Experimental investigation of spray cooling on micro-, nano- and hybrid-structured surfaces

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<ul><li><p>International Journal of Heat and Mass Transfer 80 (2015) 2637Contents lists available at ScienceDirect</p><p>International Journal of Heat and Mass Transfer</p><p>journal homepage: www.elsevier .com/locate / i jhmtExperimental investigation of spray cooling on micro-, nano-and hybrid-structured surfaceshttp://dx.doi.org/10.1016/j.ijheatmasstransfer.2014.08.0850017-9310/ 2014 Elsevier Ltd. All rights reserved.</p><p> Corresponding author. Tel.: +86 10 62772661; fax: +86 10 62775566.E-mail address: jiangpx@tsinghua.edu.cn (P.-X. Jiang).Zhen Zhang a,b, Pei-Xue Jiang a,, David M. Christopher a, Xin-Gang Liang caKey Laboratory for Thermal Science and Power Engineering of the Ministry of Education, Department of Thermal Engineering, Tsinghua University, Beijing 100084, PR Chinab Institute of Nuclear and New Energy Technology, Tsinghua University, Beijing 100084, PR ChinacKey Laboratory for Thermal Science and Power Engineering of the Ministry of Education, School of Aerospace Engineering, Tsinghua University, Beijing 100084, PR China</p><p>a r t i c l e i n f o a b s t r a c tArticle history:Received 28 May 2014Received in revised form 27 August 2014Accepted 30 August 2014</p><p>Keywords:Spray coolingMicro-Nano-Hybrid-structured surfaceLocal thermal non-equilibrium modelThe heat transfer during spray cooling was studied experimentally using deionized water to investigatethe spray characteristics and the differences between spray cooling on a smooth silicon surface andmicro-, nano- and hybrid micro/nano-structured surfaces. The spray cooling experiments show thatthe heat transfer rates were better for the nano-structured surface, followed by the smooth surfacecoated with the SiO2 film and the pure silicon surface since the contact angle was smallest on thenano-structured surface and increased on the latter two. The droplet parameter results show that mostdroplets were 4060 lm in size. The heat transfer coefficient increased and the wall temperaturedecreased on the 25G 25S surface coated with the SiO2 film compared with the 50G 50S surfacecoated with the SiO2 film as the heat transfer moved into the partial dryout region due to the SiO2 filmsstronger hydrophilicity so the heated area was more fully utilized, while the CHF was larger for the50G 50S surface. Coating the micro-structured surfaces with carbon nano-tube (CNT) films havingcharacteristic sizes smaller than the droplet size was more effective than on the surfaces with larger char-acteristic sizes. The CHF was largest on the 25G 25S surface coated with 4 carbon nano-tube films witha 75.3% increase over the smooth surface. The wall temperature increase and the temperature fluctua-tions were small in the boiling regime as the power increases for the enhanced surfaces.</p><p> 2014 Elsevier Ltd. All rights reserved.1. Introduction</p><p>Numerous industrial applications, such as electronic systems,high-energy lasers, energy weapons and aerospace satellites, havesubstantial need for more effective thermal management. Spraycooling, with its high heat dissipating capability, has been playingan important role in high heat flux applications as one of the mosteffective thermal management methods. Heat fluxes in excess of1000 W/cm2 have been reported using water spray cooling atlow coolant flow rates [1].</p><p>Spray cooling heat transfer is influenced by many factors suchas the droplet parameters [2], working fluid [3], nozzle-to-surfacedistance and inclination angle [4,5], so it has been widely investi-gated by researchers in the past two decades. Surface morphologyis another critical effect for spray cooling heat transfer enhance-ment. Enhanced surfaces, such as milli-structured surfaces [6],micro-structured surfaces [7,8] and microcavity surfaces [9,10]have been shown to effectively improve the heat transfer.Spray cooling heat transfer on smooth and micro-structuredsurfaces (characteristic sizes of 25200 lm) has been studied indetail in previous research in this group [11] with the resultsshowing that micro-structured surfaces effectively increased thespray cooling heat transfer rates in the thin film and partial dryoutregions. The effects of the groove width and stud size on the heattransfer were correlated with the droplet parameters. The micro-structured surface with larger characteristic sizes had a smallerarea enhancement factor and worse heat transfer rates, while onthe micro-structured surface with much smaller characteristicsizes, most droplets were not able to completely enter the bottomof the micro grooves since the groove size was smaller than most ofthe droplets and the heat transfer surface could not be fully wetted.However, the spray cooling heat transfer on nano- or hybrid micro/nano-structured surfaces is not clearly understood and the litera-ture has few studies on their effects to the authors knowledge.