The effects of micro-structured surfaces on multi-nozzle spray cooling

Download The effects of micro-structured surfaces on multi-nozzle spray cooling

Post on 30-Dec-2016

215 views

Category:

Documents

2 download

Embed Size (px)

TRANSCRIPT

<ul><li><p>on</p><p>. Bo</p><p>were s The heat transfer performance of cubic pin n A dimensionless number was proposed to sca Temperature uniformity was discussed.</p><p>dimensionless number (DM) is created to characterize heat transfer performance of different micro-structures in single phase heat transfer. We veried the dimensionless number using experimental re-</p><p>tions in the development of modern technologies, such as thecooling of electronic devices and high power solid-state lasers. Ithas been reported that spray cooling had been applied in thecooling of Cray X1 vector supercomputers [3].</p><p>order to obtain higher heat ux and more uniform temperature,studied. The heats signicantly de-5,6] carried out ane nozzles at sprayhat the CHF levelsh pure FC-72 and</p><p>490 W/cm2 with pure methanol respectively. Y. B. Tan et al. [7]developed a correlation based on their multi-nozzle spray coolingexperimental results to predict the dimensionless heat ux. Jia andQiu [8] made an experiment to investigate the advantage of sur-factant addition in spray cooling with ve nozzle arrays and indi-cated that the surfactant addition could result in a relativelyconstant heat removal rate near the CHF regime. Panao et al. [9]presented a thermal assessment of a multi-jet strategy for spraycooling system. The assessment considered a diverse number of* Corresponding author. Tel./fax: 86 10 8254 3108.</p><p>Contents lists availab</p><p>Applied Therma</p><p>sev</p><p>Applied Thermal Engineering 62 (2014) 613e621E-mail address: hxl@mail.etp.ac.cn (X. Huai).Among alternative solutions for cooling high-powered devices,spray cooling has the advantages of removing high heat ux andproviding uniform temperature distribution in a conned space.Traditionally, spray cooling was utilized to cool highly heated sur-faces for equipments and processes in metallurgy, chemical andnuclear industry [1,2]. Recently, it has received increasing atten-</p><p>multi-nozzle spray cooling has been widelytransfer performance of multiple nozzle spraypends on geometry and spacing [3,4]. Lin et al. [experimental investigation with eight miniaturpressure drops greater than 1.72 bar. It showed twith eight-nozzle sprays were 90 W/cm2 wit1. Introduction However, the maximum CHF was limited to 1000 W/cm2. Insults in this study and previous literature. Furthermore, the micro-structured surfaces have negligibleeffects on temperature distribution except for cubic pin ns.</p><p> 2013 Elsevier Ltd. All rights reserved.a r t i c l e i n f o</p><p>Article history:Received 13 August 2013Accepted 16 October 2013Available online 25 October 2013</p><p>Keywords:Micro-structured surfaceMulti-nozzle spray coolingHeat transferTemperature uniformity1359-4311/$ e see front matter 2013 Elsevier Ltd.http://dx.doi.org/10.1016/j.applthermaleng.2013.10.03s was the best one in zone II.le heat transfer enhancement.</p><p>a b s t r a c t</p><p>Experiments were conducted to investigate heat transfer characteristics of spray cooling with eightnozzles for micro-structured surfaces included cubic pin ns and straight pin ns of different sizes.Liquid volume ow rate ranged from 2.46 102 m3/s/m2 to 3.91 102 m3/s/m2 and the correspondedinlet pressures changed from 0.28 MPa to 0.6 MPa by keeping the inlet water temperature between20.4 C and 24.31 C. And the input power of heat block varied from 180 W to 1080 W. The results showthat the heat transfer performances of straight ns2 and straight ns3 are the best in single phase zone,but the cubic pin ns is better in two phase zone. Notably, the critical point between single phase zoneand two phase zone shifts to left with the increasing of liquid volume ow rate. Moreover, with the liquidvolume ow rate increasing, the heat transfer coefcient increases as well, but straight ns1 and pol-ished surface are not sensitive to this change. For a deeper analysis of the heat transfer enhancement, a The optimal micro-structured surfaces traight ns2 and 3 in zone I.h i g h l i g h t sThe effects of micro-structured surfaces</p><p>Yan Hou a,b, Yujia Tao a, Xiulan Huai a,*a Institute of Engineering Thermophysics, University of Chinese Academy of Sciences, P.ObUniversity of Chinese Academy of Sciences, Beijing 100049, China</p><p>journal homepage: www.elAll rights reserved.0multi-nozzle spray cooling</p><p>x 2706, Beijing 100190, China</p><p>le at ScienceDirect</p><p>l Engineering</p><p>ier .com/locate/apthermeng</p></li><li><p>l Ennozzles. Their results supported that themulti-nozzle spray coolingwas a good choice for smaller processors with more adequatethermal management system. However multi-nozzle spray coolingcannot meet the increasing cooling requirements, so more effectivemethods are required.