to 38 mmwith a binder (OB-200) and a carrier (Methyl EthylKetone, M.E.K), to develop the boiling enhancement paint andsprayed it on the heating surface to make micro porous coatings.From the pool boiling experimental results, they concluded that theheat transfer coefficient of the micro porous coated surface isaffected by (a) the number of active nucleation sites, (b) bubbleleaving resistance and (c) micro porous coating layer thermalresistance.

Metal oxide porous coating provides stable coating after con-ducting the flow boiling experiments, compared to metal porouscoating, carbon nanotube, metal nanowires, and silicon nanowires.In the case of the CNT and metal nanowire coatings, the stabilityand repeatability of experiments were major concern with respectto ageing [8,9]. The chance of the oxidization of metal coatings isvery high in the presence of high temperature fluid, which con-tributes to the erosion of the coating. Porous metal oxide coatingscan be produced by sintering, with the addition of adhesives, andchemical synthesis. Chemical synthesis can be done using the spraypyrolysis technique; it is an effective technique due to the less timetaken for coating, low cost, stability and ease of doping [25].

The desired properties of the spray pyrolysed film can be ob-tained, by varying the deposition parameters like substrate tem-perature, spraying height, solution flow rate and molarity of thesolution. This method offers a number of advantages over other

deposition processes, namely, scalability of the process, cost-effectiveness with regard to equipment costs and energy needs,ease of doping, operation at moderate temperatures(100 �Ce500 �C), which opens up the possibility of awide variety ofsubstrates, control of thickness, variation of film composition alongthe thickness, and the possibility of multilayer deposition [25].

Only limited works are available in the area of boiling heattransfer studies using porous metal oxide coatings. In the presentstudy, the spray pyrolysis technique was used to coat porousalumina on a copper substrate. Two different alumina porouscoatings were obtained by varying the deposition temperature.Flow boiling heat transfer tests were conducted for different massfluxes and inlet subcooling temperatures. The heat flux, and boilingheat transfer coefficients of the coated samples were comparedwith those of a bare copper substrate.

2. Experimental methods

2.1. Alumina coating procedure

Alumina coatings over a copper substrate were performed, us-ing the spray pyrolysis equipment (HOLMARC HO-TH-04) by thedecomposition of an aqueous Aluminium Isopropoxide precursor.Demineralized water was used as a solvent. Two grams ofaluminium Isopropoxide was mixed with 1 L of demineralizedwater. 3 ml of nitric acid in 50 ml of demineralized water was alsoprepared. The prepared aluminium Isopropoxide e demineralizedwater mixture was placed in a magnetic stirrer (REMI 1MLH) for2 h at room temperature, to form a uniform solution. A few drops ofnitric acid solution were added to the prepared solution to achievea pH value of 4. Then the solution was stirred at 80 �C, until itbecame crystal clear. The solution was kept at room temperaturefor cooling [26].

Prior to the alumina deposition, the copper heating blocks weretreatedwith diluted 50% HNO3 and HCl solutions to remove the freeoxide layer from the surface. The acid treated copper heating blockswere sand blasted to improve the adhesion between the coatingand the substrate [8]. After sand blasting, the blocks were ultra-sonically cleaned for 5min in acetone. The prepared copper heatingblock was positioned in the spray pyrolysis equipment over thesubstrate holder. The distance between the nozzle tip and the topsurface of the heater block was kept at 5 cm.

The prepared precursor solution of 0.01 M concentration wassprayed uniformly over the copper surfaces, which were main-tained at temperatures of 300 �C and 350 �C respectively [25,26].The other deposition conditions were maintained constant as fol-lows: coating time e 20 min, no. of passes e 40, carrier air pressuree 1 bar, and precursor flow rate e 1 ml/min. The thicknesses of300 �C and 350 �C spray pyrolysed samples were measured, usingthe coating thickness gauge (Make-Phynix, Model-Surfix-S), andthe corresponding thicknesses were 19 mm and 20 mm respectively.

