Spray cooling on micro structured surfaces

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    ux in the experiments was raised. Motivated by this nding, the eect of dierent micro structured surfaces, which lead to an increase inthe three phase contact line, on the spray cooling performance was studied. The micro structures consisted of micro pyramids with dif-

    metal quenching over cooling of high power electronics tomedical treatments. Using water as coolant, spray cooling

    In a pressure atomizer, only one phase is used for the atom-izing and the subsequent cooling. In the case of a air blast

    Three dierent regimes can be determined in spray cool-ing heat transfer [2], rst a low temperature regime, where

    was also not observed.Several studies were performed on the inuence of the

    spray parameters on the cooling heat ux, it was found thatthe volumetric spray ux has a dominant eect on the heattransfer [3,4] compared to other hydrodynamic parametersof the spray. Pautsch et al. [5] found that if a pressure

    * Corresponding author. Tel.: +49 0 6151 16 3159; fax: +49 0 6151 166561.

    E-mail address: pstephan@ttd.tu-darmstadt.de (P. Stephan).

    International Journal of Heat and Mass Theat uxes up to 1000 W/cm2 have been reported [1].Depending on the spray parameters and surface properties,the coolant forms a cohesive or ruptured thin liquid lm,which evaporates on the hot surface. With decreasing lmthickness the transferred heat ux at a given surface tem-perature increases [1].

    Generally, sprays can are generated in dierent ways,most frequently used are pressure and air blast atomizers.

    liquid evaporates at the free liquidvapour interface, sec-ond a high temperature regime, where liquid lms are ina lm-boiling-like state and vapour bubbles are generatedat the superheated hot wall. The third regime is the transi-tion regime from the low temperature to the high tempera-ture region. In his experiments Toda [2] observed thatsubcooling had only minor eects on single phase andnucleate boiling heat transfer. A dominant eect on CHFferent heights. A signicant enhancement in the heat transfer performance due to the surface structures could be observed, especially atlow coolant uxes.

    Additionally, high spatial resolution temperature measurements on a smooth heater using thermochromic liquid crystals wereobtained. These measurements indicate high local temperature gradients for a regime where the coolant lm on the heater is ruptured. 2007 Elsevier Ltd. All rights reserved.

    Keywords: Spray cooling; Micro structures; Contact line; TLCs; Surface temperature distribution

    1. Introduction

    Spray cooling is a very ecient means for dissipatinghigh heat uxes with low coolant mass uxes at low wallsuperheats. It is used in a wide range of applications from

    atomizer, the mass ow rate of the liquid coolant is usuallysmall compared to that of the air. Pressure atomizers areconveniently used in closed systems. The advantage of airatomizers is that they can work with lower liquid coolantow rates.Spray cooling on mic

    Christof Sodtke

    Chair of Technical Thermodynamics, Darmstadt University

    ReceivedAvailable onli

    Abstract

    Spray cooling experiments have been performed on a smoothcone nozzle into a low pressure spray chamber. Dierent coolanand heater. Observations obtained with an infrared camera indicat0017-9310/$ - see front matter 2007 Elsevier Ltd. All rights reserved.doi:10.1016/j.ijheatmasstransfer.2006.12.037structured surfaces

    eter Stephan *

    echnology, Petersenstrasse 30, 64287 Darmstadt, Germany

    uly 20066 April 2007

    ter surface. The working uid water was atomized through a fullass uxes were studied by varying the distance between nozzlehat the length of the three phase contact line increased as the heat

    www.elsevier.com/locate/ijhmt

    ransfer 50 (2007) 40894097

  • f Hatomizer is used, the lm thickness decreases with increas-ing ow rate, while Yang et al. [6] found that for an airatomizer the coolant ow rate has only little eect on thelm thickness.

    Sehmbey et al. [7] studied the eect of surface propertieson the cooling heat ux using air atomized nozzles. Theyfound that a higher surface roughness decreases the coolingperformance of the spray, due to an increased lm thick-ness. Hsieh and Yao [8] conducted experiments studyingthe inuence of surface micro structures on silicon waverson the spray cooling heat ux using a pressure atomizernozzle. Their results show an increased heat ux on struc-tured surfaces due to capillary forces of the surface actingon the lm. Smaller structures resulted in a better heattransfer. However, the improvement in heat transfer islower than the increase in surface area. Silk et al. [9] studiedthe eect of cubic ns, pyramids and straight ns on theheat transfer performance up to CHF. They found thatall surfaces tested improved the heat transfer performancecompared to a at surface, the straight ns performed best,followed by cubic ns and pyramids. The straight nsresulted in a 50% higher CHF value compared to thatof a at surface. Investigating also the inuence of dis-solved gas, Silk et al. [9] found the magnitude of theimprovement was lower for the case of a not degassed uid.