</p><p>Recently researchers have been paying more attentions to theenhanced surfaces that show great potential to further improve theperformances of spray cooling heat transfer. Various studies havebeen reported. Sehmbey et al. [10] discovered that increasing thesurface roughness had a positive effect on heat transfer through aspray cooling experiment with liquid nitrogen. Pais et al. [11]studied the surface roughness and its effect on the heat transfer</p><p>Nomenclature</p><p>A area of micro-structured surface, m2</p><p>A0 area of polished surface, m2</p><p>a the height of each n, mmBo Bond numberb the groove width, mmc the n width, mmd normal distance between the two rows of</p><p>thermocouples, mDM a dimensionless numberg gravitational constant, m/s2</p><p>h total heat transfer coefcient of heated surface, W/m2/K</p><p>k conductivity of the heat block, W/m/Kl normal distance between the heated surface and the</p><p>upper row of thermocouples, mNu the averaged Nusselt number in natural convectionPr the Prandtl numberq0 averaged heat ux of the heated surface, W/m2</p><p>Q 0loss the heat transferred to surrounding air from the part ofheat block between two rows of thermocouples, W</p><p>Qv inlet volume ow rate, m3/s</p><p>Y. Hou et al. / Applied Therma614mechanism in spray cooling using an air atomizing nozzle. Theyfound that the nucleate boiling played a major role in the heattransfer when the surface roughness was greater than 1 mm.However, for lms of the order of 0.1 mm, heat was conductedthrough the lm and evaporated on the surface, yielding very highheat uxes of the order of 1200 W/cm2 at very low superheat. Kimet al. [12] built the microporous structures on the heated surfacesand studied the effect of particle size on the heat transfer co-efcients experimentally using the air-atomized nozzle. The resultsshowed that the heat transfer coefcient increased by up to 400%relative to that of uncoated surface cooled by dry air, and thisenhancement was maintained at high heat uxes. They attributedthis enhancement to the increased capillary forces betweenmicrostructures.</p><p>Bostanci et al. [13] conducted the experiments to investigatespray cooling onmicro-structured surface with ammonia using twovapor atomized spray nozzles, and a smooth surface was also testedfor comparison. Results suggested that the heat transfer coefcientsincreased by 112% and 49% for treated surfaces with protrusionsand indentations respectively, in comparison with smooth surface,when the heat ux over heated surface was 500 W/m2. Stephanet al. [14] studied the spray cooling heat transfer performance onmicro-structured surfaces consisted of micro pyramids withdifferent heights. They found that a signicant enhancement in theheat transfer performance due to the surface structures could beobserved, especially at low coolant uxes. The authors attributedthis to the increase of the three phase contact line, which leads tomore effective thin lm evaporation. Bostanci et al. [15] had studiedspray cooling with ammonia on structured surfaces to determinethe CHF limits. The results showed that the maximum heat ux ofmulti-scale structured surface increased by 18% over smooth sur-face, up to 910 W/cm2 at nominal ow rate. And the multi-scalestructured surface with pyramidal ns and protrusions achievedthe highest CHF value of 1090 W/cm2, so did the surface withprotrusions. de Souza et al. [16] conducted an experiment to studythe spray cooling on copper-foam enhanced surface with R134a.The enhancement factor of copper-foam surface is as high as 1.39.In sum, treated surfaces can enhance heat transfer signicantly. It isessential to understand the heat transfer mechanism to obtain the</p><p>RaL the Rayleigh numberTlower averaged temperature of the lower row of</p><p>thermocouples, CTupper averaged temperature of the upper row of</p><p>thermocouples, CTw the averaged temperature of the heated surface, CTl temperature of inlet water, CTc averaged vapor temperature in spray chamber, C</p><p>Greek symbolsg surface tension of water, N/mdk error in conductivity, W/m/Kdq0 the uncertainty of the averaged heat ux, W/m2</p><p>dT error in temperature measurement, CdTw the uncertainty of the averaged temperature,</p><p>Cdx error in thermocouple location, mT heat loss percentage between two rows of</p><p>thermocouplesr density, kg/m3</p><p>SubscriptsV vapor phasel water phase</p><p>gineering 62 (2014) 613e621optimized micro-structure, however, few studies were found.Chien et al. [17] investigated multi-nozzle jets cooling with FC-</p><p>72 on cubic pin ns and straight pin ns. They indicated that theheat transfer performance increased with the increasing of liquidvolume ow rate or surface area enhancement ratio. Their datashows that the heat transfer performance of two-phase jets isdependent on Re, Bo and surface enhancement ratio. Hsieh et al.[18] investigated evaporative heat transfer characteristics of a wa-ter droplet spray on the plain and square micro-studs silicon sur-faces at very low spray mass uxes up to 4.41 mL/cm2. Theyindicated that the Bond number of the microstructures was animportant factor to explain the heat transfer enhancement ofevaporative spray cooling on micro-structured silicon surfaces.Moita et al. [19] studied the impact of droplets onto micro-structured surfaces and scaled the effects of surface topographyon secondary atomization. They indicated that wetting propertieswere responsible for different characteristics of the thermal-induced atomization. The results also show good correlation be-tween the mean sizes of the secondary droplets generated bythermal-induce atomization and the ratio of the mean height of thepeaks and the pitch between them.