International Journal of Heat and Mass Transfer 75 (2014) 718–725

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International Journal of Heat and Mass Transfer

journal homepage: www.elsevier .com/locate / i jhmt

High heat flux spray cooling with ammonia: Investigation of enhancedsurfaces for HTC

http://dx.doi.org/10.1016/j.ijheatmasstransfer.2014.04.0190017-9310/� 2014 Elsevier Ltd. All rights reserved.

⇑ Corresponding author. Address: University of North Texas, Department ofEngineering Technology, UNT Discovery Park, 3940 North Elm St. F115, Denton, TX76207, United States. Tel.: +1 940 369 5101; fax: +1 940 565 2666.

E-mail address: huseyin.bostanci@unt.edu (H. Bostanci).

Huseyin Bostanci a,⇑, Daniel P. Rini a, John P. Kizito b, Virendra Singh c, Sudipta Seal c, Louis C. Chow d

a RINI Technologies, Inc., 582 S. Econ Circle, Oviedo, FL 32765, United Statesb Department of Mechanical and Chemical Engineering, North Carolina A&T State University, Greensboro, NC 27411, United Statesc Department of Materials Science and Engineering, Advanced Materials Processing Analysis Center, Nanoscience Technology Center, University of Central Florida,Orlando, FL 32816, United Statesd Department of Mechanical and Aerospace Engineering, University of Central Florida, Orlando, FL 32816, United States

a r t i c l e i n f o a b s t r a c t

Article history:Received 5 November 2013Received in revised form 3 March 2014Accepted 8 April 2014Available online 12 May 2014

Keywords:Spray coolingThermal managementHigh heat fluxHTCEnhanced surfaces

An experimental spray cooling study was carried out to investigate the effect of enhanced surfaces onheat transfer performance. Test surfaces involved; (a) micro scale indentations and protrusions, (b) macro(mm) scale pyramidal, triangular, rectangular, and square pin fins, and (c) multi-scale structures thatcombine macro and micro scale structures, along with a smooth surface that served as reference. Testswere conducted in a closed loop system using vapor atomized spray nozzles with ammonia as theworking fluid. Cooling performance data for each enhanced surface were obtained applying heat fluxesof up to 500 W/cm2, and using flow rates of 1.6 ml/cm2-s of liquid and 13.8 ml/cm2-s of vapor. Typicaltemperature readings with embedded thermocouples were verified using infrared thermographymethod. Data indicated that the multi-scale structured surface achieved the highest heat transfer coeffi-cient (HTC) of 772,000 W/m2 �C, corresponding to 161% enhancement over the reference surface. Theresults suggest that the multi-scale structured surface can combine the unique benefits of the microand macro scale structures, and provide some insights to the understanding of the spray cooling heattransfer mechanisms by emphasizing the importance of boiling through surface nucleation. Thereforeammonia spray cooling, with the utilization of enhanced surfaces, offers significant cooling performancefor high heat flux thermal management applications that target to maintain low device temperatureswith a compact and efficient cooling system.

� 2014 Elsevier Ltd. All rights reserved.

1. Introduction

Thermal management plays a critical role for the reliable andefficient operation of many high power devices in electronics,optics and aerospace fields, among others. Beyond the device levelrequirements, the choice of thermal management technique alsoaffect the system level design parameters, such as weight and size.Advanced cooling methods that involve two-phase heat transfermechanisms continue to get attention due to their inherent advan-tages, with spray cooling being one of the most promisingtechnologies.

Earlier spray cooling research that focused on heat transferenhancement included various surface modifications. Pais et al.[1] and Sehmbey et al. [2] examined the effects of surface

roughness and contact angle using water with air atomized nozzle,and reported enhancement in HTC with decreasing surfaceroughness and increasing contact angle. Kim et al. [3] investigatedmicro-porous coated surfaces using water and found that thecritical heat flux (CHF) increased 50% relative to the uncoated sur-face at low coolant rates. Stodke and Stephan [4] studied micro-structured and micro-porous surfaces using water and observeda heat flux level with the micro-pyramid surface that was muchlarger than the surface area enhancement. Amon et al. [5] andHsieh and Yao [6] performed experiments with water on siliconmicro-studs and attributed the higher heat transfer in the interme-diate regimes to more effective spreading of the liquid by capillaryforces. Silk et al. [7] investigated the effect of surface geometrywith PF-5060 using 1-mm size embedded structures and com-pound extended structures, and obtained up to 62% CHF enhance-ment. Coursey et al. [8] evaluated the straight fin geometry usingPF-5060 and found that fin heights between 1 and 3 mm wereoptimum for heat fluxes up to 124 W/cm2.

