第五章(第四次課)干涸后傳熱課件

第四課干涸后傳熱

尚智上海交通大學(xué)核工系第四課干涸后傳熱尚智1Chapter5.LessonFour

Post-CHFHeatTransferShangZhiDepartmentofnuclearscienceandsystemengineeringShanghaiJiaotongUniversityChapter5.LessonFour

Post-C2IntroductionandObjectivesInsomesituations,heatexchangerequipmentisoperatedinatwo-phaseregionbeyondthecriticalheatfluxpoint;e.g.,fossilfueledboilersorsteamgenerators,understeadyconditionsandnuclearreactorcoresundertransientconditions.Becauseoftheseandothersimilarsituationsthereexistsacontinuingneedformoreaccurateinformationaboutheattransfercoefficientsinpost-CHFflowregimes.Significantimprovementsinunderstandinghavebeenmadeoverthepastfewyears.Inparticular,thedevelopmentofexperimentaltechniqueshasallowedthedeterminationofthecompleteforcedconvectionboilingcurveincludingthe"transitionboiling"regionforparticularvaluesoflocalvaporqualityandmassvelocity.IntroductionandObjectivesIn3TheobjectiveofthissectionistoreviewthesubjectofPost-CHFheattransferandindicatethe"state-of-the-art."Althoughmuchofthisworkisspecificallyrelevanttoaqueousfluidsthetechniquesusedarebeneficialinthedesignofotherheatexchangeequipmentinthepowerandprocessindustries.Theobjectiveofthissection4Post-CHFHeatTransferModelsandCorrelationsThreetypesofmodellingapproacheshavebeenattempted:1.Correlationsofanempiricalnaturewhichmadenoassumptionwhateveraboutthemechanisminvolvedinpost-CHFheattransfer,butsolelyattemptsafunctionalrelationshipbetweentheheattransfercoefficientandtheindependentvariables.Thisassumesthatthevaporandliquidareatthesaturationtemperatureandinthermodynamicequilibrium.Post-CHFHeatTransferModels52.Correlationswhichrecognizethatdeparturefromthermodynamicequilibriumconditioncanoccurandattempttocalculatethe"true"vaporqualityandvaportemperature,differentfromTsat.Aconventionalsingle-phaseheattransfercorrelationforthevaporisthenusedtocalculatetheheatedwalltemperature.2.Correlationswhichrecogniz63.Semi-theoreticalmodelswhereattemptshavebeenmadetoexamineandderiveequationsforthevariousindividualhydrodynamicandheattransferprocessesoccurringintheheatedchannelandrelatethesetothewalltemperature(orheatfluxdependingontheboundaryconditions).3.Semi-theoreticalmodelswhe7Groeneveld(1973)hascompiledabankofcarefullyselecteddatadrawnfromavarietyofexperimentalpost-dryoutstudiesintubular,annularandrodbundlegeometriesforsteam/waterflows.Groeneveld(1973)hascompiled8EmpiricalCorrelationsThisheattransfercoefficientistobeusedforthedeterminationofthewalltemperaturegiventhewallheatfluxorviceversaforaknownwalltemperature.Aconsiderablenumberofempiricalequationshavebeenpresentedbyvariousinvestigatorsfortheestimationofheattransferratesinthepost-dryoutregion.Almostalloftheseequationsaremodificationsofthewell-knownDittus-Boeltertyperelationshipforsingle-phaseflowandtakenoaccountofthenon-equilibriumeffectsdiscussedabove.Ratherthermodynamicequilibriumisassumedbetweenthevaporandliquid.Variousdefinitionsofthe"two-phasevelocity"andphysicalpropertiesareusedintheseempiricallymodifiedformsandanumberofcorrelationsresult.EachofthesecorrelationsisbasedononlyalimitedamountofexperimentaldataandGroeneveld,therefore,proposedanewcorrelationforeachgeometryoptimizedusinghisbankofselecteddata.Thisistherecommendedcorrelationifoneistousethesimplifiedequilibriumapproachtoapost-CHFanalysis.EmpiricalCorrelationsThishe9第五章(第四次課)干涸后傳熱ppt課件10Non-EquilibriumEmpiricalModelsConsiderthecaseofaconstantheatflux.Nowinthisnon-equilibriumapproachweassumethattheheatfluxcanbedividedintotwoportions;onedirectlyheatingthevapor,qg,andonedirectlyintotheliquid,qf,causingittoevaporate.Non-EquilibriumEmpiricalMode11wherewedefineeasGroeneveldhassuggestedsuchacorrelationforegivenbywherewedefineeas12Semi-TheoreticalModelsAcomprehensivetheoreticalmodelofheattransferinthepost-CHFregionmusttakeintoaccountthevariouspathsbywhichittransferredfromthesurfacetothebulkvaporphase.Sixseparatemechanismsmaybeidentified:1.heattransferfromsurfacetoliquiddropletswhichimpactwiththewall("wet"collisions)2.heattransferformthesurfacetoliquiddropletswhichenterthethermalboundarylayerbutwhichdonot"wet"thesurface("dry"collisions).3.convectionheattransferfromthesurfacetothebulkvaporSemi-TheoreticalModelsAcomp134.convectiveheattransferfromthebulkvaportosuspendeddropletsinthevaporcore5.radiationheattransferfromthesurfacetotheliquiddroplets6.radiationheattransferfromthesurfacetothebulkvapor.4.convectiveheattransferf14Oneofthefirstsemi-theoreticalmodelsproposedwasthatofBennettetal.,(1964)whichisaone-dimensionalmodelstartingfromknownequilibriumconditionsatthepointofCHF.ItwasoriginallyassumedthatthereisnegligiblepressuredropalongthechannelbyGroeneveld(1974)revisedtheequationstoallowforpressuregradientandflashingeffects.Itwasalsoassumedthatdropletscouldnolongerapproachthesurface.Oneofthefirstsemi-theoreti15TransitionBoilingVariousattemptshavebeenmadetoproducecorrelationsforthe"transition"filmboilingregioncorrespondingtotheleftoftheminimumintheboilingcurve(Figure).Thesecorrelationshaveusuallybeencombinedwithexpressionsforthestablefilmboilingregion.Thedifficultywiththisapproachisthatinflowboilingthephysicalmeaningofthisminimumisdifficulttounderstand.TransitionBoilingVariousatte16Tong(1974)suggestedthefollowingequationforcombinedtransitionandstablefilmboilingat2000psiawithwalltemperatureslessthan800oF,andTw-Tsat<300oF.Tong(1974)suggestedthefoll17TransitiontoFilmBoilingConsiderthecasewhenthemassvelocitydecreasestoverylowvalue.AsGdecreasestheflowsituationbecomesmorelikepoolboilingwherethepost-CHFregimebecomesfilmboiling.Becausetheliquidisdisplacedfromtheheatingsurfacebyavaporfilmandtheuncertaintiesassociatedwithbubblenucleationareremoved,filmboilingisveryamenabletoanalyticalsolution.Thisissimilartocondensationwherestratifiedflowoccurwithaliquidfilm.Ingeneral,theproblemistreatedasananalogueoffilm-wisecondensationandsolutionsareavailableforhorizontalandverticalflatsurfaces,andalsoinsidetubesunderbothlaminarandturbulentconditionswithandwithoutinterfacialshear.Itisnot,however,proposedtoreviewallthesesolutionshereandthereaderisreferredtothecomprehensivereview