第五章(第四次课)干涸后传热

第四课干涸后传热尚智上海交通大学核工系Chapter5.LessonFourPost-CHFHeatTransferShangZhiDepartmentofnuclearscienceandsystemengineeringShanghaiJiaotongUniversityIntroductionandObjectivesInsomesituations,heatexchangerequipmentisoperatedinatwo-phaseregionbeyondthecriticalheatfluxpoint;e.g.,fossilfueledboilersorsteamgenerators,understeadyconditionsandnuclearreactorcoresundertransientconditions.Becauseoftheseandothersimilarsituationsthereexistsacontinuingneedformoreaccurateinformationaboutheattransfercoefficientsinpost-CHFflowregimes.Significantimprovementsinunderstandinghavebeenmadeoverthepastfewyears.Inparticular,thedevelopmentofexperimentaltechniqueshasallowedthedeterminationofthecompleteforcedconvectionboilingcurveincludingthetransitionboilingregionforparticularvaluesoflocalvaporqualityandmassvelocity.TheobjectiveofthissectionistoreviewthesubjectofPost-CHFheattransferandindicatethestate-of-the-art.Althoughmuchofthisworkisspecificallyrelevanttoaqueousfluidsthetechniquesusedarebeneficialinthedesignofotherheatexchangeequipmentinthepowerandprocessindustries.Post-CHFHeatTransferModelsandCorrelationsThreetypesofmodellingapproacheshavebeenattempted:1.Correlationsofanempiricalnaturewhichmadenoassumptionwhateveraboutthemechanisminvolvedinpost-CHFheattransfer,butsolelyattemptsafunctionalrelationshipbetweentheheattransfercoefficientandtheindependentvariables.Thisassumesthatthevaporandliquidareatthesaturationtemperatureandinthermodynamicequilibrium.2.Correlationswhichrecognizethatdeparturefromthermodynamicequilibriumconditioncanoccurandattempttocalculatethetruevaporqualityandvaportemperature,differentfromTsat.Aconventionalsingle-phaseheattransfercorrelationforthevaporisthenusedtocalculatetheheatedwalltemperature.3.Semi-theoreticalmodelswhereattemptshavebeenmadetoexamineandderiveequationsforthevariousindividualhydrodynamicandheattransferprocessesoccurringintheheatedchannelandrelatethesetothewalltemperature(orheatfluxdependingontheboundaryconditions).Groeneveld(1973)hascompiledabankofcarefullyselecteddatadrawnfromavarietyofexperimentalpost-dryoutstudiesintubular,annularandrodbundlegeometriesforsteam/waterflows.EmpiricalCorrelationsThisheattransfercoefficientistobeusedforthedeterminationofthewalltemperaturegiventhewallheatfluxorviceversaforaknownwalltemperature.Aconsiderablenumberofempiricalequationshavebeenpresentedbyvariousinvestigatorsfortheestimationofheattransferratesinthepost-dryoutregion.Almostalloftheseequationsaremodificationsofthewell-knownDittus-Boeltertyperelationshipforsingle-phaseflowandtakenoaccountofthenon-equilibriumeffectsdiscussedabove.Ratherthermodynamicequilibriumisassumedbetweenthevaporandliquid.Variousdefinitionsofthetwo-phasevelocityandphysicalpropertiesareusedintheseempiricallymodifiedformsandanumberofcorrelationsresult.EachofthesecorrelationsisbasedononlyalimitedamountofexperimentaldataandGroeneveld,therefore,proposedanewcorrelationforeachgeometryoptimizedusinghisbankofselecteddata.Thisistherecommendedcorrelationifoneistousethesimplifiedequilibriumapproachtoapost-CHFanalysis.Non-EquilibriumEmpiricalModelsConsiderthecaseofaconstantheatflux.Nowinthisnon-equilibriumapproachweassumethattheheatfluxcanbedividedintotwoportions;onedirectlyheatingthevapor,qg,andonedirectlyintotheliquid,qf,causingittoevaporate.fgqqqwherewedefineeasGroeneveldhassuggestedsuchacorrelationforegivenbyqqef5211ln674.0402.0CFHFgwaterxpDGeSemi-TheoreticalModelsAcomprehensivetheoreticalmodelofheattransferinthepost-CHFregionmusttakeintoaccountthevariouspathsbywhichittransferredfromthesurfacetothebulkvaporphase.Sixseparatemechanismsmaybeidentified:1.heattransferfromsurfacetoliquiddropletswhichimpactwiththewall(wetcollisions)2.heattransferformthesurfacetoliquiddropletswhichenterthethermalboundarylayerbutwhichdonotwetthesurface(drycollisions).3.convectionheattransferfromthesurfacetothebulkvapor4.convectiveheattransferfromthebulkvaportosuspendeddropletsinthevaporcore5.radiationheattransferfromthesurfacetotheliquiddroplets6.radiationheattransferfromthesurfacetothebulkvapor.Oneofthefirstsemi-theoreticalmodelsproposedwasthatofBennettetal.,(1964)whichisaone-dimensionalmodelstartingfromknownequilibriumconditionsatthepointofCHF.ItwasoriginallyassumedthatthereisnegligiblepressuredropalongthechannelbyGroeneveld(1974)revisedtheequationstoallowforpressuregradientandflashingeffects.Itwasalsoassumedthatdropletscouldnolongerapproachthesurface.TransitionBoilingVariousattemptshavebeenmadetoproducecorrelationsforthetransitionfilmboilingregioncorrespondingtotheleftoftheminimumintheboilingcurve(Figure).Thesecorrelationshaveusuallybeencombinedwithexpressionsforthestablefilmboilingregion.Thedifficultywiththisapproachisthatinflowboilingthephysicalmeaningofthisminimumisdifficulttounderstand.Tong(1974)suggestedthefollowingequationforcombinedtransitionandstablefilmboilingat2000psiawithwalltemperatureslessthan800oF,andTw-Tsat300oF.satWTBTTh01.0exp16860890TransitiontoFilmBoilingConsiderthecasewhenthemassvelocitydecreasestoverylowvalue.AsGd