Nomenclature

A base area, cm2

CHF critical heat flux, W/cm2

h heat transfer coefficient, W/m2 �Chfg latent heat of vaporization, kJ/kgHTC heat transfer coefficient, W/m2 �CI current, Ak heater wall thermal conductivity, W/m �Ckl liquid thermal conductivity, W/m �Cq00 heat flux, W/cm2

Tl liquid temperature, �CTres resistor temperature, �CTres

0 normalized resistor temperature, �C

Tsat saturation temperature, �CTsurf surface temperature, �CTCavg average thermocouple reading, �Crc,max maximum cavity mouth radius, lmrc,min minimum cavity mouth radius, lmRa average surface roughness, lmV voltage, Vx TC to spray surface distance in heater wall, mDTsat surface superheat, �Cqv vapor density, kg/m3

dt thermal boundary layer thickness, mr surface tension, N/m

H. Bostanci et al. / International Journal of Heat and Mass Transfer 75 (2014) 718–725 719

Among the recent spray cooling studies, Thiagarajan et al. [9]performed tests with HFE-7100 dielectric liquid on copper micro-porous, and copper nanowire coated surfaces. At a subcooling con-dition of 30 �C, their CHF was �135 W/cm2, and they found up to300% increase in HTC compared to that of a plain surface. De Souzaand Barbosa Jr. [10] evaluated copper foam and radially-groovedsurfaces in their study with R-134a refrigerant. They determineda CHF level of 30 W/cm2, and up to 1.35� increase in HTC over aplain surface. Xie et al. [11] did experiments with micro-structured(wire-cut EDM), macro-structured (mm scale finned surfaces), andmultiscale-structured (combination of micro- and macro-scalestructures) copper surfaces using R134a coolant. Their data withmultiscale-structured surface showed 65% HTC enhancement overa smooth reference at heat fluxes of up to 210 W/cm2. Thisenhancement was mainly attributed to more nucleation sites andincreased wetted surface, with a particular emphasis on finarrangement. Zhang et al. [12] studied spray parameters as wellas enhanced surfaces with de-ionized water. They fabricatedvarious micro scale square pin-fins using wire cutting, and roughsurfaces using sandpapers, and observed HTC enhancement of upto 67% and 50% over their flat reference surface, respectively, atsaturation condition. Yang et al. [13] reported an ammonia spraycooling research using surfaces with sub-mm scale cylindrical cav-ities, and examined the capillary effect in heat transfer perfor-mance. When compared to a flat reference surface, theirenhanced surface provided up to 1.3� higher HTC of 145,000 W/m2 K at heat fluxes reaching 450 W/cm2.

In their previous work [14–16], authors conducted spray cool-ing research with ammonia and tested surfaces featuring indenta-tions, protrusions, and pyramidal fins. They observed HTCenhancement of up to 112% at 500 W/cm2 over a reference smoothsurface. The present work also focused on high heat flux spraycooling with ammonia, but evaluated a larger spectrum ofenhanced surfaces. The objectives were to investigate the effectof surface enhancement on spray cooling HTC by varying structuregeometry and size, and contribute to the physical explanations ofspray cooling heat transfer mechanisms. The experimental domainof enhanced surfaces included; (a) micro scale indentations andprotrusions, (b) macro (mm) scale pyramidal, triangular, rectangu-lar, and square pin fins, and (c) multi-scale structured surfaces,combining macro and micro scale structures. Tests were conductedat heat fluxes of up to 500 W/cm2 to represent high powerapplications.