    Recently, Horacek et al. [10] performed a study on thebasic heat transfer mechanisms of spray cooling using a

    Nomenclature

    d10 average droplet diameter (lm)_m mass ux (kg/h)p pressure (bar)_q heat ux (W/cm2)Ra surface roughness (lm)t temperature (C)Dt temperature dierence (K)k thermal conductivity (W/(m K))s time constant (s)

    4090 C. Sodtke, P. Stephan / International Journal otransparent microheater array consisting of 96 square heat-ing elements, each having an edge length of 100 lm. Theyfound that apparently dry spots appear in the otherwisecohesive liquid lm covering the cooled surface at wallsuperheats in excess of 10 C. According to [10], the heatux is directly related to the contact line length of the threephases gasliquidsolid and not to the wetted area.

    There is some discussion about the temperature varia-tions encountered on a spray cooled surface. While Guet al. [11] found that compared to other cooling techniques,e.g., jet cooling, spray cooling results in a relatively uni-form temperature distribution over the cooled surface,Shedd and Pautsch [12] found quite signicant temperaturevariations over their test heater reaching as high as 17 C.

    Despite the advantages of spray cooling over competingcooling techniques, there is a reluctance to incorporatespray cooling in demanding cooling situations, which isattributed to the very limited understanding of the heattransfer mechanisms and practical concerns such as thelimited repeatability of cooling performance for seeminglyidentical spray nozzles [13].

    For the present work experiments were conducted tostudy the eect of surface structures and coolant mass uxon the heat transfer in the low surface temperature regimeat low system pressures. Additionally, the inuence of thecoolant behavior on the heat transfer performance wasinvestigated by observing the heater surface with a high-speed infrared camera and performing high resolution tem-perature measurements on a thin spray cooled foil heater.In the following section, the experimental set-up isdescribed, in Section 3 experimental results are presentedand discussed.

    2. Experimental set-up and procedure

    2.1. Experimental set-up

    A schematic of the experimental set-up used is shown inFig. 1. A Plexiglas spray chamber was constructed, whichallows spray cooling at system pressures as low as0.005 bar. The spray is generated using deionised water, agear pump and a full cone pressure atomizer (SprayingSystems, TG SS 0.3). A heat exchanger inside the spray

    Subscripts

    con experimental containercyl heaterres reservoirw wall

    eat and Mass Transfer 50 (2007) 40894097chamber ensures constant temperature for all experiments(Dtcon = 1 K), independent of the ambient temperature.A second heat exchanger inside the reservoir is used to con-trol the subcooling of the coolant (Dtres = 0.4 K). Thecoolant mass ow rate pumped through the nozzle is mea-sured with a coriolis ow meter and can be accuratelyadjusted using a bypass controlled by a needle valve. Ahighspeed infrared imaging system (Phoenix Midas) canbe used to observe the coolant distribution and behavioron the heater. Alternatively, the infrared camera can bereplaced by a standard CCD-camera.

    The heater surfaces are the upper end faces of dierentlytreated copper cylinders which are placed on the bottom ofthe spray chamber one at a time. A xation with a conicaltop ensures that excess coolant can run o towards theintercepting tank and does not accumulate on the cylinder

  • 1mm

    4mm

    7mm

    conical

    g

    f Heat and Mass Transfer 50 (2007) 40894097 4091P

    nozzle

    Plexiglas chamber

    CCD / IR

    P F

    vacuum pumppump

    flowmeter

    bypass

    heater

    reservoir

    heat exchanger 1

    heatexchanger 2

    isolation

    C. Sodtke, P. Stephan / International Journal osurface. That way, only the upper end face of the coppercylinders get in contact with the coolant and the coolantdoes not accumulate on the heater surface. The cylindersare electrically heated by cartridge heaters and are wellinsulated at the circumference. The upper end face in con-tact with the spray has a diameter of 20 mm. The dissipatedheat ux and the surface temperature are calculated usingthree thermocouples spaced 3 mm apart from each otheralong the axis of the copper cylinder (see Fig. 2). The upperthermocouple is positioned 1 mm below the heater surface.The thermal conductivity of the copper cylinders was mea-sured via their electrical conductivity using the relationshipbetween the electrical and the thermal conductivities to bekcyl = 394 W/(m K). A 3D heat conduction calculationwith Fluent has shown that the temperature prole is veryclose to a one dimensional temperature prole in the upperpart of the copper cylinder. Hence, the assumption of a onedimensional temperature prole can be used to calculatethe dissipated heat ux based on the three temperaturemeasurements and their distance. The surface temperatureis extrapolated using the measured temperature at the ther-mocouple closest to the surface and the calculated heatux. Additionally, the temperature and the pressure insidethe spray chamber are monitored, as well as the tempera-tures, of the working uid in the reservoir and just in frontof the nozzle.