2.2. Heat transfer measurement

2.2.1. Test sectionThe constructional details of the test section are shown in

Fig. 1(a). The important components of the test section were, thecopper testing block (copper 145 alloy of 99.5%) in which four-cartridge heaters were inserted, a thermally insulated PEEK hous-ing, and a fibre glass top plate through which the flow boilingbehaviour was observed.

The testing plate was inserted into the housing, as repre-sented in Fig. 1(b); a mini-channel of 20 mm width and 0.4 mmheight was formed after fixing the top plate to the housing. Boththe entry to and exit of the 20 mm plenum were provided, to

Fig. 1. (a) Important components, and (b) sectional view of test section.

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develop a uniform flow through the channel. The copper testingblock consisted of three parts. The top part of 20 � 30 � 7 mm indimensions corresponded with the middle and bottom di-mensions of 30 � 40 � 5 mm and 40 � 50 � 45 mm respectively.Four cartridge heaters (each 350 W) were inserted into thebottom part of the copper testing block. The heating rate andtemperature of the copper test block were controlled by a PIDcontroller of 1500 W stabilized auto transformer. Surface tem-peratures were measured at three locations, using 1 mm diam-eter K-type thermocouples. One thermocouple was placed at thecentre of the copper testing block, and the other two 12.5 mmapart from the centre along the length and 1.5 mm below the topsurface of the copper testing block. Three K-type thermocoupleswere inserted 6 mm below the thermocouple placed formeasuring the surface temperature. Using these 6 thermocou-ples, the heat fluxes supplied to the working fluid weremeasured. Two K-type thermocouples were placed at themiddle of the copper testing block to limit the maximum tem-perature to a particular value using the PID controller. To mea-sure the inlet and outlet temperature, K-type thermocoupleswere placed just before the inlet and immediately after theformed mini channel. The instantaneous temperatures werecaptured by a computerized data taker (Data taker, DT 85). Athick layer of glass wool insulation was provided to avoid heatloss to the surroundings.

2.2.2. Experimental setupFig. 2 shows the experimental setup and flow loop that gives the

desired flow rate and constant inlet temperature. It consists of areservoir, micro gear pump, water filter, water preheater, test sec-tion and a chiller unit. All the experiments were conducted usingde-mineralized water as the working fluid. A micro gear pump wasused for pumping the water from the constant temperature reser-voir maintained at 45 �C. A water filter of 5 mm sieve diameter wasused. The inlet temperature of the water at the entry of the testsection was maintained, using a 1000 W Nichrome wire heater,

which was controlled by a PID controller. Boiling water from thetest section was cooled using the chiller unit, and circulated to-wards the reservoir.

2.2.3. Data reduction and uncertainty analysisThe input heat flux supplied to the flowing water in the channel

was calculated using the temperature gradient in the vertical di-rection of the test section: [3]

q ¼ �kDTDx

(1)

where DT is the difference between the average of 3 surface tem-peratures and average of 3 temperatures measured Dx below thesurface temperature thermocouples.

The average boiling heat transfer coefficient was determinedusing the formula [9]

havg ¼ q�Tsavg � Tfavg

� (2)

The outlet local surface temperature and fluid temperaturewerecalculated by extrapolating the measured surface and fluid tem-peratures. The outlet quality was calculated as: [16]

x ¼ iin þ ðQ=GAchÞ � ilsatilg

(3)

where

Q ¼ qðAsurÞ (4)

The local two phase heat transfer coefficient at the channeloutlet was obtained as: [16]

htp ¼ q�Tsout � Tfout

� (5)

Fig. 2. (a) Schematic of experimental setup, and (b) photograph of main experimental apparatus.

S.K. C.S. et al. / Applied Thermal Engineering 71 (2014) 508e518512

The outlet surface temperature Tsout was found out by extrap-olating three surface temperatures. Tfout is the saturation temper-ature of the fluid [14,16].