2. Experimental setup and procedure

A closed loop spray cooling system was used to run theexperiments. Schematic diagram in Fig. 1 illustrates the main

components of the system: a reservoir, 1 � 2 nozzle array, a subco-oler, a condenser, and a pump. Anhydrous ammonia (Refrigerant717) was selected as the working fluid. The favorable propertiesof ammonia, such as high latent heat of vaporization and low sat-uration temperature (1368 kJ/kg and �33.4 �C, respectively, atatmospheric pressure) make it an attractive coolant for high heatflux applications that also require low temperature operation.Ammonia cooling systems however, require careful material selec-tion, as ammonia is not compatible with some of the commonlyused engineering materials, and should be sealed very well toavoid any leaks. In the setup, the reservoir supplies ammonia liquidand vapor to the nozzle array. Liquid and vapor streams mix in theatomizer nozzles and the resulting sprays impinge on a 1 � 2 cmheater. Thick film resistors mounted on this heater work as a con-trolled heat source. Two-phase exhaust from the 1 � 2 nozzle arrayslightly subcools the incoming liquid supply in a small heatexchanger, before entering into the larger heat exchanger to con-dense. Finally, the pump takes the ammonia and returns it backto the reservoir, providing the required pressure lift to driveammonia in the cycle and generate the spray. A separate air-cooledR-22 cycle absorbs the heat from the ammonia cycle and rejects itto the ambient. The test setup allows controlling flow rates andpressures across the nozzle array and is equipped with computercontrolled data acquisition system for accurate data recording.

2.1. Spray Nozzle

A compact vapor atomized nozzle array (Fig. 2a). manufacturedby RINI Technologies, Inc. and composed of two side-by-side noz-zles, was used in all tests. In these nozzles a fine liquid stream isinjected into a high velocity vapor stream. The shear force createdby the vapor stream atomizes the liquid into fine droplets that areejected from the nozzle orifice. Fig. 2a also shows the pressure andtemperature measurement ports on the nozzle array that wereneeded to determine the driving pressures across the nozzle, andsaturation condition of the coolant. Nozzle-to-surface distance of11 mm was found to be adequate for each spray to cover 1 cm2

area, and was held constant.

2.2. Heaters and spray surfaces

The heater design is shown in Fig. 2b–c. The heater was made ofAl 6061 (aluminum alloy 6061), a material compatible with ammo-nia. One side of the heater had a total heated area of 1 � 2 cm. Two1-cm2 thick film resistors were soldered onto this side to simulatea heat generating device. The other (opposite) side of the heaterfeatured smooth or enhanced surfaces, and was sprayed by thenozzle array. Heat flux was determined from the total power

R22R22

Two-phase pump

NH3 vapor

NH3 liquid

Reservoir

R22Subcooler

NH3 exhaustNozzle array

PT P

P

P

Heater

Fig. 1. Schematic diagram of the spray cooling setup.

Fig. 2. RINI 1 � 2 vapor atomized nozzle array (a), heater sample-spray side (b), heater sample-resistor side (c).

720 H. Bostanci et al. / International Journal of Heat and Mass Transfer 75 (2014) 718–725

supplied into the thick film resistors per unit base area (q00 = V � I/A).Heater temperature was monitored with four type-T thermocou-ples embedded halfway in the heater wall and spaced equallyacross the 2 cm2 area as illustrated in Fig. 2c. Temperature spreadresulted from the four thermocouples did not exceed ±1 �C at200 W/cm2, and ±1.5 �C at 500 W/cm2 indicating uniform coolingacross the surface. Spray surface temperature was calculated byextrapolating average of four thermocouple readings through theknown distance to the surface (consistently taken as half of heaterwall thickness for all types of samples) and assuming steady 1-Dconduction through the heater wall (Tsurf = TCavg � (q00 �x)/k).

Infrared (IR) thermography was utilized in this work as an alterna-tive temperature measurement technique. Besides the embeddedTCs in the heater wall, an Inframetrics PM290 ThermaCam modelIR camera allowed measuring temperatures at the thick film resis-tor surface, the outermost surface of the heater assembly. Thishelped validating the temperature readings for a better perfor-mance comparison between various enhanced surfaces. Use of IRthermography in the current context is detailed in Ref. [17].

