Houston Ugm Phase Change Modeling

8/13/2019 Houston Ugm Phase Change Modeling

1/45

2011ANSYS,Inc. June21,20131

PhaseChangeModeling

Vedanth Srinivasan


2/45


qqtqqq

n

p

qpqpqqqqqqqqq

qqqmp

t ,vm,lif

1

FFFuRguuu

MultifluidConservationEquations

Continuity:

Momentum for qth phase:

The inter-phase exchange forces are expressed as:

Energy equation for the qth phase can be similarly formulated.

n

p

pqqqq

qqm

t 1u

qppqpq K uuR

transient convection pressure shear

interphase

forces

exchange

interphasemass

exchange

body external, lift, andvirtual mass forces

Volume fraction for the qth phase

Solids pressure term is

included for granular model.

PhaseChange!!


3/45


Solvesoneequationforcontinuityofmixture

Solvesoneequationforthemomentumofthemixture

Solvesforthetransportofvolumefractionofeachsecondaryphase

MixtureModelConservationEquations

r

k

r

kk

n

kkm

T

mmmmm

m

uuFguupuut

u

1eff

0

mm

m ut

).().()( rpppmpppp uut

PhaseChangesourcesadded

+Sp


4/45


Interfacialmasstransfer

Masstransferrateperunitofvolume sourcetermsinphasemass

conservationequation

secm

kg

312 iiS Am

Mass flux vector,

kg/(m2 sec)

Interfacial area

density, 1/m

Phase 1 Phase 2

Interface

12S

Mass transfer


5/45


Masstransferdefinedthroughphaseinteractionpanel

MasstransfermodelsavailablewithmixtureandEulermodels Cavitation

Evaporationcondensationmodel

Userdefinedmasstransfer

Boiling

Masstransferduetoheterogeneousreactions

Nucleationandgrowthinpopulationbalancemodels

Mass transfer


6/45


7/45


Latentheatisaccountedforwhenmasstransferis

prescribedthroughstandard

means. Latentheatiscalculatedfrom

standardstateenthalpyof

species/phaseparticipatingin

massexchange.

Materialtypemustbefluid

Beawareofvaluesofstandard

stateenthalpy only

enthalpydifferencematters. For

example,vaporenthalpy

mustbelargerthanliquid

enthalpy.

Mass transfer


8/45


CavitationModeling


9/45


Cavitationoccursinmanyengineeringdevices.

Aliquidatconstanttemperaturecanbesubjectedtoadecreasing

pressure,whichmayfallbelowthesaturatedvapourpressure Theliquidalsocontainsnoncondensablegases(dissolvedoringested)

Hydrofoils,Propellers,Inducers,Nozzles,Biomedical,

Needforcavitationmodelswhichaccountfor

Nphaseflowswithmultiphase

speciestransport.

Effectsofslipvelocitiesbetween

theliquidandgasphases.

Thermal

effects

and

compressibilityofbothliquid

andgasphases.

CavitationModels


10/45


PhysicalChallenges

PhaseChange(bubblegeneration&collapse)

Largedensityratioofliquidtovapor(e.g. water300K,theratiois4e+4)

Strongdependenceofgeometryandflowconditions

Incavitating zones,staticpressureremainsaconstant (=saturationpressure)

Turbulenceeffects

Thermalinfluence

Numericalchallenges:

Handlethelargeliquidtovapordensityratios

Dealwithcavitationmasstransferandpossiblyheattransfer

Phasictransitionswithinthedomain(vaporflooding,liquid/vaporregimes)

CavitationCharacteristics& NumericalChallenges


11/45


Cavitationzonesareprevalentlynotedinfluidpumps,valves,sharpedgedorifices,injectorsetc.

Cavitationisanundesirable(mostly)andcancause: Significantdegradationinperformance,asmanifestedbyreducedmass

flowrates,lowerheadriseinpumps,loadasymmetry,vibrationand

noise.

Physicaldamagetoadevice(duetobubbleimpactonsurfacesCavitationErosion)whichcanultimatelyaffectstructuralintegrity.