An uncertainty analysis was done using the Kline and McClin-tock [27] method. The estimated uncertainty of position was foundto be ±3.33% and that of thermal conductivity was ±1%. Themaximum uncertainty in temperature measurement was ±0.5 �C.The experimental uncertainties for the wall heat flux and boilingheat transfer coefficient were found to be ±7.11% and ±8.91%respectively, and that of the vapour quality is 12.16%.

2.2.4. Experimental procedurePrior to the experiment, the water was boiled vigorously

for 30 min to remove the dissolved gases in order to obtain ac-curate subcooled flow boiling data. A micro gear pump was usedto pump the water into the test section. The water inlet tem-perature at the test section was maintained at constant tem-peratures: 70 �C and 50 �C using the water pre-heater. The heatwas supplied to the testing plate in small increments by cartridgeheaters inserted in the copper testing block. Once the tempera-ture in the copper block reaches the steady state all the tem-perature data from the thermocouples were recorded using adata logger. After all the measurements were taken, the heat

flux to the testing block was increased, and the measurementprocedure repeated. The experiment was then repeated fordifferent values of heat flux up to the commencement of theannular flow.

3. Experimental results and discussion

3.1. Characterization

The morphology of the bare copper substrate and aluminacoated copper substrate was observed, using the Scanning ElectronMicroscope (Model: JSM 6010), and represented in Fig. 3 (a)e(c).Fig. 3(a) represents the SEM image of the substrate while, Fig. 3(b)and (c) represent the SEM images of the spray pyrolysed aluminacoated copper substrates, deposited at 300 �C& 350 �C respectively.The deposit exhibited amesh like porous structure, but the porositydecreased with the increase in the deposition temperature. Thismay be attributed to the fine particle deposition occurring duringhigh temperature deposition.

The X-ray diffraction (XRD) patterns of the alumina coatedsubstrates are shown in Fig. 4. Fig. 4(a) corresponds to the XRDpattern of the spray pyrolysed alumina sample heated at 300 �C,which exhibited a major peak at 42.28�, while the 350 �C spray

Fig. 3. SEM images of (a) bar copper (b) 300 �C spray pyrolysed alumina coated copper, and (c) 350 �C spray pyrolysed alumina coated copper.

Fig. 4. X-ray diffraction pattern of (a) 300 �C spray pyrolysed alumina coated copper,and (b) 350 �C spray pyrolysed alumina coated copper.

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pyrolysed alumina sample exhibited the major peak at43.01�(Fig. 4(b)), both corresponding to b Al2O3. The slight shift inthe peaks may be due to the mechanical stress induced during thedeposition process. The shift in peak of the 350 �C spray pyrolysedalumina coated sample was higher than that of the 300 �C spraypyrolysed alumina coated sample, due to the higher mechanicalstress invoked by the higher deposition temperature. The crystalsize of the deposit was found out using the Scherrer formula [28].The substrates coated at 300 �C exhibited an average crystallitesize of 22 nm, while those coated at 350 �C exhibited an averagecrystallite size value of 14 nm. This reduction in the crystal sizemay be due to the fine deposition taking place at highertemperatures.

The surface roughness and particle size of the coated substrateswere measured using the Atomic Force Microscope (AFM). Fig. 5(a)and (b) shows the deflection and topographical range of the twocoated samples and their roughness values are 474 nm and 335 nmrespectively, corresponding to the 300 �C spray pyrolysed aluminacoated copper and 350 �C spray pyrolysed alumina coated copper.The roughness value of the 300 �C spray pyrolysed alumina samplewas found to be higher than that of the 350 �C spray pyrolysedalumina sample, due to the coarse and non-uniform coatingformation.