</p><p>As above mentioned, the surface technologies improve heattransfer performance greatly. However the enhancement mecha-nisms of treated surface are not clear yet. Besides, previous studieson micro-structured surface enhancing multiple nozzle spraycooling and inuencing temperature uniformity are limited. Themain objective of the current work is to investigate the effects of</p></li><li><p>independent wires and plug, so it is convenient to set series or</p><p>rime</p><p>l Enmicro-structured surface on heat transfer performance and tem-perature uniformity. A dimensionless number, DM, was proposedto scale the heat transfer enhancement on cubic-nned surfacesand straight-nned surfaces. We veried the dimensionless num-ber using experimental results in this study and previous literature.Surface modication techniques were used to obtain microscalecubic pin ns and the straight ns of different sizes on the heatersurfaces. A smooth surface was also tested to have baseline data forcomparison. Tests were conducted in an open loop system withwater as the working uid, using pressure atomized spray nozzles.The heat uxes and the heat transfer coefcients under differentliquid volume ow rates with micro-structured and smooth sur-faces were obtained.</p><p>2. Experimental system</p><p>2.1. Test facility</p><p>The experimental setup consists of four parts: liquid delivery,spray, heater assembly and data acquisition system. The open-loopuid delivery system is comprised of two liquid reservoirs, a rotaryvane pump, a lter and the ow channels with control valves as</p><p>Fig. 1. Expe</p><p>Y. Hou et al. / Applied Thermashown in Fig. 1. The de-ionized water pumped from the reservoirpasses through the lter, the three-way valve, and then reaches thespray chamber. In the spray chamber, water is split into dropletswhen it passes through a multi-nozzle assembly, then the dropletsimpact onto the heated surface. A heat transfer process iscompleted when the liquid outows from the spray chamber andreturns to the other reservoir. In this open-loop experimental sys-tem, the liquid volume ow rate is regulated by a three-way valve,and the heat power supplied to the surface is regulated by the ACvoltage, respectively.</p><p>2.2. Spray system</p><p>The structure of the multi-nozzle assembly is shown in Fig. 2.There are eight miniature nozzles in the multi-nozzle plate. Eachnozzle has a swirl insert, a swirl chamber and a discharge orice.The swirl insert is mounted onto the multi-nozzle plate. The waterows into the swirl chamber through three orices with thediameter of 2 104 m, generates a swirl ow pattern, and thenows out from the discharge orice with diameter of 3 104 mand breaks up into ne droplets. Besides, the distance between thetwo discharge orices is 8 103 m.parallel electric circuit. The upper surface of the heater with an areaof 3.2 102 m 1.6 102 m is used as the test surface. Sixteen K-type thermocouples with probe diameter of 0.5 mm are embeddedinto the holes drilled along the two planes in the block heater asshown in Fig. 3. The distance between the two thermocouplelocation planes is 5 mm. The distance between the test surface andthe upper plane of the thermocouple locations is 31 mm. Theheated surface is sealed in the bottom plate of the spray chamber.The heater block is placed in a stainless steel shell with berfrax tominimize the heat transfer to the ambience.</p><p>2.4. Enhanced surfaces</p><p>Four enhanced surface geometries are shown in Fig. 4. Themicro-structure in Fig. 4(a) is consisted of 512 cubic pin ns, and themicro-structures in Fig. 4(bed) are consisted of straight ns of2.3. Heater assembly</p><p>A copper block heater consisted of six rod-type heaters isemployed as the heat source in the experiment as shown in Fig. 3.The maximum power of each rod-type heater is 250 W. Each has</p><p>ntal setup.gineering 62 (2014) 613e621 615different sizes. Fig. 5 shows the cross sectional views of the micro-structures. The corresponding dimensions are presented in Table 1.</p><p>2.5. Data acquisition system</p><p>The liquid volume ow rate, the liquid inlet pressure, the aver-aged vapor temperature in spray chamber and two rows of tem-peratures in block heater could be measured directly in theexperiments. And temperatures were collected by 34970A dataacquisition unit. However, the test surface temperature was ac-quired by indirect measurement due to the thin liquid lm on thesurface and the continuous impact of droplets. As the heat lossbetween the two rows of thermocouples is small, the heat con-duction between lower row of thermocouples and heated surfacecan be simplied into one-dimensional condition. The averagedheat ux of heated surface can be calculated by expression (1) onthe basis of Fourier law of heat conduction:</p><p>q0 kTlower Tupperd (1)The averaged temperature of heated surface can be calculated</p><p>by:</p></li><li><p>Tw Tupper l=dTlower Tupper</p><p>(2)</p><p>3. Test conditions and procedure</p><p>All tests wer...</p></li></ul>

Recommended

View more >