The description and surface characteristics of the test heatersare provided in Table 1. Five of the heaters (the surfaces s1–s5)with machine finished-smooth surface, served as reference for all

Table 1Heater descriptions.

Heater ID Surface condition, enhancement type, structuregeometry, structure size*

s1, s2, s3, s4, s5 Smooth, plain, machine-finished (Ra � 0.3 lm)mi-f Micro structured, indentations, fine (Ra � 2 lm)mi-m Micro structured, indentations, medium (Ra � 3 lm)mi-c Micro structured, indentations, coarse (Ra � 5 lm)mp-f Micro structured, protrusions, fine (Ra � 5 lm)mp-m Micro structured, protrusions, medium (Ra � 13 lm)mp-c Micro structured, protrusions, coarse (Ra � 20 lm)Mpf(�0.75,�0.50, �0.25)

Macro structured, pyramidal fins, 0.75/0.50/0.25 H, 0.88/0.59/0.29 B, 0.88/0.59/0.29 P

Mtf-0.75 Macro structured, triangular fins, 0.75 H, 0.88 B, 0.88 PMrf-0.75 Macro structured, rectangular fins, 0.75 H, 0.50 B, 1.00 PMspf-0.75 Macro structured, square pin fins, 0.75 H, 0.50 B, 1.00 PMpf-0.25mi-f Multi-scale structured, pyramidal fins/indentations,

0.25 H, 0.29 B, 0.29 P & fine (Ra � 3 lm)Mpf-0.25mp-c Multi-scale structured, pyramidal fins/protrusions,

0.25 H, 0.29 B, 0.29 P & coarse (Ra � 20 lm)

* H: height, B: base length, P: pitch (all in mm).

H. Bostanci et al. / International Journal of Heat and Mass Transfer 75 (2014) 718–725 721

enhanced surfaces in obtaining quantitative performance compar-ison. Enhanced surfaces involved micro-, macro-, and multi-scalestructures, and were investigated by varying their structure geom-etry and size. At micro level, structure geometries were indenta-tions and protrusions, and structure size was classified with theroughness as fine, medium, and coarse. At macro level, structuregeometries were pyramidal, triangular, rectangular, and squarepin fins, and structure size was varied between 0.25 and0.75 mm. At multi-scale level, best performing micro- and macro-structures were simply combined to investigate the potential offurther heat transfer improvement. Fig. 3 includes some schemat-ics, scanning electron microscope (SEM) images, and solid modelsof micro- and macrostructured surfaces to illustrate the surfacecharacteristics. The microstructured spray surfaces featuringindentations and protrusions were fabricated using a particleblasting and a thermal spray coating process, respectively, asexplained in Ref. [16]. While the smooth surface had a roughnesslevel (Ra) of 0.3 lm, the microstructured surfaces with indenta-tions and protrusions resulted in a Ra of 3–20 lm that correspondsto surface area enhancement of 1.4–2.8� over the projected basearea, as determined with measurements by a confocal laser scan-ning microscope [16]. The macrostructured surfaces featured0.25–0.75 mm high fins in pyramidal, triangular, straight andsquare pin forms that were fabricated on a precise CNC mill. Thefinned surfaces provided an area increase of 1.5–1.6�.

2.3. Test conditions and procedure

All the tests were conducted at 550–570 kPa (65–68 psig) and7–8 �C conditions using saturated ammonia as the working fluid.Cooling curves, in the form of surface superheat (DTsat = Tsurf � Tsat)vs. heat flux (q00), were generated for each test heater by increasingthe heat flux gradually in steps of 50 W/cm2 up to 500 W/cm2. Theresulting heater temperatures were recorded every 3 s over 3 mindurations at each heat flux level. A series of tests were performedin order to find the optimum flow rates that balance high heattransfer rate, and reasonably low coolant usage and pumpingpower, as detailed in Ref. [17]. Based on these tests, the optimumflow rates were determined to be 1.6 ml/cm2-s for liquid, and13.8 ml/cm2-s for vapor (corresponding to a pressure drop of48 kPa (7 psi) across the nozzle).