The particle size distributions of the 300 �C and 350 �C spraypyrolysed alumina samples are shown Fig. 6(a) and (b) respectively,and the corresponding values are 87.5 nm and 16.5 nm. It isobserved, that the particle size of the higher temperature, coatedsurface is much lower than that of the lower temperature coatedsurface, due to the fine deposition process taking place. The surfaceporosities of the two coated samples were measured from the AFMimages, as shown in Fig. 7(a) and (b) respectively, using Gwyddionsoftware. The measured porosity of the 300 �C spray pyrolysedalumina sample was almost 52%, much higher than that of the350 �C spray pyrolysed alumina sample (18%). The adhesionstrength of coatings was measured using a pull off tester (Elc-ometer-506 pull off adhesion tester UK) and the obtained valueswere 0.85 MPa and 0.9 MPa for the 300 �C and 350 �C spraypyrolysed alumina samples respectively.

Fig. 8 shows the static contact angles of water on the threetested surfaces at three different time intervals. The static contactangle of the bare copper surface was higher than that of the othertwo surfaces for all time intervals. This may be due to the hydro-philic nature of the spray pyrolysed alumina coating. The initialcontact angles of both 300 �C and 350 �C spray pyrolysed sampleswere similar. However, for the other two time intervals, themeasured contact angle of the 300 �C spray pyrolysed sample wasfound to be lower than that of the 350 �C spray pyrolysed sample.This behaviour may be attributed to the higher surface porosity ofthe 300 �C spray pyrolysed sample.

3.2. Flow boiling heat transfer studies

The subcooled flow boiling heat transfer experiments wereconducted for three mass fluxes of 88 kg/m2s, 248 kg/m2s and346 kg/m2s, with inlet fluid temperatures of 50 �C and 70 �Crespectively. The subcooled flow boiling heat transfer results of the300 �C and 350 �C spray pyrolysed alumina coated copper were

Fig. 5. Deflection and topographical view of (a) 300 �C spray pyrolysed alumina coated copper, and (b) 350 �C spray pyrolysed alumina coated copper.

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compared with that of bare copper. For all the tested samples, thecommencement of nucleation occurred in the downstream regionof the channel and propagated to the upstreamwith the increase inthe heat input. At a low mass flux (88 kg/m2s), the nucleatedbubbles grew rapidly into a single slug type bubble inn thedownstream region, due to the higher channel width compared tothe height. This formed bubble is then found to propagate towardsthe upstream regionwith respect to the increasing heat flux. In thecase of the other two mass fluxes, the formed bubble elongationwas a slow process due to its high velocity. The alumina coatedcopper sample spray pyrolysed at 300 �C, showed a maximumamount of bubble nucleation density, compared to the other two

Fig. 6. AFM images showing the particle size of (a) 300 �C spray pyrolysed alu

samples due to its higher pore density. The obtained flow patternsfor the coated samples were different from the bare copper sampledue to its hydrophilicity and porosity. The transition from thebubbly to the annular flow over the coated samples was slow,compared to the bare copper, which led to a high heat carryingcapacity. In the case of the coated samples, due to the mesh-likepores the initial density of bubbles was higher than in the barecopper samples. Due to the hydrophilicity of the coated samples,the water gets trapped inside the mesh-like pores, which hindersthe growth of the initiated bubbles by their local pumping action.Under the high heat flux conditions, the water that gets trappedinside the pores gets heated, and moves across the direction of the

mina coated copper, and (b) 350 spray pyrolysed alumina coated copper.

Fig. 7. AFM images showing porosity measured by Gwyddion software of (a) 300 �C spray pyrolysed alumina coated copper, and (b) 350 �C spray pyrolysed alumina coated copper.

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growth of the bubbles [11]. For the coated samples, a lowmass flux(88 kg/m2s), achieved the maximum enhancement inwall heat fluxcompared to the copper sample. This was due to the fact that at thismass flux, theworking fluid gets enough time to propagate throughthe mesh like pores.