</p><p>Only Alvarado [12] has investigated spray cooling on nano-structured surfaces and observed lower minimum wall tempera-tures for similar heat fluxes, better heat transfer curves, and lowertemperature gradients on the nano-structured surfaces than on thebare surface. Nano-structured surfaces have also been used for</p><p>http://crossmark.crossref.org/dialog/?doi=10.1016/j.ijheatmasstransfer.2014.08.085&amp;domain=pdfhttp://dx.doi.org/10.1016/j.ijheatmasstransfer.2014.08.085mailto:jiangpx@tsinghua.edu.cnhttp://dx.doi.org/10.1016/j.ijheatmasstransfer.2014.08.085http://www.sciencedirect.com/science/journal/00179310http://www.elsevier.com/locate/ijhmt</p></li><li><p>Nomenclature</p><p>G groove width, lmS stud size, lmD groove depth, lmTW silicon top surface temperature, CTPt platinum temperature, CTf water temperature, CT average temperature in an x cross section of the silicon,</p><p>Cq heat flux, W/cm2</p><p>h heat transfer coefficient, W/(m2 C)H silicon thickness, lmHc critical thickness, lmL length, lme volume fraction, ei = Li/(Lh + Luh)k thermal conductivity, W/(m C)R100 standard resistance, XRx platinum resistance, XU100 standard resistance voltage, VUx platinum voltage, VQx electrical heat source in the platinum heater</p><p>in each cell, W</p><p>Qelec total electrical heat source on the silicon, WQ heat dissipated by the spray, WD10 average diameter, lmD32 Sauter mean diameter, lmv droplet velocity, m/sAS stud top surface area directly impinged by the water</p><p>spray, mm2</p><p>AB base area, 7.4 mm 7.4 mmAT total area of the micro-structured surface, mm2</p><p>AS/AB directly impinged surface percentageAT/AB area enhancement factor compared to the smooth</p><p>surface</p><p>Superscriptsh heateduh unheatedc critical0 silicon bottom conditionssi silicons loss</p><p>Z. Zhang et al. / International Journal of Heat and Mass Transfer 80 (2015) 2637 27pool boiling with the results helping understanding of the spraycooling heat transfer on such surfaces. Young Lee et al. [13] inves-tigated the nucleate pool boiling heat transfer and long-term per-formance of a nano-porous surface fabricated by anodizing withthe results showing that the nucleate boiling heat transfer coeffi-cient of the nano-porous coating surface was higher than that ofthe non-coated surface particularly at low heat fluxes with higherheat transfer coefficients remaining throughout 500 h of operation.Im et al. [14] fabricated copper nanowire arrays on a silicon sub-strate by electro-chemical deposition and observed that the coppernanowires increased the pool boiling Critical Heat Flux (CHF) andreduced the wall superheat compared to a smooth silicon surface.An optimum CHF was found at a nanowire height of 2 lm. Kwarket al. [15] found that a nanoparticle coated heater consistently pro-duced dramatic CHF enhancement relative to an uncoated surfacefor all tested conditions. The authors postulated that the betterwettability in the nanocoating was the main cause of the enhance-ment. Thus, all the experiments show that the nano-structures onthe surface improve the heat transfer rates.</p><p>In addition, hybrid micro/nano-structured surfaces have beenused in pool boiling to further enhance the heat transfer in recentyears. Various micro-, nano- and hybrid-structures were fabricatedon copper surfaces with the corresponding pooling boiling heattransfer performance studied by Li et al. [16]. The authors claimedthat the CHF of the hybrid-structured surfaces was about 15%higher than that of the surfaces with nanowires only andmicro-pillars only, and that the superheat at CHF for the hybrid-structured surface was about 35% smaller than that of the micro-pillared surface. Launay et al. [17] studied pool boiling on smooth,nano-, micro- and hybrid-structured surfaces using PF5060 andwater. The highest heat fluxes were obtained using the 3D micro-structures without CNTs and the experimental results indicate thatthe heat transfer rates were higher on the purely nano-structuredinterfaces only at very low superheats compared to the smoothsurfaces, but still lower than those on the conventional Si-etchedmicrostructures for all cases.</p><p>The primary objective of the current study is to investigatespray cooling heat transfer on smooth and nano-, micro- andhybrid-structured surfaces with accurate measurements of thespray droplet parameters using the shadowgraph technique toexplain the heat transfer mechanism.2. Experimental system and parameter measurements</p><p>2.1. Experimental system</p><p>The spray cooling system shown in Fig. 1 included spray, heat-ing and measurement sections. Deionized water driven by a Fluid-o-Tech magnetic drive gear pump flowed from the constant tem-perature water bath through the filter to remove impurities beforebeing sprayed on the heated surface through a full cone pressureatomizer (Spraying Systems Co.) with a nozzle orifice of0.51 mm. The nozzle was fixed in a bracket with the orifice-to-sur-face distance adjusted by an accurate micrometer with a position-ing accuracy of 0.01 mm. A mechanical pressure gauge was usedto measure the nozzle inlet pressure which was assumed to beequal to the spray pressure with one OMEGA 0.125 mm diameterT-type thermocouple imbedded in the flow tube just before thenozzle to measure the deionized water temperature. The experi-ments used a water spray pressure of 0.3 MPa with an orifice-to-surface distance of 30 mm with subcoolings of 82 to 80 C.A flow rate measurement container with a square hole the samesize as the heated surface was made to measure the water flowrate impinging the target surface as in Ref. [11]. The flow ratewas 1.239 kg/m2 s in all cases.