CavitationModeling


12/45


EstimationoftherateofvapourproductionisbasedontheasymptoticgrowthrateofRayleighPlessetequation

Zwartetal.Model

Schnerr andSauerModel

SinghalModel (Mixturemodelonly)

TransportEquations

cevv

v RRvt

)(

InCavitation,theliquidvapor masstransfer(evaporationand

condensation)isgovernedbythevapor transportequation

Masstransfer

due

to

growth

and

collapse

of

vapor

bubbles


13/45


Singhals FullCavitationmodel

CavitationModelCapability

vPP

vPP

l

vvl

gv

vap

PPkFRe

3

20.1,1max

l

vvl

vcond PPkFRc

32,1max

kPP satv 39.02

1

cevv

v RRVt

).(

)(

Fvap=0.02,Fcond=0.01

/solve/set/expert Singhal etal.module[No]Yes

Variableproperties

Turbulenceeffectsonsaturatio


14/45


Zwarts &Schnerrs CavitationModel

VariablePsat


15/45


Zwarts andSchnerrs CavitationModels

Cavitation Models Vapor Volume Fraction Transport Equation

Zwart-Gerber-Belamri Model Schnerr-Sauer Model

cevv

v RRV

t

).(

)(

l

v

B

vvnuc

vape

PPFR

)(

3

2)1(3

vPP

l

v

B

vv

condc

PPFR

)(

3

23

v

PP

01.0

50

105

10

4

6

cond

vap

nuc

B

F

F

m

l

v

B

lv

e

PPR

)(

3

23)1(

vPP

vPP

l

v

B

lv

c

PPR

)(

3

23)1(

1310

1

4

3

1

B

)3

41/(

3

4 33BB


16/45


17/45


ForSchnerr andZwarts model

Keepunderrelaxationforvaporto0.5orhigher

KeepDensity/Vaporizationmassto1.0 Ifcoupledsolverisused,keepcourantnumberaround200(default),onlyfor

complicatedgeometry/cases,considerreducingittotherangeof20 50

ForSinghals model

momentumrelaxationfrom0.050.4 Pressurerelaxation:0.2 0.4

Vaporizationmass:0.1 1.0

Tips/Tricks


18/45


NoneofthecavitationmodelscanbeusedwiththeexplicitVOFoptionbecausethesurfacetrackingschemesareincompatiblewiththe

interpenetratingcontinuaassumptionofthecavitationmodels.

Theycanonlybeusedforasinglecavitationprocess.

TheSinghaletal.modelrequirestheprimaryphasetobealiquidandthesecondaryphasetobeavapour.

Singhals modelisonlycompatiblewiththemultiphasemixturemodel.

TheSinghaletal.modelisnotcompatiblewiththeLESturbulencemodel.

LimitationofCavitationModels


19/45


WallBoilingModels


20/45


Threeboilingmodelavailable:

RPIBoilingmodel Applicabletosubcoolednucleateboiling

NonequilibriumBoiling ExtensionofRPItotakecareofsaturatedboiling

CriticalHeatFlux ExtensionofRPItotakecareofboilingcrisis

Contours of vapor volume fraction

in a nuclear fuel assembly

Boiling Model Options


21/45


BoilingModels

Boilingmodels:

RPIboilingmodel

Nonequilibriumboiling

CriticalHeatFlux

InterfacialAreaandBubbleDiameter

AlgebraicformulationsandUDFoptions

IACequationcompatiblewithboilingmodelsInterfacialTransfermodels

Arangeofsubmodelsfordragandlift,and

turbulentdispersion

Liquid/vaporinterfaceheatandmasstransfermodels

Flowregimetransitions frombubblytodroplets

Contours of vapor volume fraction

in a nuclear fuel assembly

Current ANSYS

Capabilities

Transitional

or Unstable Film

boiling

Critical

Heat

Flux Minimum

Heat Flux

StableHea

tFlux

Wall Superheat (Twall - Tsat)

Subcoole

d

Nucleate

boiling

Sa

turated

Single

Phase

3.0V


22/45


RPIWallBoilingModel

Tw

Tbulk

Tsat

bwdBubble nucleating site

Departing bubbleHeated wall

Thin superheated layer

Subcooled boilingoccurswhenwallandthinliquidboundarylayer

havetemperaturehigherthansaturationtemperatureatlocal

pressure,i.e.,Superheated

Ex:Heatexchangers,Heatedpipeflows,Coolingjackets,Quenchheat

treatment..


23/45


ThemodelwasdevelopedatRensselaerPolytechnicInstituteandiscalledRPImodelforsubcooled boiling

ThemodelisimplementedwithinEulermodel

Itincludesatleasttwophases:liquid(primaryorcontinuousphase)andvaporbubbles(secondaryordiscretephase)

TheRPIWallBoilingModelwasdevelopedduetothefailureofpurelysinglephaseconvectiveheattransferforwallboiling.

RPIaccountsforsteambubblegenerationaswellasquenchingeffects:

Nucleation,bubblegrowthanddeparturefromthewall

KeystepsintheRPImodel

Partitionthewallheatfluxintothreecomponents.

Closureofeachcomponentusingsubmodels

Canbeusedwithdifferentboundaryconditions.