3.3. Effect of porous alumina coatings

The present study compared the effect of the spray pyrolysedalumina porous coated copper samples, with respect to the barecopper substrate, on flow boiling heat transfer. The amount of heatflux supplied to thewater, and the effective heat transfer coefficientwere calculated using Eqs. (1) and (2) respectively. Fig. 9(a)e(c)shows the subcooled flow boiling curves of three different sur-faces for three mass fluxes of 88 kg/m2s, 248 kg/m2s and 346 kg/m2s respectively, under an inlet temperature of 50 �C. Themaximum enhancements of the heat flux of 28.3% and 17.03% wereobtained for the 300 �C and 350 �C spray pyrolysed alumina coatedcopper respectively, compared to that of the bare copper, for the amass flux of 88 kg/m2s. For the other twomass fluxes of 248 kg/m2sand 346 kg/m2s, the corresponding enhancements in the heat fluxof the 300 �C and 350 �C spray pyrolysed alumina coated copperwere 22.2% and 16.08%, and 15.0% and 13.2% respectively. The

Fig. 8. Static contact angles for different time intervals of (a) bar copper (b) 300 �C spray py

percentage increases in the heat flux of both the coated sampleswere due to the combined effect of hydrophilicity and porosity.Hydrophilicity increased the area of contact of water with a surface,which led to the increase in the heat carrying capacity [16]. In thecase of the two phase flow, the heat carrying capacity of the fluidincreases with hydrophilicity in the high heat flux condition, due tothe slow transition from the bubbly to the annular flow [7]. Porosityincreased the bubble nucleation density, which led to an early onsetof boiling on the surface. For the mass flux of 88 kg/m2s, the cor-responding surface temperatures at which the bubble nucleationtook place were 105.08 �C, 104.14 �C and 104.72 �C respectively, forbare copper, 300 �C spray pyrolysed alumina coated copper and350 �C spray pyrolysed alumina coated copper samples. In the caseof the higher heat flux, the growth of the formed bubble was hin-dered by the local pumping action of the water inside the pores,which led to an increase in the wall super heat for the same surfacetemperature. Porosity also increased the total area of heat transferby increasing the roughness of the surface as mentioned earlier,which led to a higher heat capacity of the surface. The percentageincrease in the heat flux of the 300 �C spray pyrolysed aluminacoated copper sample for all mass fluxes is higher than that of the350 �C spray pyrolysed alumina coated copper sample, due to thehigher porosity and hydrophilicity (as shown in Figs. 7 and 8). The

rolysed alumina coated copper, and (c) 350 �C spray pyrolysed alumina coated copper.

Fig. 9. 50 �C inlet temperature subcooled flow boiling curve for different mass fluxes(a) 88 kg/m2s (b) 248 kg/m2s, and (c) 346 kg/m2s.

Fig. 10. Variation of average boiling heat transfer coefficient with heat flux at 50 �C fordifferent mass fluxes (a) a) 88 kg/m2s (b) 248 kg/m2s, and (c) 346 kg/m2s.

S.K. C.S. et al. / Applied Thermal Engineering 71 (2014) 508e518516

experimental results show that the % increase in the heat fluxes ofboth the coated samples was reduced, with respect to the increasein themass fluxes. This wasmainly due to the partial wetting actionof the pores at high mass fluxes, and the reduction in the %

enhancement of the 350 �C spray pyrolysed alumina coated samplewas lesser compared to that of the 300 �C spray pyrolysed alumina,due to its lower porosity.

The average heat transfer coefficient is calculated using Eq. (2).Fig. 10(a)e(c) represents the variation of the heat transfer coeffi-cient with heat flux, for the three mass fluxes of 88 kg/m2s, 248 kg/m2s and 346 kg/m2s respectively, under an inlet temperature of50 �C. The enhancement in the heat transfer coefficient corre-sponding to the maximum enhancement in the heat flux is 22.2%

S.K. C.S. et al. / Applied Thermal Engineering 71 (2014) 508e518 517

and 15.4% respectively for 300 �C spray pyrolysed alumina coatedcopper and 350 �C spray pyrolysed alumina coated copper,compared to that of the bare copper, under a mass flux of 88 kg/m2s. The enhancement was slightly lesser than that in the heat flux,due to the higher surface temperature. This effect is mainly due tothe hydrophilicity of the surface, which shifted the boiling curvetowards the right. Comparing the results for different mass fluxes,the heat transfer coefficient increasing with the increase in themass fluxes, is due to the increase in the heat carrying capacity ofthe fluid with increases in the mass flux [7].

