(2011)Pool-Boiling-Critical-Heat-Flux-in-Dielectri

PoolBoilingCriticalHeatFluxinDielectricLiquidsandNanofluidsMEHMETARIK1,ALIKO¸SAR2,HUSEYINBOSTANCI3,andAVRAMBAR-COHEN41DepartmentofMechanıcalEngıneerıng,OzyeginUniversity,Istanbul,Turkey2MechatronicsEngineeringProgram,SabanciUniversity,Tuzla,Istanbul,Turkey3RINITechnologies,Inc.,Oviedo,Florida4DepartmentofMechanicalEngineering,UniversityofMaryland,CollegePark,MDI.IntroductionBoilingonasurfacethatisimmersedinapoolofstagnantliquidiscalled“poolboiling”heattransfer.Boilingtakesplacewhenevaporationoccursincavities(nucleationsites)ataliquid
solidinterfaceandproducesasuccessionofvaporbubbles.Thismodeofheattransferoccursinavarietyofapplica-tions,suchasheatexchangers,immersioncooledelectronicsystems,metallur-gicalquenchingprocesses,andwaterheatersfordomesticuse.Therateofboilingheattransferfromtheheatedsurfacetothepoolvariesstronglywiththeliquidtemperature,pressure,thermophysicalpropertiesoftheliquid,heatergeometry,heatersurface,andheaterorientation.Innucleatepoolboiling,theheatfluxincreasessteeplywiththesurfacesuperheat,butwhenthegeneratedvaporbubblesblanketthesurface,alargetemperatureincreaseontheheatersurfaceresults,leadingtothecriticalheatflux(CHF)conditionandthetermi-nationofnucleatepoolboiling.Thisconditionisalsoknownastheburnoutpointandthepeakormaximumheatflux,andisassociatedwithatemperatureriseofapproximatelyseveralhundreddegreeCelsiusforwaterand100CforFC-72.Inspiteofthelast70yearsofresearchandnumerouspublishedpapers,CHFstilldefiesanaccurateprediction.OneoftheearliestCHFmodelswasproposedbyRohsenowandGriffith[1].Itwasbasedonabubblepackingmodel.Accordingtothismodel,CHFisinitiatedbythecoales-cenceofdiscretebubblesorcolumnsintoalargevapormushroomleadingtodryoutoftheliquidfilmontheheatedsurface.Theempiricaldata1AdvancesinHeatTransferVolume43ISSN0065-2717DOI:10.1016/B978-0-12-381529-3.00001-3Copyright©2011ElsevierInc.Allrightsreserved.ADVANCESINHEATTRANSFERVOL.43showedalinearrelationshiponlog
logcoordinates,leadingtoaCHFcorrelationgivenas:CHFR2G5143ðft=hÞðρl2ρvÞρvg
0:6ρvhfgag0:25(1)TheindicatedgravitationaltermwassubsequentlyintroducedbyRohsenow[2]onthebasisofobservedbubbledeparturefrequencyanddiameter.Thiscorrela-tionhasbeenwidelyusedbymanyresearcherstopredictCHFinsaturatedliquids,butdoesnotaccountforheatergeometryorproperties,nordoesitdealwithfluidmixtures.Later,pioneeringstudiesservedtoprovideatheoreticalunderpinningforthepredictionoftheCHFinnucleateboiling.TwomajorschoolsofthoughthaveemergedonthecontrollingmechanismofCHF,classifiedashydrody-namicinstabilitymodelsandsurface-controlledCHFmodels,respectively.ThefirsttheoreticalmodelofCHFistodayknownastheKutateladze
Zubermodel[3].Inthismodel,CHFisdictatedbyinstabilityinthevapor
liquidinterfaceofthevaporjetsemanatingfromtheheatersurfaceinthehighfluxregionofthenucleateboilingcurve.Itispostulatedthattheverticalcoa-lescenceofvaporbubblesinfullydevelopednucleateboilingresultsintheformationofcylindricalvaporcolumnsorjets.Withinthesecolumns,thereisanupwardflowofvapor,andthejetsareseparatedbycounterflowingcolumnsofliquid,asseeninFig.1.Inthismodel,jetbreakdownleadstovaporblanketingofthesurfaceandresultsinCHF.Inapower-controlledsystem,thismechanismcausesadras-ticsurfacetemperatureincrease,whileinatemperature-controlledsystem,itdictatesareductionintheheatremovalrate.KutateladzeobtainedarelationshipforCHFbysolvingthemomentumandenergyequationsforD/2DVaporcolumnsHeaterDgFIG.1.Idealizedliquid
vaporconfigurationintheKutateladze
Zubermodel.2M.ARIKETAL.two-phaseflowneartheheater.Later,ZuberassumedthatCHFwascon-trolledbyhydrodynamicinstabilityandderivedanexpressionforCHF.Thismodelassumesahorizontalheatersurfaceofinfiniteextent,sothereisnocharacteristiclength.Accordingly,CHFcanbeexpressedas:CHFðK2ZÞ5π24hfvffiffiffiffiffiρvpσfgðρf2ρvÞρv2
1=4(2)Inthisearly,butverymuchacceptedmodel,CHFisassumedtobeonlyafunctionofthehydrodynamicinstabilityandtheheatersurfacepropertiesandtemperaturedonotenterintoconsideration.Inthatera,boilingresearchwasdrivenlargelybynuclearreactorcooling,usingheat-flux-controlledhea-ters,andplacedtheemphasisonpreventingcatastrophicfailure.Controllingsurfacetemperatureanddeterminingthepost-CHFchangesinheatfluxwereoflessinterestthancontrollingheatfluxandmeasuringtheresultingsurfacetemperatures.Someoftheearlystudies[4,5]interestinglydescribedtheexistenceofathinliquidfilmunderthelargevaporcolumns/mushroomswhichwereobservedontheheatersurfaceandsuggestedthatthisthinliquidfilm,ormacrolayer,mightaffecttheCHFphenomena.ThesecondCHFmodelwasproposedbyHaramuraandKatto[6]andisknownasthemacrolayermodel.Thismodelfocusesontheexistenceofavapormushroomhoveringabovetheheatersurface,causedbythecoalescenceofvaporbubbles,andpostu-latestheexistenceofathinlayerofliquidtrappedunderthevapormush-roomandseparatingthe“vaporstems”emanatingfromactivenucleationsitesandfeedingvaportothemushroom.Thetimeperiodbetweenthestartofgrowthofthevapormushroomanditsdeparturefromthesurfaceiscalledthehoveringperiod.Themacrolayerthicknesswasdeterminedtobe25%oftheHelmholtzwavelength.CHFisthoughttooccurwhenthemacrolayeriscompl