CFX12_Multiphase_Nuclear_06_Phase_Change.ppt

Lecture6ModelsforPhaseChange CFXMultiphasefortheNuclearIndustry Overview PhaseChangePhenomenaThermalPhaseChangeWallBoilingModelCavitationCavitationPhenomenaCavitationModelingCavitationExamples Introduction Interphasemasstransfercanproceedbybulkexchangeofmaterialbetweenphases boilingcondensationsublimationfreezingmeltingcavitationflashingInterphaseexchangeofmassmaybeeitherthermallyormechanicallydriven PhaseChangeModels Thermal DrivenbytemperaturedifferencesApplications CondensationBoilingPoolBoilingWallBoilingMelting Mechanical DrivenbypressuredifferencesApplications CavitationFlashing PhaseChangePhenomena ThermalPhaseChange AdditionorremovalofheattoaphaseinducesaphasechangeasthetemperaturerisesaboveorfallsbelowthesaturationtemperatureEnergybalanceatphaseinterfaceInterfacialmasstransferratesNote theremustbeinterfacialareaforthermalphasechangetooccur i e bothphasesmustbepresent InterfacialEnergyBalance Steady stateenergybalance Liquid Vapour Interface Interfacialheattransferrates InterfacialEnergyBalance Balanceequation Liquid Vapour Interface Interfacialenthalpies saturationvaluesHeattransferratesfromempiricalcorrelationsDifferentoncontinuousanddispersedside HeatTransferCoefficents Continuousside v Ranz Marshallcorrelation Continuous Correlations casedependentHughmark HeatTransferCoefficents Dispersedside l Zeroresistance Continuous Droplets approximatesolutionofheattransferequationinasphere 1 CondensationExample ExperimentatLehrstuhlf rThermischeKraftanlagen TUM nchenCondensationontofreesurfaceforsteam liquidwater 128 790 Thermocouples Adiabaticwall Adiabaticwall Steam Water TemperatureProfileMeasuredasaFunctionofHeight CondensationExample Vapour Saturatedvapouratconstantpressure Liquid Large heattransfercoefficientVapour laminarLiquid turbulent k Turbulencedampingatfreesurface Noenergyequationinvapourphase CondensationExample Liquidvolumefraction Grid121 60 y 40 CondensationExample VerticalTemperatureprofileatthermocouplelocation T C Increaseduetolatentheatfromcondensation VapourLiquid CondensationExample StationaryDroplet Vapour Saturatedvapouratconstantpressure Liquid Heattransfercoefficientbasedon Noenergyequationinvapourphase Initiallysubcooledliquiddropletincontactwithsaturatedvapor heatsupascondensationtakesplaceuntilsaturationtemperatureisreachedDimensionlesstemperatureofdroplet q predictedasafunctionofdimensionlesstime ascharacterizedbytheFouriernumber Fo CondensationExample StationaryDroplet WallBoilingModelinANSYSCFX BetaFeaturein11 0 RPIWallBoilingModel Forsubcooledflowswithsuperheatedwalls standardthermalphasechangemodelsforbulkboiling condensationwillunderpredictrates bubblesformattinynucleationcavitiesonwallsandgrowTheRPIWallBoilingmodelprovidesamechanisticmodelforwalldrivenboiling originallydevelopedbyPodowskiandco workersTheRPIwallboilingmodelisfullyreleasedinANSYSCFX12 0afterhavingbeenpreviouslyavailableonlyasabetafeature RegimeChangesforaBoilingLiquid RPIWallBoilingModel RPIWallBoilingModel Sub GridScaleModelforNucleateBoilingSimilartoTurbulentWallFunctionsSimilartomodelimplementedinCFX 4Plusimprovementstoachievegridindependence EgorovandMenter CustomisedimplementationexistedinCFX 11Plususabilityimprovements AllwallheattransferBC ssupported SpecifiedHeatFlux SpecifiedTemperature SpecifiedHeatTransferCoefficient BoilingatCHTboundariessupportedUseslaggedwalltemperature hencenotasrobustasdomainwalls RPIWallBoilingModel DeterminesHeatFluxPartitionatWall Q Qc Qq QeQc ConvectiveHeatTransferDeterminedbyTurbulentWallFunctionQq QuenchingHeatTransferDepartureofabubblefromheatedsurface coolingofsurfacebyfreshwater Qe EvaporativeHeatTransferDeterminedbyphysicalsub modelsonthesub gridscale RPIWallBoiling SubModels SeveralSubModels Nucleationsitedensity LemmartandChawla BubbleDepartureDiameter