3.4. Effect of inlet subcooling temperature

Fig. 11(a) demonstrates the influence of the liquid inlet sub-cooling on the boiling curves, for a mass flux of 88 kg/m2s. Thesubcooling effect was found by keeping the inlet temperature at50 �C and 70 �C. The incipience of boiling took place at a loweraverage surface temperature of 50 �C inlet fluid temperature. This isdue to the subcooling effect of the working fluid on the surface,which led to the high heat carrying capacity for the same outputvapour quality. The corresponding values of the surface tempera-tures at which the boiling incipience took place were 106.20 �C,105.21 �C, and 105.79 �C respectively, for bare copper, 300 �C spraypyrolysed alumina coated copper and 350 �C spray pyrolysedalumina coated copper samples, for a mass flux of 88 kg/m2s at70 �C inlet temperature. Depressions in the nucleation onset

Fig. 11. Effects of inlet subcooling temperature on flow boiling heat transfer (a) boilingcurve, and (b) outlet local two phase heat transfer coefficient as a function of heat flux.

temperatures were almost the same, for both the subcooling tem-peratures. This revealed that the liquid subcooling exerts a minorimpact on the early onset of boiling, using porous coating.

The effect of subcooling temperatures on the local two phaseheat transfer coefficient at the outlet of the test section is shown inFig. 11(b). For all the samples, the local two phase heat transfercoefficient of the 50 �C inlet temperature subcooled experimentalresults showed higher values than that of the 70 �C inlet temper-ature subcooled experimental results. This is due to the incrementin the adequate heat flux for all the surfaces, so as to maintain afully developed subcooled boiling. For both the subcooled condi-tions, the 300 �C spray pyrolysed alumina coated copper showedthe maximum enhancement in the heat transfer coefficient, due toits high porosity and hydrophilicity.

4. Conclusion

The subcooled flow boiling heat transfer studies were con-ducted on two porous alumina coated copper samples, and a barecopper sample, under three different mass fluxes at 50 �C and 70 �Cinlet temperatures. The important findings from the present studyare as follows:

1. For all flow boiling heat transfer experiments, the initiation ofboiling took place in the mini channel's downstream region.Early onset and high density of nucleation were observed inboth the coated samples compared to the bare copper sample,due mainly to the porosity of the surface. The transition of thebubbly to the slug flow in the case of the coated samples wasslow, compared to the bare copper sample, due to the combinedeffect of porosity and hydrophilicity.

2. Spray pyrolysed alumina coating provides an appreciable flowboiling heat transfer enhancement, especially at a lowmass flux.The 300 �C spray pyrolysed alumina coated copper provided amaximum enhancement of 28.3% in the wall super heat flux,compared to that of the bare copper surface for a particularsurface temperature at 88 kg/m2s mass flux. For the other twomass fluxes, the obtained heat flux enhancements are 22.2% and15% respectively. Higher enhancement in the wall super heatflux at a lower mass flux was mainly due to the area enhance-ment and the local pumping action of the mesh-like pores onthe surface. The reduction in the percentage enhancement inthe boiling heat flux at higher flow rates, is mainly due to thesudden movement of the fluid from the pores, that leads to areduction in the local pumping action. With an increase in thedeposition temperature of the spray pyrolysis process from300 �C to 350 �C the porosity of the surface decreased from 50%to 10%. This leads to a lower enhancement of the flow boilingheat flux in the case of the 350 �C spray pyrolysed aluminacoated copper, compared to that of the 300 �C spray pyrolysedalumina coated copper.