    intercepting tank

    Fig. 1. Schematics of the experimental set-up.The spacing between heater surface and nozzle can bevaried between 20 mm and 100 mm. For spacings largerthan 25 mm, only a fraction of the spray hits the heater,while for smaller distances the total spray volume ux isdeposited on the heater.20mm

    35m

    m

    thermocouples

    holder

    cartridge heaters

    coppercylinder

    insulation

    Fig. 2. Copper heater embedded in the xation system for the spraychamber.2.2. Micro structured surfaces

    The heat transfer performance of three dierent surfaceswith micro pyramids as surface structures has been com-pared to that of a smooth polished surface. The surfacestructures dier in height and width of the pyramids. Thedimensions of the structures can be inferred from Table 1.All surface structures were machined into the end face ofcopper cylinders. One of the surfaces is shown exemplarilyin Fig. 3. All structures investigated caused an increase insurface area compared to the smooth surface by a factorof

    2p

    . The surface roughness of the smooth polished sur-face was found to be Ra < 0.3 lm using a perthometer.The structured surfaces were xed in their holders such thatthe bottom of the structures was on the same level as theedge of the conical holder.

    One set of calibrated thermocouples is used for eachcopper cylinder. The thermocouples were checked for

    Table 1Dimensions of surface structures

    Structure Height (in lm) Width (in lm)

    S1 75 150S2 150 300S3 225 450

  • by using an infrared transmissive CaF2-window. The

    of the spray chamber with a CCD-camera. The foil heaterhas a thickness of 10 lm, a 20 lm thick layer of a glue witha high thermal conductivity is used to have a good contactbetween the backside of the heater and the TLC sheet. TheTLC sheet itself consists of 3 layers, a layer of black paint(5 lm), a layer of encapsulated TLC (240 lm) and amylar foil as protection (125 lm). TLCs reect a distinctcolour depending on their temperature when illuminatedby white light. Below and above their active temperaturerange they are transparent so that the black backgroundcan be seen, within their working range the reected light

    f Himages were taken with a frame rate of 300 Hz. The spatialresolution of the camera system was 100 lm 100 lm/pixel. Using the Matlab-image processing toolbox, theinfrared images were converted to binary matrices consist-ing of ones where the heater surface is covered with waterand zeros elsewhere. The distortion of the images caused byrange of temperatures between 25 C and 60 C to deviateless than Dt = 0.05 K from each other in order to minimizethe error in the heat ux calculation. The temperaturesmeasured during the experiments were in a range from30 C to 55 C.

    2.3. Infrared images

    The aforementioned high speed infrared imaging systemhas been used to obtain images of the coolant behavior onthe smooth unstructured copper heater. The emissivity ofwater diers strongly from that of copper so that informa-tion what portion of the heater surface is covered with awater lm and where that lm is ruptured can be easilyextracted from the infrared images. As shown in Fig. 1,the images were taken from the side under an observationangle of 60. Optical access to the heater was permitted

    Fig. 3. Top view of micro structured surface (pyramids).4092 C. Sodtke, P. Stephan / International Journal othe viewing angle was removed as well by an algorithmwhich deskews the elliptical image of the circular heaterface so that the nal image shows a circular shape. After-wards, an edge detection algorithm was used to calculatethe surface area of lm covered portions of the heaterand their perimeter, which corresponds to the three phasecontact line length.

    2.4. High resolution temperature measurements

    To obtain high spatial resolution temperature measure-ments at a heated surface, the copper cylinder is replaced insome experiments by a thin stainless steel foil heater withthermochromic liquid crystals (TLCs) applied to the lowersurface. A schematic of the foil heater and the appliedTLCs is shown in Fig. 4. The colourplay of the TLCs isobserved from below through the Plexiglas bottom plateelectrode electrodePlexiglas

    foil heater

    glue

    black background

    TLC layer

    mylar foil

    nozzle

    ring light

    objectivecoonection topower supply

    Fig. 4. Schematic of TLC heater.

    eat and Mass Transfer 50 (2007) 40894097changes from red to green to blue with increasing temper-ature. The TLCs used for the current experiment are sup-plied in the form of a foil such that they are protectedfrom environmental inuences. Images of the TLCs arecaptured using a three chip colour CCD-Camera and thecolour information is converted from the RGB-signal ofthe camera to the HSV-colourspace (Hue, Saturation,Value). In the HSV-colourspace, the hue-value is meas...

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