TolubinskyandKostanchuk BubbleDepartureFrequency Cole BubbleDepartureWaitingTime RPIWallBoiling SubModels Wallareafractioninfluencedbybubbles clipped WallareafractioninfluencedbysinglephaseconvectionConvectiveHeatTransfer TurbulentWallFunction QuenchingHeatTransferEvaporativeHeatTransfer RPIWallBoiling SubModels MeshIndependenceDefaultbubbledeparturediameterdependsona nearwallliquidtemperature ExperimentalcorrelationusespipecentrevalueCFX 4usesvalueatcentreofcontrolcelladjacenttowall meshdependent CFX 5useslogarithmicwallfunctiontocorrectthistoanestimatedvalueatfixedyplusvalue 250 useradjustable hencemeshindependentSimilarcorrectionprocedureisappliedtoquenchingheattransfercoefficient RPIWallBoiling SubModels Sub ModelDefaultsOriginallytunedforpressurisedwaterMayneedretuningforothersituationsPossibletochangedefaultmodelconstantsPossibletoover ridewithuser sownsub model usingCEL Bothmaybedoneonaper domainorper wallbasis OthermodelstypicallyappliedforvariablesoutsidetheRPImodelitselfBubblediameterinthebulk usuallyassumedasafunctionoflocalliquidsubcooling Includednon dragforces WallHeatPartitioning IterativeSolution HeatFluxPartitiondetermineswallheatfluxasacomplexnon linearfunctionofwallsuperheatSpecifiedWallTemperature UsespecifiedTwalltodetermineindividualwallheatfluxesSpecifiedHeatFlux ComputeTwallfromQspecusingnon linearequationsolutionalgorithmUsecomputedTwalltodetermineindividualwallheatfluxesSpecifiedHeatTransferCoefficient SameasforSpecifedHeatFlux CHTWallSameasforSpecifedTemperature usinglaggedinterfacialtemperature WallBoilingSetupProcedure DomainLevel IfwallboilingisrequireditmustbedefinedfirstatthedomainlevelOptionalSub ModelofThermalPhaseChangeMassTransferDefaultsub modelsareassumedunlessalteredbytheuser WallBoilingSetupProcedure WallBoundaries Wallboilingmustbeswitchedonexplicitlyoneachwallwhereisexpectedtooccur DefinedunderFluidPairobjectsunderboundarydetailsOptionnotavailableiffluiddependentheattransferBCsarerequestedN B DifferenttoCFX 11customisedimplementation wherewallboilingtookplaceonallwalls Wallboilingwillnotoccurifyouforgettodothis User definedSub models Sub modeldefaultsmaybeoverriddenonthedomainortheboundarylevelMaychangemodelconstantsMayproveduser definedsub modelsviaCELDomainlevelmodificationsapplyonallwallswherewallboilingisdefined unlessoverriddenlocallyBoundarylevelmodificationsonlyapplylocallytothespecificwall CurrentWallBoilingModelLimitations Sub CooledNucleateBoilingModelNotapplicabletoFilmBoilingorCriticalHeatFlux CHF regimes TurbulentFlowAllturbulencemodelssupportedinprincipal Laminarflownotsupported Robustnessproblemsobservedforfinenear wallmeshesusingAutomaticwallfunctions Mono DispersedBubblyFlowWallboiling MUSIGnotsupported OtherRestrictionsWallboiling radiationnotsupported WallheattransferBC smustbedefinedperwall notperfluid WallBoilingValidation ValidationTestCaseSubcooledBoilinginPipewithHeatedWallBartolomejetal 1967 1982 ConxitaLifante 2008 largenumberofexperimentaltestcaseconditionswithdatasteam waterpipeflowwithwallboilingliquidsub coolingdefinedtohavesteaminceptionalwaysatthesamewallheightDifferentconfigurationswerestudiedinthepaper Mainparameters MassinflowratePressureWallheatfluxPipediameter BartolomejTestCase Description 2Daxialsymmetry steadysimulation1degreeextrusionSpecifiedheatfluxatthewallSymmetryb c atplanesandaxisInletb c withgiveninletmassflowOutletb c withaveragestaticpressure BartolomejTestCase Models Steam Water2 phaseflow Water continuousphaseWaterSteam dispersebubblesLiquidtemperaturedependentbubblediameterEquation of state IAPWS IF97water watersteampropertiesInterfacialTransferGracedragFADTurbulentdispersionforceTomiyamaliftforceWalllubricationforce Antal Tomiyama none Tworesistanceheattransfermodel withRanzMarshallonthecontintinuousphasesideRPIwallboilingmodelwithdefaultsub