3. The higher inlet fluid temperature provided a lower averagesurface temperature for all the tested samples, due to the sub-cooling effect of fluid that gave a higher heat carrying capacity.For all the samples the local two phase heat transfer coefficientat the outlet for a lower inlet temperature was higher, than thatof the other for a particular heat flux. This is mainly due to thehigh heat flux requirement. Enhancement of heat transfer co-efficient for the 300 �C spray pyrolysed alumina coated copper isalmost the same for both subcooling temperatures. So theporosity and the hydrophilicity did not have any effect on thesubcooling temperatures.

The spray pyrolysed porous coated surfaces in the mini-channelprovided the combined effect of high density nucleation site and

S.K. C.S. et al. / Applied Thermal Engineering 71 (2014) 508e518518

hydrophilicity, and gave a remarkable enhancement in flow boilingheat transfer. This technique is a cutting edge option for high fluxapplications.

Acknowledgements

The authors wish to thank the Department of Science andTechnology (DST Sanction letter No. SR/FTP/ETA-0019/2012 dated19.07.2012) Government of India for its financial support to thiswork. The authors are also grateful to the Director, National Insti-tute of Technology (NIT), Tiruchirappalli, for his continuousencouragement and support.

Appendix A. Supplementary data

Supplementary data related to this article can be found at http://dx.doi.org/10.1016/j.applthermaleng.2014.06.060.

References

[1] I. Mudawar, Assessment of high-heat-flux thermal management schemes,IEEE Trans. Compon. Packag. Tech. 24 (2001) 122e141.

[2] I. Mudawar, E. Maddox, Critical heat flux in subcooled flow boiling of fluo-rocarbon liquid on a simulated electronic chip in a vertical rectangularchannel, Int. J. Heat. Mass Transf. 323 (1989) 379e394.

[3] X. Dai, F. Yang, R. Fang, T. Yemame, J.A. Khan, C. Li, Enhanced single- and two-phase transport phenomena using flowseparation in a microgap with copperwoven mesh coatings, Appl. Therm. Eng. 54 (2013) 281e288.

[4] S. Kottoff, D. Gorenflo, E. Danger, A. Luke, Heat transfer and bubble formationin pool boiling: effect of basic surface modifications for heat transferenhancement, Int. J. Therm. Sci. 4 (2006) 217e236.

[5] M.S. Sarwar, Y.H. Jeong, S.H. Chang, subcooled flow boiling CHF enhancementusing porous surface coatings, Int. J. Heat. Mass Transf. 50 (2007) 3649e3657.

[6] N. Singh, V. Sathyamurthy, W. Peterson, J. Arendt, D. Banerjee, Flow boilingenhancement on a horizontal heater using carbon nanotube coatings, Int. J.Heat. Fluid Flow. 31 (2010) 201e207.

[7] V. Khanikar, I. Mudawar, T. Fisher, Effects of carbon nanotube coating on flowboiling in a micro-channel, Int. J. Heat. Mass Transf. 52 (2009) 3805e3817.

[8] C.S. Sujith Kumar, S. Suresh, L. Yang, Q. Yang, S. Aravind, Flow boiling heattransfer enhancement using carbon nanotube coating, Appl. Therm. Eng. 65(2014) 166e177.

[9] A.K.M.M. Morshed, F. Yang, M.Y. Ali, J.A. Khan, C. Li, Enhanced flow boiling in amicrochannel with integration of nanowires, Appl. Therm. Eng. 32 (2012)68e75.

[10] D. Li, G.S. Wu, W. Wang, Y.D. Wang, Dong Liu, D.C. Zhang, Y.F. Chen,G.P. Peterson, R. Yang, Enhancing flow boiling heat transfer in microchannelsfor thermal management with monolithically-integrated silicon nanowires,Nano Lett. 12 (2012) 3385e3390.

[11] F. Yang, X. Dai, Y. Peles, P. Cheng, C. Li, Can multiple flow boiling regimes bereduced into a single one in microchannels? Appl. Phys. Lett. 103 (2013),043122-(1-5).

[12] F. Yang, X. Dai, Y. Peles, P. Cheng, J. Khan, C. Li, Flow boiling phenomena in asingle annular flow regime in microchannels (II): reduced pressure drop andenhanced critical heat flux, Int. J. Heat. Mass Transf. 68 (2014) 716e724.