RPI Wall Boiling Model


24/45


Activating/WorkingwithRPIBoilingModel


25/45


DecidewhichBoilingmodeltochoose IfTbulkbelowTsat Subcooled flow

UseRPIwallboilingmodel

IfTbulkcloseto(within3K)Tsat Saturatedflow Usenonequilibriumwallboilingmodel

ForCriticalHeatFlux/Burnout/DeparturefromNucleateBoiling(DNB)situation

Usecriticalheatfluxmodel

CommonmistakesinBoilingModel EnsuregravityisONtoseeanyheattransfer

Ensuresurfacetensionisspecified

Neededfornucleationandgrowthofbubbles

Ensurecorrectphasesinmasstransfermechanism

BoilingModels Tips&Tricks

VaporVolumefraction

Vertical location(m)

dia

meter=15.4mm

length=2000mm

Subcooled

water

Gravity

Bartolemei & Chanturiya Validation


26/45


Solutionstrategies UselowerenergyURF(~0.6)

Y+forRPIcanbe>30(standardwallfn apply)

Youcanrunboilingcalculationinsteadystate

CangoforcoupledsolverwithratherlowerCourantnumberslike10orevenlessattimesorpseudo

transientsolver

Ifyoufaceproblemswithcoupledsolvers,lowertheexplicitrelaxationfactorsforbothpressureand

momentumto0.75orevenupto 0.5.

Ifcoupledsolverdoesnotworkatall,goforSIMPLE.

Ifsteadycalculationiscausingissues,checkifthe

reverseflowiscausinganytrouble. Ifyouaresurethatthereisnoreverseflowinthefinal

solution,specifyreverseflowquantitiessuchthatthey

helpconvergence.

BoilingModels Tips&Tricks

VaporVolum

efraction

Vertical location

(m)Bartolemei & Chanturiya Validation


27/45


Evaporation/Condensation


28/45



Evaporation/Condensationisasurfacephenomenaunlikeboiling(volumetric)

Condensationisthetransformationofasubstancefromvaportoliquidresultingfromenergyremovalfromthevaporphase.

Incondensationprocesses,thevaportemperatureisatorbelowthesaturationtemperature.

Evaporationisthetransformationofasubstancefromliquidtovaporresultingfromenergyaddition.

Condensationoccursinvariousmodes.

Dropletformation

in

vapor

Liquiddropletformationonacooledsurface

Liquidfilmcondensationonacooledsurface


29/45


Mechanisticmodelwithaphysicalbasis

Evaporation/CondensationModel

lvvlvv

v

mmvt

)(

Tl>Tsat

sat

satvvvclv

sat

satlllcvl

T

TTm

T

TTm

Tv


30/45


Evaporationcondensationmodel

Positivemasstransferrateisdefinedas

being fromliquidtovapor.

Saturationtemperaturecanbeprovidedas

functionofpressure.

Ifsaturationtemperatureisafunctionof

othervariablesuchasvolumefraction,

pressureandothersolutions,aUser

DefinedFunctions(UDFs)maybe

necessarytodefinetheentirephase

changemechanism

Mass transfer


31/45


Masstransferfromliquidtovapor

SpecifyLatentHeatasStandardStateFormationEnthalpy Standardstateenthalpyofvapor=latentheat(inj/kgmolunits)

Standardstateenthalpyofliquid=0

Samemolecularweightforliquidandvapor

Referencetemperature=298.15K

Calculationstrategy UsecoupledsolverwithlowCourant

numbers

Lowertheexplicitrelaxationfactors

forpressureandmomentumto0.5

Ensurereverseflowvolumefraction

properlydefinedatoutletboundaries

EvaporationCondensation Tips&Tricks


32/45


Tuningevaporationandcondensationfrequency

Comparethenumericalresultswithexperimentalresults

Usesimplecalculationtoestimateevaporation Evaporationexpected=(Htotal Hsensible)/LatentHeat

Adjustevaporation/condensationfrequencies (0.001 100)

InEvaporationCondensationModel,departurefromsaturationdeterminestherateofmasstransfer

(TcellTsat)isthedrivingforce

Formasstransfertohappen,Tcell>or


33/45


Dropletformationandvaporization(inpipelines,mixingTs,Valves,Pumpscompressingandexpandinggases)

Volatilephasebehavior(evaporation/condensationintanksstoringvolatilecompoundsthatchangephasebasedonstoragepressure)

ModelingFlashingbothusingCavitationmodel(Pdriven)andalsousingEvaporation/Condensation(Tdriven,superheatedflows)

L.N.GFlashing CryogenicLiquidFlashing

RefrigerantFlashing

FuelTankflashing

FlashTank

PhysicsModelingusing



34/45


WetSteamModel


35/45


Duringtherapidexpansionofsteam,acondensationprocesswilltakeplaceshortlyafterthestatepathcrossesthevaporsaturationline.