</p><p>The heating sections were made of 7.4 mm 7.4 mm, double-side polished, 490 lm thick silicon dies. One CNT film or fourCNT films laid in the cross direction with tube diameters of 8090 nm were laid on the top surface. Then, Plasma Enhanced Chem-ical Vapor Deposition (PECVD) was used to deposit a SiO2 film hav-ing a thickness 50 nm on the CNT films to prevent the nanotubesfrom being washed away by the spray. These surfaces are referredto as nano-structured surfaces. A smooth surface having just a50 nm thick SiO2 film, called the SiO2 film surface, was used tostudy the wettability effect in comparison with the smooth surfaceand to study the nano-scale structures effect compared with thenano-structured surfaces.</p><p>A three-dimensional white light surface interference profilome-ter was used to measure the roughnesses of the smooth surfaceswith a resolution of 752 489 pixels, RMS repeatability precisionof 1 nm and calibration accuracy 0.1%. Sa was defined as theaverage difference in the profile height from the average heightwithin the sample length, which is widely used to characterize</p></li><li><p>Fig. 1. Spray cooling system.</p><p>(a)</p><p>(b) Smooth +1CNT (c) Smooth +4CNT </p><p>Fig. 2. Roughness distribution on the smooth surface (a) and surface appearances observed by an atomic force microscope on the nano-structured surfaces (b) and (c).</p><p>28 Z. Zhang et al. / International Journal of Heat and Mass Transfer 80 (2015) 2637the surface roughness. The roughness distribution for the smoothsurface is shown in Fig. 2(a) with Sa = 0.451 nm. The nano-struc-tured surface appearances were observed by an atomic forcemicroscope as shown in Fig. 2(b) and (c).</p><p>Various size micro-studs were fabricated on the top surface ofthe smooth silicon dies by deep reactive ion etching (DRIE). FourCNT films were then laid in the cross direction on the top surfacewith the CNT films between the micro-studs ablated by a laser toensure that the spray could flow into the grooves. Thus, the CNTfilms were only deposited on the top surface of the micro-studs.A PECVD SiO2 film was also deposited on the micro-structured sur-faces with the micro-structured silicon dies just having a SiO2 filmfor reference. Ten test surfaces were investigated with their sizeparameters listed in Table 1.</p><p>Four platinum resistors were designed on the bottom surface ofeach silicon die to reduce the voltage input for safety consider-ations with each platinum resistance being 300400 X. Chromium,platinum and titanium (thickness proportions of approximately1:10:1) were applied to the bottom surface of the silicon dies usingpositive photoresist lift-off in a serpentine pattern with a total</p></li><li><p>Table 1Test surface parameters.</p><p>Surface Abbreviation in the analysis</p><p>1 Smooth silicon Smooth2 Smooth silicon + a SiO2 film Smooth + SiO23 Smooth silicon + one CNT film + a SiO2 film Smooth + 1CNT4 Smooth silicon + four criss-crossed CNT films + a SiO2 film Smooth + 4CNT5 25G 25S 100Da + a SiO2 film 25G 25S + SiO26 25G 25S 100D + four criss-crossed CNT films + a SiO2 film 25G 25S + 4CNT7 50G 50S 100D + a SiO2 film 50G 50S + SiO28 50G 50S 100D + four criss-crossed CNT films + a SiO2 film 50G 50S + 4CNT9 100G 100S 100D + a SiO2 film 100G 100S + SiO210 100G 100S 100D + four criss-crossed CNT films + a SiO2 film 100G 100S + 4CNT</p><p>a G = groove width, lm, S = stud size, lm, D = groove depth, lm. 25G 25S 100Dmeans that the groove width between the studs is 25 lm, the stud size is 25 lm and thegroove depth is 100 lm.</p><p>HLh Luh</p><p>q0</p><p>hT</p><p>0hT 0uhT</p><p>x</p><p>y</p><p>Unit cell</p><p>0</p><p>wT</p><p>Hc</p><p>qh0</p><p>uhT</p><p>quh0</p><p>Fig. 3. Silicon temperature profile in a unit cell.</p><p>Z. Zhang et al. / International Journal of Heat and Mass Transfer 80 (2015) 2637 29thickness of 241 nm. A PECVD SiO2 film having a thickness 150 nmwas also added on the metals for protecting. The SiO2 film and tita-nium on just the eight pads were removed by wet etching toexpose the platinum before the silicon die was mounted on a tem-perature resistant PCB circuit board. Eight 40 lm gold wires con-nected the four platinum resistors with the circuit board by wirebonding. A DC stabilized voltage source was then used to supplypower to the platinum through eight wires soldered on the circuitboard.</p><p>A synthetic glass sleeve filled with calcium silicate cellucotton(thermal conductivity = 0.05 W/mK) was glued to the bottom ofthe circuit board to reduce the heat losses. The system was consid-ered to be at steady state after the voltage and the temperatureremained constant for at least 15 min with the results calculatedusing the last 5 min.2.2. Temperature and heat flux measurements</p><p>The platinum was used both as the heat source and a resis...</p></li></ul>

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