[13] F. Yang, X. Dai, Y. Peles, P. Cheng, J. Khan, C. Li, Flow boiling phenomena in asingle annular flow regime in microchannels (I): characterization of flowboiling heat transfer, Int. J. Heat. Mass Transf. 68 (2014) 703e715.

[14] P. Bai, T. Tang, B. Tang, Enhanced flow boiling in parallel microchannels withmetallic porous coating, Appl. Therm. Eng. 58 (2013) 291e297.

[15] R. Wang, Y. Li, A. Gu, S. Hu, The experimental study of liquid nitrogen boilingheat transfer in vertical narrow channel of porous layer surface, Cryogenics 32(Suppl. 1) (1992) 249e252.

[16] Y. Sun, L. Zang, H. Xu, X. Zhong, Subcooled flow boiling heat transfer frommicroporous surfaces in a small channel, Int. J. Therm. Sci. 50 (2011) 881e889.

[17] H.J. Jo, S.H. Kim, H. Kim, J. Kim, M.H. Kim, Nucleate boiling performance onnano/microstructures with different wetting surfaces, Nanoscale Res. Lett. 7(2012) 242e250.

[18] H.J. Jo, H. Kim, H.S. Kim, S. Kim, S.H. Kang, J. Kim, M.H. Kim, Experimentalstudy of boiling phenomena by micro/milli hydrophobic dot on silicon surfacein pool boiling, in: Proceedings of the Seventh International Conference onNanochannels, Microchannels and Minichannels, A.S.M.E, Pohang, New York,2009, pp. 93e97.

[19] Y. Takata, S. Hidaka, M. Kohno, Enhanced nucleate boiling by super-hydrophobic coating with checkered and spotted patterns, in: Proceedings ofthe Sixth International Conference on Boiling Heat Transfer, Spoleto, CurranAssociates Inc, New York, 2006, pp. 240e244.

[20] Y. Nam, J. Wu, G. Warrier, Y.S. Ju, Experimental and numerical study of singlebubble dynamics on a hydrophobic surface, J. Heat. Transf. 131 (2009)120e124.

[21] H.T. Phan, N. Caney, P. Marty, S. Colassan, J. Gavillet, Flow boiling of water ontitanium and diamond like carbon coated surface in a microchannel, Front.Heat. Mass Transf. 2 (0103002) (2011) 1e4.

[22] B.J. Zhang, K.J. Kim, H. Yoon, Enhanced heat transfer performance of aluminasponge-like nano-porous structures through surface wettability control innucleate pool boiling, Int. J. Heat. Mass Transf. 55 (2012) 7487e7498.

[23] C.C. Hsu, P.H. Chen, Surface wettability effects on critical heat flux of boilingheat transfer using nanoparticle coatings, Int. J. Heat. Mass Transf. 55 (2012)3713e3719.

[24] C.Y. Yang, C.F. Liu, Effect of coating layer thickness for boiling heat transfer onmicroporous coated surface in confined and unconfined spaces, Exp. Therm.Fluid Sci. 47 (2013) 40e47.

[25] A.B. Khatibani, S.M. Rozati, Synthesis and characterization of amorphousaluminum oxide thin films prepared by spray pyrolysis: effects of substratetemperature, J. Non-cryst. Solids 363 (2013) 121e133.

[26] G. Ruhi, O.P. Modi, I.B. Singh, A.K. Jha, A.H. Yegneswaran, Wear and electro-chemical characterization of sol-gel alumina coating on chemically pre-treated mild steel substrate, Surf. Coat. Tech. 201 (2006) 1866e1872.

[27] J.P. Holman, Experimental Methods for Engineers, seventh ed., Tata McGrawHill Education Private Limited, 2007.

[28] B.D. Culity, S.R. Stock, Elements of X-ray Diffraction, third ed., Prentice Hall,2000.