第五章(第三次课)--CHF

第三课临界热流密度（CHF）尚智上海交通大学核工系Chapter5.LessonThreeCriticalHeatFlux(CHF)ShangZhiDepartmentofnuclearscienceandsystemengineeringShanghaiJiaotongUniversityIntroductionandObjectivesThecriticalheatfluxconditionischaracterizedbyalargedecreaseintheheattransfercoefficientfromtheheaterwalltothebulkfluid.Thereasonforthereductionisduetotheflowregimetransitionwhencontinuousliquidisreplacedbycontinuousvaporattheheaterwall,whetherlocallyoroveralargerarea.Letusconsiderthecriticalheatfluxfortwodifferentboundaryconditions.Forthepurposeofthissection,CHFisdefinedasfollows:1.Forasurfacewithacontrolledheatflux(e.g.,withelectricalheating,radiantheatingornuclearheating),CHFisdefinedasthatconditionunderwhichasmallincreaseinthesurfaceheatfluxleadstoalargeincreaseinthewalltemperature.2.Forasurfacewhosewalltemperatureiscontrolled(e.g.,oneheatedbyacondensingvapor),CHFisdefinedasthatconditioninwhichasmallincreaseinwalltemperatureleadstoalargedecreaseinheatflux.Thetermburnout'issometimestakenasbeingsynonymouswiththetermscriticalheatflux(CHF),departurefromnucleateboiling(DNB),boilingcrisisanddryout.Inthisdiscussiontheimplicationofburnoutisthattheriseinsurfacetemperatureissufficienttocausephysicaldamagetotheheatsurface.EffectofSystemParametersonCHFInatypicalCHFexperimentwithauniformheatfluxonaheatertube,CHFfirstoccursattheendofthechannel.Foragivenpressure(P),fixedmassflux(G),tubelength(L)andtubeinsidediameter(D)thecriticalheatflux(CHF)increasesapproximatelylinearlywiththeinletsubcooling(i.e.,thedifferenceinenthalpy,,betweensaturatedliquidandinletliquid).Thisrelationshipoccursoverfairlywideranges,buthasnofundamentalsignificance,excepttoindicatemoreenergygoesintosaturatingthefluid.Ifaverywiderangeofinletsubcoolingisused,thendeparturesfromlinearityareobserved.ForfixedP,D,Land,CHFrisesapproximatelylinearlywithGatlowvaluesofGbutthenrisesmuchlessrapidlyforhighGvalues;thisisdiscussedlater.ForfixedP,D,G,and,criticalheatfluxdecreaseswithincreasingtubelengthL.However,thepowerinputrequiredforburnout,,increasesatfirstrapidly,andthenlessrapidly,alsoasshowninthefigure.Forverylongtubes,thecriticalpowermayappeartoasymptotetoaconstantvalueindependentoftubelengthinsomecases.Again,thisonlyappliesoveralimitedrangeoflength.ForfixedP,G,andL,CHFincreaseswithtubediameter,D,therateofincreasedecreasingasthediameterincreases.subisubisubisubiCorrelationMethodsforCHFinRoundTubesWithUniformHeatingForagivenmassflux,fluidphysicalproperties(i.e.,pressureforagivenfluid),tubediameterandforuniformheatflux,itisfoundthatthedataforarangeofthetubelengthandinletsubcoolingcanberepresentedapproximatelybyasinglecurveofcriticalheatfluxagainstmassqualityattheCHFlocation.TheinitialimplicationoftherelationshipbetweenCHFandqualityatthislocationisthatthelocalqualityconditionsgovernthemagnitudeoftheburnoutheatfluxatthatlocality;thisistermedthelocalconditionshypothesis.ThesamedatacanalsobeplottedintermsofthequalityofCHF,XCHF,andboilinglengthattheCHFpoint,ZCHF.fgfifefgCHFiiiLzxLiDGq4Thevastmajorityofcorrelationsforcriticalheatfluxfallintoeitherofthesetwocategories.Althoughthesecorrelationsareequivalentforuniformlyheatedchannels,theygivequitedifferentresultswhentheheatfluxisnonuniform.ThequestionobviouslyarisesastowhichofthetwoformsisbestsuitedtothepredictionofCHFwithnonuniformheating,thecasewhichoccursmostofteninpracticalapplications.Aswillbeseenlater,thereisaconsiderabledifferenceincriticalheatfluxatagivenlocalqualityfortheuniformandnonuniformheatedtubesrespectively.Notethat,withthenonuniformheating,CHFcouldoccurfirstupstreamoftheendofthechannel.Rememberthatthisisonlyanexampleofnonuniformheatingintheaxialdirection.Onemayalsohavecircumferentialvariationofthefluxorvariablefluxwithinarodbundle.Thesemustalsobehandledbasedonempiricaldata.LimitsontheCriticalHeatFluxTounderstandthelimitsonthecriticalheatfluxitisusefultocomputeitsupperandlowerbounds.Thesimplestsituationtoconsideristhecaseofauniformheatfluxontheheaterwall.Foravariableheatfluxtheselimitsapplybutonemustnumericallyintegratetheequations.ThelowerboundonCHFwouldbethatheatfluxwhichfirstcausestheheaterwalltorisetothefluidsaturationtemperature.wherehisthesinglephaseheattransfercoefficient.hCDGzTTqpffisatMINCHF14,Theupperboundonthecriticalheatfluxisthatuniformheatfluxwhichwouldcausethefluidtocompletelyevaporate(Xe=1).Onceagainwecanusetheone-dimensionalenergybalanceforthefluidandsetthethermodynamicequilibriumqualitytooneandsolveforthemaximumCHFforagivenaxiallocation.fgfisatpffgMAXCHFiTTCziDGq14,MechanismsofCriticalHeatFluxAlargenumberofalternativemechanismsforCHFhavebeenproposedbutfourconceptswhichappeartohavebeenreasonablywellestablishedexperimentallyareillustratedinFigure(a)Formationofhotspotundergrowingbubble(Figure(a)).Here,whenabubblegrowsattheheatedwall,adrypatchformsunderneaththebubbleasthemicro-layerofliquidunderthebubbleevaporates.Inthisdryzone,thewalltemperaturerisesduetothedeteriorationinheattransfer.Whenthebubbledeparts,thedrypatchmayberewettedandtheprocessrepeatsitself.Howeve