2.4. Uncertainty analysis

Uncertainties were estimated mainly for heat flux and temper-ature measurements that are essential in performance evaluation.

Error involved in the heat flux measurement (considering varia-tions in voltage, current, and area) was ±1.3% at 500 W/cm2. Errorin the temperature measurements from the embedded thermocou-ples in the heater wall was calibrated to be ±0.2 �C. Spray surfacetemperature had an uncertainty of approximately ±0.5 �C involvinguncertainty in temperature extrapolation from the TC hole to theheater surface at 500 W/cm2. The HTC included ±3.4%, 5.4%, and4.6% uncertainty at 500 W/cm2 for the surfaces s, mp-c, and Mpf-0.25, respectively. Flow rate measurements had ±5% uncertaintydictated by the flow meter characteristics. Heat loss from the thickfilm resistors to surrounding heater body via conduction was esti-mated to be approximately 2.5% based on finite element analysisresults. Heat loss from the thick film resistors to the ambientenvironment was negligibly small (<1 W) based on calculationsconsidering natural convection and black body radiation from100 �C heater surface to 20 �C stagnant air.

3. Results and discussion

3.1. Reference surfaces

Since the performance of the reference surface is critical in com-parisons, five different heaters (surfaces s1–s5) were tested at leasttwice to obtain a database. These heaters provided a consistentperformance and their surface superheats varied approximately±1 �C [17]. The averaged data from ten tests were then plotted asthe reference cooling curve, and called as s-ref in Fig. 4.

Validation of temperature readings with IR thermographyrequired establishing a resistor temperature for the reference sur-faces that was normalized for saturation temperature. Therefore,all available IR temperature readings from the reference surfacetests were processed to determine the normalized resistor temper-ature (Tres

0 = Tres � Tsat) data from the surfaces s1–s5 at 500 W/cm2.These temperatures lay within a ±1.5 �C range resulting in an aver-age value of 136.3 �C [17]. Thus, the data independently confirmthat the five reference surfaces perform very consistently, and sug-gest that the IR thermography can be a valuable tool in verifyingheater performances.

3.2. Microstructured surfaces

The microstructured surfaces involved two types of surfacegeometries, indentations and protrusions. Testing of the surfacemi aimed to evaluate the effect of micro scale surface indentationson the heat transfer enhancement. A range of roughness level,referred as fine, medium and coarse (mi(-f,-m,-c)) was generatedsequentially on the same heater surface. Data in Fig. 4 clearly showthat the surfaces mi(-f,-m,-c) (solid data points) provide a signifi-cant improvement over the surface s-ref. Although the surfacesmi(-f,-m,-c) resulted in nearly the same surface superheat valueof 9 �C at 500 W/cm2, examination of the cooling curves at lowerheat fluxes indicates that roughness level still have an effect onthe performance, and the lower roughness level helps reducingsuperheat. Normalized resistor temperatures based on IR imagesin Fig. 4 inset further illustrates the nearly identical performanceof the surfaces mi(-f,-m,-c) at 500 W/cm2.

The surface mp was the other type of microstructured surfacefeaturing protrusions, and its heat transfer enhancement was alsoevaluated for a range of roughness level, referred as fine, medium,and coarse (mp(-f,m,c)). Data in Fig. 4 shows a substantial enhance-ment for the surfaces mp(-f,-m,-c) (hollow data points) over thesurface s-ref as well, resulting in a surface superheat of 9.5–10.5 �C at 500 W/cm2. There is a clear distinction between thethree surfaces at heat fluxes up to 300 W/cm2, where the surfacemp-c performs best, followed by the surface mp-m and then the

Fig. 3. Schematics and SEM images of microstructured surfaces (a), and solid models of macrostructured surface (b).

722 H. Bostanci et al. / International Journal of Heat and Mass Transfer 75 (2014) 718–725

surface mp-f. At higher heat fluxes, >300 W/cm2, however, allcurves merge together emphasizing a similar cooling performance.Fig. 4 inset also includes normalized resistor temperatures at500 W/cm2 and confirms the performance of the surfacesmp(-f,-m,-c).