Theexpansionprocesscausesthesuperheateddrysteamtofirstsubcoolandthennucleatetoformtwophasemixture

Theformationofliquiddropletsinahomogeneousnonequilibrium

condensationprocess,isbasedontheclassicalnonisothermalnucleationtheory.

Assumptions

Thevelocityslipbetweenthedropletsandgaseousphaseisnegligible.

Theinteractionsbetweendropletsareneglected. Massfractionof thecondensedphaseissmall(


36/45


37/45


WetSteamModel

Ivt

vt

v

1

TTCRTh

Pdtdr

eM

qI

op

llv

TK

r

ml

vc b

21

2

2

1

3

4

3

22

Nucleationrate

Dropletradius

(growthrate)

Mixturedensity

MassFractionTransport

NumberdensityTransport

Droplettemperature

NonEquilibriumCondensationProcess

LoadMaterialbyTextCommand:Define/models/multiphase/wet steam/compileuserdefinedwetsteamfunctions


38/45


39/45


Adjusttemperaturelimits,minimumof273K

Makesuremaximumwetnessfactorisnotbeyond0.2sincethepresent

modelassumeslowwetnessfactor Withwetnessfactor,>0.1,solutionbecomeslessstable

Forwetsteammodels,solveflowsolutioninitiallywithoutcondensationandoncepropersolutionisachieved,switchoncondensation

SwitchingoffcondensationcanbedonebydeselectingWetsteamequationsinthesolutioncontrolpanel

SolutionStrategiesforWetSteamModel


40/45


Melting/Solidification


41/45


SolidificationandMelting

Solidificationisthetransformationofasubstancefromliquidtosolid

Temperaturedecrease(morefrequentlyencountered)andchangeofstateoccursatthefreezingpoint

Pressureincrease(inthiscasetemperatureremainsconstant)

Thesolidificationprocessstartswithsmallsolidnucleationintheliquidthatincreasesinnumberwithtime(untilliquidiscompletelysolidified)

Application Casting,Crystallization..

Meltingisthetransformationofasubstancefromsolidtoliquid

Temperatureincreases,thechangeofstateoccursatthemeltingpoint

Ingeneral,themeltingpointisrelativelyinsensitivetopressure

Application Decrystallization in

pipes

under

low

temperatures,

Deicing

windshields..

Freezing&meltingpointareoftenequal(certainmaterialscanhavedifferentvalues)


42/45


43/45


Melting/SolidificationModeling

Procedure Possibletomodelmeltingorsolidificationinasinglephaseorin

multiplephases

Forphasesthatarenotchangingphase,setLatentHeat,Liquidus andSolidusTemperaturetoZero

ModelingThermal&Solutal Buoyancy

ThermalandSolutal Buoyancy

optionavailableonlywhensolving

thephasechangeproblemwith

SpeciesTransport

TUIdefine/models/solidification-melting? yes

Include Thermal Buoyancy? yes

Include Solutal Buoyancy? yes

Use reference mass fraction of solutes? yes

Reference mass fraction of the species-i "value"


44/45


Recommendations

MushyZoneParameter:Valuerangesaround104 to107.Highervalueswillassistin

bringingdownlocalcellvelocitytozeroasmaterialsolidifiesbutmaycause

oscillationsinsolutions.

Notnecessarytousepullvelocitywithinthesolution moreimportantwhenmodelingcontinuouscastingwherevelocityboundaryconditionsarespecified

Convergencedifficultiescanbeexpectedinsteadystatecalculations,continuous

castingsimulations,simulationsinvolvingmulticomponentsolidification,and

simulationswherealargevalueofthemushyzoneconstantisused.Inthatcase,considerreducingliquidfractionupdateinthesolvercontrols


45/45


UserDefinedFunctions

DEFINE_CAVITATION_RATE(ModifyCavitationSourceterms)

DEFINE_BOILING_PROPERTY(Modifyparameterslikebubbledepature diameter,

frequencyofdeparture,nucleationsitedensity,areacoefficient&liquidref

temperature)

DEFINE_LINEARIZED_MASS_TRANSFER(Userdefinedmasstransfersourceterms)

DEFINE_SOLIDIFICATION_PARAMS(modifymushyzoneandbackdiffusionparameters)

Houston Ugm Phase Change Modeling

Documents

Transcript of Houston Ugm Phase Change Modeling