modelsUserdefinedInterfacialareadensityaccountingforhighervolumefractionofthesteamphase BartolomejTestCase Conditions R 7 7mm Z 2m q 0 57MW m2 Gin 900kg sm2 NumericalGrids NumericalParameters Results Grid1 Axialdevelopmentofwatertemperatureandsteamvolumefraction Results Grid2 Axialdevelopmentofwatertemperatureandsteamvolumefraction Results Grid3 Axialdevelopmentofwatertemperatureandsteamvolumefraction ComparisontoDataandGridIndependence TotalGasContent EffectofParameter ModelVariation Wallheatfluxinfluence Walllubricationforcemodelinfluence ReactorSafety BoilinginRodBundle 3 3rodsymmetrysectionfromanuclearreactorfuelassemblywithguidevanesPeriodicBC satallsidesWallheatfluxofqwall 106W m2ReferencePressurep 15 7MPaWaterinlettemperatureTInlet 607K 12Kwatersubcooling ReactorSafety BoilinginRodBundle CFX 12 0 VapourVF 25 isosurfaces VaporVolumeFractioninChannels Cavitation VaporousandGaseousCavitation Vaporouscavitationoccurswhenthelocalpressurefallsbelowthevaporpressureofaliquid Thiscausingveryrapidboilingoftheliquidatcavitationnucleilocatedwithinthefluid Asthevaporbubblesmovetoregionswherethevaporpressureisexceeded theycollapseorcondense Thiscondensationcanbeaccompaniedbylocalhydraulicshocksandtheformationofhighvelocitymicrojets Thistypeofcavitationisresponsibleforthemechanicaldamagethatcanoccuronship spropellers Gaseouscavitationoccursforaliquidthatcontainsdissolvedgasessuchasoxygenornitrogen Whenthelocalpressurefallsbelowthesaturationpressureofthedissolvedgas itcancomeoutofsolutionasbubbles Gaseouscavitationisadiffusionprocessandismuchslowerthanvaporouscavitation Gaseouscavitationisresponsibleforthebends ortheformationofnitrogenbubblesinthebloodofscubadiverswhoascendtooquickly WillCavitationOccur ThetendencyforaflowtocavitatecanbecharacterizedbytheCavitationNumberwhichexaminesthedifferencebetweentheinletpressureandthevaporpressurerelativetothedynamicpressurehead Ca AsCadecreases thetendencyforcavitationtooccurincreases p pV rU2 2 Cavitation Thermodynamics 1 2 Pressuredropintheliquid2 3 Vapourgeneration cavitation 3 2 Pressureincreasebecauseofvolumeincreasefollowedbycondensation3 Intermediatestatebecauseofnon equilibriumeffects h P Pv 1 2 3 3 VaporousCavitation CFD ImportantcavitationeffectstomodelinCFDModificationofthepressurefieldAbsolutepressurereductionduetoBernoullivelocity pressureeffectsshouldbeboundedbytheliquidvaporpressure effectoncomputeddrag lift etc Presenceofvaporintheflowdomainandthelocationswhereit sformedCangiveguidanceintopossibility liklihoodofdamagetosurfacesbycavitationCavitationeffectscurrentlyoutsidethescopeofCFDActualmodellingofbubblecollapseandthepressurepulseandmicrojetformationthatresultsDirectsimulationofthedamagetosurfacesresultingfromcavitationbubblecollapse InterfacialMassTransfer Interfacialarea unitvolume interfacialareadensityObtainfromAssumptiononflowmorphology spheres Modeltransportequation Masstransferrateperunitarea Interfacialarea unitvolume VapourMassGenerationRate Sphericalbubbles Rayleigh PlessetEquation Linearbubblegrowthrate neglect2ndorderterm Mechanicalforcebalance Rapidprocess thermaleffectsneglectedviscous surfacetensioneffectsneglected InterfacialAreaDensity SphericalbubblespresentatvolumefractionaV VaporizationandCondensationRates Availabilityofnucleationsitesdecreaseasvaporvolumefractionincreases neighborsitescouldbeabsorbedbyasinglebubble Amodifiedinterfacialareadensityisappliedforvaporization Empiricalcalibrationcoefficients Fvap 50 andFcon 0 01 Nucleationsitedensity nuc 5 10 4 andRadiusRB 10 6m CavitationExamples CavitationatHydrofoil 1 angleofattackCavitationatmidchordCavitationNumbers 0 34 0 43 Ca 0 34 Cavitation Hydrofoil 4 angleofattackCavitationatleadingedgeCavitationNumbers 0 84 1 0 Ca 0 84 CavitationExample PumpInducer Geometryanddata LEMFI ENSAM ParisBakiretal 2004