Microstructured surfaces in this study are believed to greatlyenhance one of the spray cooling heat transfer mechanisms,namely, boiling through surface nucleation, in addition to otherexisting mechanisms of free surface evaporation and boilingthrough secondary nucleation, in two phase regime. Based on thedata presented in this section, further observations can be made.The surface mi possesses abundant indentations that promotesurface nucleation. Results suggested that in the region up to450 W/cm2, lower roughness provides better performance. As longas the size of indentations are within the range of active cavitysizes for a certain surface superheat, having smaller indentationsin a fixed spray area leads to more potential nucleation sites (orhigher nucleation site density). The model developed by Hsu [18]can be used to predict the range of active cavity sizes, in terms ofthe minimum and maximum cavity mouth radius, rc,min and rc,max,respectively, as

rc;min

rc;max

� �¼ dt

41� ðTsat � TlÞðTsurf � TlÞ

f�g

ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi1� ðTsat � TlÞðTsurf � TlÞ

� �2

� 12:8 � r � Tsat

qv � hfg � dt � ðTsurf � TlÞ

s24

35

where, Tl is the bulk liquid temperature, r is the surface ten-sion, qv is the vapor density, hfg is the latent heat of vaporiza-tion, and dt is the thermal boundary layer thickness that can beestimated as

dt ¼kl

h

where, kl is the liquid thermal conductivity, and h is the heat trans-fer coefficient, calculated as h = q00/DTsat.

Fig. 7 presents the range of active cavity sizes for the case of sat-urated ammonia at 7 �C. Here, dt was estimated using an h valuebased on the cooling curve s-ref in Fig. 4–6 at the inception oftwo phase regime. Although the geometry of real cavities is highlyirregular, it is reasonable to idealize the micro scale indentations inthis study as conical cavities. The mouth radius of these conicalcavities then can be assumed to be nearly half of the Ra value. Aslisted in Table 1, Ra values of the surfaces mi-f, mi-m and mi-c are2.1, 3.2 and 4.6 lm, respectively. At 7 �C surface superheat corre-sponding to the early part of two phase regime, for instance, thepredicted range of active cavity sizes is 0.35–1.40 lm in Fig. 7.Therefore, even the smallest cavity mouth radius found in the sur-face mi-f falls within the predicted range, and supports the conclu-sion that among the surfaces mi(-f,-m,-c), the surface mi-f wouldhave more potential nucleation sites in a fixed area and performbetter than the surfaces mi-m and mi-c.

Fig. 4. Heat transfer performance, and IR thermography based normalized resistor temperatures for the microstructured surfaces mi(-f,-m,-c) and mp(-f,-m,c).

H. Bostanci et al. / International Journal of Heat and Mass Transfer 75 (2014) 718–725 723

The surfaces mp(-f,-m,-c) resulted in an opposite trend wherehigher roughness provides better performance. As surface charac-terization efforts revealed, the surface mp in general features largesurface area, and many randomly sized re-entrant cavities that canentrap vapor and facilitate nucleation at low surface superheats.Hence, the surface mp-c with its larger surface area is expectedto have more of the re-entrant cavities and enter the two phaseregime earlier than the surfaces mp-m and mp-f.

3.3. Macrostructured surfaces

Macrostructured surfaces featured mm-scale fins positionednormal to the spray nozzle. Initially, a finite element analysis(FEA) was conducted to roughly evaluate the effect of macrostruc-ture size and geometry on spray cooling performance, and deter-mine the proper scope for actual test surfaces used in theexperiments. For this analysis, pyramidal, triangular, rectangular,and square pin fins were selected as structure geometries, andthe heater material was already determined as Al 6061. The heaterbottom surface received a heat flux of 500 W/cm2. The wettedspray surface including base and all fin surfaces were exposed toa temperature dependent heat transfer coefficient calculated fromthe reference cooling curve for the surface s-ref in Fig. 4–6. First, aunit building block of rectangular fin with 1 � 1 mm base area wasused to check the effect of fin height, where fin heights above0.75 mm showed minimal contribution. Next, four structure geom-etries at a fixed 0.75 mm fin height were evaluated utilizing quar-ter models of the test heaters with 5 � 5 mm base area. The resultsshowed up to 5% performance enhancements corresponding to anarea increase of �1.5–1.6� due to fins. The FEA however was notexpected to be capable of capturing the complex spray coolingmechanisms, and the results (as detailed in Ref. [17]) were consid-ered as an initial step in the design process.

