A study of local heat transfer mechanisms along th

InternationalJournalofThermalSciences45(2006)269–283∗,T.Lüttichb,W.MarquardtbaInstitutfürEnergietechnik,TechnischeUniversitätBerlin,Marchstr.18,KT1,D-10587Berlin,GermanybLehrstuhlfürProzesstechnik,RWTHAachen,Turmstr.43,D-52064Aachen,GermanyReceived22December2003;receivedinrevisedform24August2004;accepted24August2004Availableonline12October2005AbstractAnarrayof36microthermocouples(38µmdiameter)embeddedinahorizontalcopperheater(distancetothesurface3.6µm),amicroopticalprobe(tipdiameter∼1.5µm)andamicrothermocoupleprobe(tipdiameter∼16µm),bothmoveableabovetheheatersurface,areappliedtostudyheattransfermechanismsalongtheentireboilingcurveundersteady-stateconditions.TestfluidsareisopropanolandFC-3284.Innucleateboiling,verylocalizedandrapidtemperaturedropsareobservedindicatinghighheatfluxesatthebottomofthebubbles.AlreadybeforereachingCHF,hotspotsoccurthesizeofwhichincreasestowardstheLeidenfrostpoint.Intheentiretransitionboilingregimewettingeventsareobserved,butnoonesinfilmboiling.Inlowheatfluxnucleateboilingsmallvaporsuperheatsexistinthebubblesandstrongsuperheatsinthesurroundingliquid.Thischaracteristicchangescontinuouslywithincreasingwallsuperheat:theliquidsurroundingthevaporapproachessaturationwhereasthevaporbecomesmoreandmoresuperheated.Infilmboilingthebubblesleavingthevaporfilmcanreachsuperheatsof40Knearthesurface.Theopticalprobesconfirmaliquidrichlayernearthesurfacebetweennucleateboilingandhighheatfluxtransitionboiling.Thevoidfractioninthislayerincreasescontinuouslywiththedistancetothesurfaceuntilamaximumvaluewhichseemstobelinkedtothebubbledeparturediameter.2005ElsevierSAS.Allrightsreserved.Keywords:Microthermocouple;Opticalprobe;Microthermocoupleprobe;Poolboiling;Transitionboiling;Surfacewetting;Drypatch;Surfacetemperature;Boilingmechanism;Voidfraction;Isopropanol;FC-3284;PF-5052;Temperaturefield;Temperaturecontrol;Twophaseflow1.IntroductionInspiteofthehugeamountofstudiesonboilingheattransferinthepast,thenumberofinvestigationsdoesnotdecrease,in-dicatingthatthiscomplexprocessisstillnotwellunderstood.Neverthelesssignificantprogresshasbeenachievedinrecentyears.Thisisbecausemoresophisticatedexperimentaltech-niquesanddataacquisitionsystemsaswellassignificantlyim-provedmathematicaltoolsarenowavailable.Hence,ourabilitytoexplorethebasicmechanismsofboilinghasbeenimprovedandthoseresultsareincreasinglyusedtodevelopphysicallybasedheattransfermodels.Theyaremoregenerallyvalidthanempiricalcorrelationsand,lastbutnotleast,adeeperknowl-edgeofthegoverningphysicalmechanismsgivesusthechance*Correspondingauthor.E-mailaddress:[e-mail](H.Auracher).todevelopenhancedheattransfersurfacessystematicallyandnotbytrialanderrormethods.Thegoverningmechanismsofboilingtakeplaceveryneartothesurface.Ifathinelectrically-heatedplateisusedastestheater,bubbleandvaporspotdynamicsatthesurfacecanbestudiednonintrusivelybyliquidcrystals[1]orinfrared-thermographyontherearsurface.Bymeansofliquidcrystalsitwaspossibletoconfirmqualitativelyintensiveevaporationinthemicroregionatthebottomofabubble[2].Furthermore,firstattemptshavebeenmaderecentlytodevelopmechanisticmod-elsfornucleateboilingbasedonliquid–crystaldata[3].Thisstudyrevealedthatliquidcrystalthermographyhasalimitedtemporalresolution.Thisshortcomingmightbeovercomebyhighspeedinfraredthermography.Buteventhough,validationofaboilingmodelalsoneedsdataonthetwo-phasecharac-teristicsabovethesurface,suchasinformationaboutbubblegrowth,bubblecoalescenceetc.Moreover,realheatingsurfaces1290-0729/$–seefrontmatter2005ElsevierSAS.Allrightsreserved.doi:10.1016/j.ijthermalsci.2004.08.007270M.Buchholzetal./InternationalJournalofThermalSciences45(2006)269–283NomenclatureCHFcriticalheatfluxEMFelectromotiveforce.........................VGgain,outputvoltage/inputvoltageKpppeak-to-peaktemperaturefluctuations(noise)..KMOPmicroopticalprobeMTCmicrothermocoupleMTCPmicrothermocoupleprobep∗reducedpressure,p/pcPIFphaseindicatorfunctionPtroughnessparameteraccordingtoDINENISO4287.................................µm˙qheatflux.............................W·cm−2˙q∗dimensionlessheatflux˙q/˙qCHF
Theatersurfacesuperheat,Tsurface−Tsat........K
TpMTCPsuperheat,Tprobe−Tsat...............KTCthermocouplezdistanceperpendiculartotheheatersurface.............................mmorµmαvvoidfractionarethickandtheboilingmechanismmaydifferfromtheoneonthinfoils,see[4,5]forexample.Inparticular,thetemperaturefielddynamicsintheheaterbodyplaysaroleandshouldbetakenintoaccount.Hence,acomplexthree-dimensionalmul-tiphaseflowproblemconnectedtoathree-dimensionalheatconductionproblemhastobetackled.Ifthickheatersaretobestudiedasinthepresentreport,non-intrusivetemperaturemeasurementtechniquessuchasliquidcrystalsorthermographyarenotapplicable.Othernonintrusivetechniquesforinvestigatingthetwo-phaseflowcharacteristicslikehigh-speedvideo,x-rayattenuationetc.areoflimitedben-efitbecausethesemethodscannotaccessthemostimportantparametersdeterminingtwo-p