After the FEA study, a set of experiments was done to evaluatethe actual performance of the macrostructured surfaces. First, theheaters featuring four types of fin geometries with a fixed finheight of 0.75 mm were tested for the effect of structure geometry.As the cooling curves in Fig. 5 reflect, these surfaces experienced anextended single phase regime, and entered the two phase regimequite late at higher surface superheats. Therefore, none of thetested geometries were able to provide an enhancement over thesurface s-ref. While the results from the surface Mpf-0.75 are com-parable to the surface s-ref at 500 W/cm2, the surfaces Mtf-0.75,Mrf-0.75, and Mspf-0.75 yielded higher surface superheats.

Comparison of normalized resistor temperatures of four types ofthe macrostructured surfaces generated consistent data, andmarked the pyramidal fins as the best structure geometry. In thenext step of the evaluation of macrostructured surfaces, two moreheaters with shorter, 0.50 and 0.25 mm high pyramidal fins weretested. Data in Fig. 5 clearly illustrate that shorter pyramids helptransition to two phase regime earlier, and provide lower super-heats throughout. At 500 W/cm2, the surfaces Mpf-0.50 andMpf-0.25 lower the superheat by 2.6 and 5.3 �C, respectively, com-pared to the initially tested surface Mpf-0.75. However, the surfacesuperheats at low heat fluxes are still higher than that of thesurface s-ref. IR thermography based normalized resistor tempera-tures decrease as structure height decrease, and thus validates theperformance of the surfaces Mpf(�0.75, �0.50, �0.25).

Based on these data, we can conclude that even though themacrostructured surfaces provide surface area enhancement, tem-perature gradient along the fins causes higher surface superheats,with the temperature at the tip of the fin being near saturationtemperature. Additionally, the fins on the macrostructured sur-faces would increase resistance to the liquid flow over the heatersurface. Thus, compared to the reference surface, macrostructuredsurfaces could diminish the single phase heat transfer performanceand delay transition to two phase regime. The initial evaluation ofmacrostructured surfaces for the effect of geometry indicated thatthe surface Mpf-0.75 was slightly better than the others. Mostlikely the pyramidal fins offer a favorable temperature distributionamong the considered structures. When the pyramids were inves-tigated further by varying the structure size, but specifically keep-ing their surface area enhancement constant, the shortestpyramids performed the best. This can be attributed to someadvantages shorter structures have, such as lower added thermalresistance (over the surface s-ref), and more of direct liquid accessto the substrate due to the longer boundary around the structurebase.

3.4. Multi-scale structured surfaces

Third group of the enhanced surfaces, multi-scale structuredsurfaces, incorporated the combination of best performing macroand micro scale structures. Based on the results, the surfaces Mpf-0.25, mi-f, and mp-c were considered to form the two new sur-faces of Mpf-0.25mi-f and Mpf-0.25mp-c for the evaluation. Datafrom these surfaces in Fig. 6 indicates that for the surface Mpf-0.25mi-f, two phase effects starts to dominate after 100 W/cm2

Fig. 5. Heat transfer performance, and IR thermography based normalized resistor temperatures for the macrostructured surfaces Mtf, Mrf, Mspf and Mpf(�0.75, �0.50, �0.25).

Fig. 6. Heat transfer performance, and IR thermography based normalized resistor temperatures for the multi-scale structured surfaces Mpf-0.25mi-f and Mpf-0.25mp-c.

0.0

0.4

0.8

1.2

1.6

2.0

0 5 10 15 20

ΔTsat (oC)

Cav

ity M

outh

Rad

ius,

r c ( μ

m)

Saturated NH3

Tsat=7 oC, δt≈3.5 μm

Range of Active Cavity Sizes

Fig. 7. Prediction of the range of active cavity sizes using Hsu’s analysis.

724 H. Bostanci et al. / International Journal of Heat and Mass Transfer 75 (2014) 718–725

and the performance is characterized by a very steep curveresulting in a wide range of heat flux removal at nearly constantsurface temperature. For the surface Mpf-0.25mp-c, boiling

through surface nucleation appears to pick up even earlier, after50 W/cm2, and results in lower superheat at heat fluxes of upto 450 W/cm2. However, at 500 W/cm2, performance of the twomulti-scale structured surfaces match, achieving the lowest sur-face superheat of 6.5 �C in this study. IR thermography based nor-malized resistor temperatures confirm the results, indicatingthese two heaters have the equivalent, and the lowest resistortemperatures obtained so far.

Superior performance of the surface Mpf-0.25mp-c over the sur-face Mpf-0.25mi-f at low to medium heat fluxes is believed to bedue to its mp component with complex structural form that inher-ently offers more surface area and reentrant cavities, compared tothe mi. Such characteristics enable the surface Mpf-0.25mp-c toenter the two phase regime earlier resulting in higher HTC. In gen-eral, performance enhancement level of multi-scale structured sur-faces exceeded those from both micro- and macrostructuredsurfaces individually. Actually, results quantitatively indicate thatthe enhancement level in terms of heat transfer coefficient is addi-tive. Therefore it can be concluded that the previously outlinedenhancement mechanisms for micro- and macrostructured sur-faces work simultaneously, allowing the multi-scale structuredsurfaces to take advantage of all.

Fig. 8. Performance comparison of micro-, macro-, and multi-scale structured surfaces based on HTC at heat fluxes of up to 500 W/cm2.

H. Bostanci et al. / International Journal of Heat and Mass Transfer 75 (2014) 718–725 725

Finally, Fig. 8 provides a summarized performance comparisonof all three groups of the enhanced surfaces in terms of HTC (orh). The surfaces Mpf-0.25mi-f and Mpf-0.25mp-c provide HTC of772,000 and 741,000 W/m2 �C at 500 W/cm2, respectively, corre-sponding to 161% and 150% improvement over the surface s-ref.On the other hand, the level of enhancement for the micro-, andmacrostructured surfaces, surfaces mi-f, mp-c, and Mpf-0.25, corre-spond to 81%, 70% and 44% improvement, respectively, over thesurface s-ref at 500 W/cm2.

4. Conclusions

The present study focused on high heat flux spray cooling withammonia. Various heater surfaces featuring; (a) micro scaleindentations and protrusions, (b) macro (mm) scale pyramidal, tri-angular, rectangular, and square pin fins, and (c) multi-scale struc-tures that combine macro and micro scale structures were testedto investigate the effect of surface enhancement on spray coolingperformance at heat fluxes of up to 500 W/cm2. Based on theresults, the following conclusions can be reached:

� The microstructured and macrostructured surfaces offered upto 81% and 44% increase in HTC, respectively, over the referencesurface, s-ref. Multi-scale structured surfaces achieved up to161% enhancement in HTC (corresponding to 772,000 W/m2 �Cat 500 W/cm2) over the surface s-ref, and revealed additiveenhancement mechanisms.� This performance enhancement can be attributed to the

increase in surface area and stronger contribution of one ofthe spray cooling phase-change mechanisms, namely, boilingthrough surface nucleation, in addition to the free surface evap-oration and secondary nucleation mechanisms. Both types ofthe microstructured surfaces, mi and mp, provide a spectrumof cavity sizes and thus have the potential to generate addi-tional surface nucleation sites.� Overall, the present study, through extensive experimental

data, emphasized the importance of boiling through surfacenucleation as a heat transfer mechanism that can greatlyenhance spray cooling performance.

Conflict of interest

None declared.

Acknowledgments

We acknowledge Air Force Research Laboratory (AFRL) Propul-sion Directorate and Universal Technology Corporation for theirfinancial support.

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