Cavitation-CFD-using-STAR-CCM+

CavitationCFDusingSTAR-CCM+ofanAxialFlowPumpwithComparisontoExperimentalDataEdwardM.Bennett,Ph.D.VicePresidentofFluidsEngineeringMarch17,2014TheProject•MechanicalSolutions,Inc.(MSI),anengineeringconsultancy,wasapproachedbyamajorpumpmanufacturertoundertakearedesignofalineofaxialpumps•AclassofaxialpumpsrequiresredesigntoachievereducedNetPositiveSuctionHeadRequired(NPSHr)•ThecustomerwishestovalidateSTAR-CCM+againsttheexistingconfigurationbeforeproceedingwithredesign•AnInternalResearchandDevelopmentEffortwasundertakentoexaminetheefficacyofthecavitationmodelinSTAR-CCM+•Theaxialpumpconfigurationwasanalyzedusingseveralturbulencemodelsandassociatedcavitationparameters•Asecondcomplextestconfigurationforwhichexacttestconditionswereknownwasalsoanalyzed•ConclusionsweremaderegardingSTAR-CCM+capabilitytoresolvecavitationproblemsincomplexpumpconfigurations2Definitions•Thephenomenonofcavitationoccurswhenthenetpressureinthefluiddecreasesbelowthevaporpressure,e.g.3170Paforwaterat25°C•Thenetpressureinthefluidisafunctioninletpressure,whichiscommonlystatedintermsofhead,i.e.NetPositiveSuctionHead(NPSH)•NetPositiveSuctionHeadAvailable(NPSHa)istheactualfluidenergyattheinlet,definedasthedifferencebetweentheinlettotalheadandvaporpressureexpressedintermsofhead•NetPositiveSuctionHeadRequired(NPSHr)istheNPSHpointatwhichthepumpperformancedropsbelowsomeacceptablelevel,oftendefinedasapointatwhichthetotaldynamichead(TDH)producedbythepumpdropsby3%•CavitationcanthusbereducedbyincreasingNPSHaviainletconditionsordecreasingNPSHrviageometrymodifications3BreakdownCurve•Theoccurrenceofcavitationisvisuallypresentedviathebreakdowncurve,whereNPSHaisplottedagainstTDH•AsNPSHaislowered,theonsetofcavitationismarkedbyadropinTDH–thevalueofNPSHaatthe3%droppointdefinesNPSHr4AxialPumpConfiguration5AxialPump–FlowpathGeometry6InletPropellerPipeFlowpathMeshinSTAR7MeshDetails8MeshStatistics9DomainVertexCountCellCountInlet107,15426,848Propeller2,095,119796,104Pipe624,174230,902TOTAL2,826,4471,053,854AxialPump–ModelSetup•Realizablek-εturbulencemodel•Segregatedflowsolver•2nd-orderconvectionscheme•Multi-phaseVolumeofFluid(VOF)model•Rayleigh-Plessetcavitationmodel•Boundaryconditions:-Variableinlettotalpressureviapressurereferencepoint-785rpmrotatingspeed-755.236kg/sinletandexitmassflow(12000gpm)•Transienttimestepof2.123e-4s(360perrev)•15-20iterationsperstepIMPORTANT:MSIdidnotreceiveanydetailsregardingtestconditions,suchastemperatureorexperimentalsetup10AlternativeSetups•SSTk-ωturbulencemodel•SSTmodelwithdoubledcavitationseeddensity(2e12/m3)•Spalart-Allmarasturbulencemodel•Finermesh(remeshedwithallmeshersizingvalueshalved)11FlowfieldandPressureContours12Streamlines13VaporFractionContours14InletTotalPressure–137.9kPaInletTotalPressure–82.74kPaInletTotalPressure–55.16kPaInletTotalPressure–37.92kPaCavitationBreakdownResultsNssNPSHa[ft]InletTotalPressure[psi]OutletTotalPressure[psi]TotalPressureRise[psi]TDH[ft]TDHDrop[%]485646.220.47427.2066.73215.594.1%524841.618.50225.6547.15216.5100.0%573936.916.47523.3036.82815.795.5%633732.414.49421.7357.24216.7101.3%712127.712.47119.6237.15216.5100.0%816423.110.47017.5587.08716.399.1%883420.89.47116.5247.05316.398.6%964918.58.47115.6257.15416.5100.0%1066516.27.47014.5977.12716.499.7%1127015.06.97213.7746.80215.795.1%1196213.96.47513.0536.57815.292.0%1275412.75.98212.1756.19314.386.6%15MainModelCavitationBreakdownResultsNPSHa[ft]TDH[ft]NPSHa[ft]TDH[ft]NPSHa[ft]TDH[ft]NPSHa[ft]TDH[ft]46.215.746.215.546.215.746.215.634.714.834.615.334.615.134.616.823.115.623.115.630.015.723.115.918.515.518.515.323.116.018.516.116.216.016.215.418.515.217.315.415.114.715.114.916.215.416.215.312.812.313.913.715.014.915.015.012.811.913.914.112.712.312.813.516AlternativeModels:SSTModelSSTmodel–doubleseeddensitySpalart-AllmarasmodelrKEmodel–finermeshTurbulenceModelResults17SeedDensityResults18MeshRefinementResults19AxialPumpCFDConclusions•STAR-CCM+performedwellinpredictingthetrendofthecavitationbreakdown•Furthermeshrefinementmaybringresultsevenclosertodata•Turbulencemodeldidnotgreatlyimpacttheresults•Bubbleseeddensitydidnothaveamajorimpact•MSIdidnothaveaccesstotheexperimentalrigsetupandthiscouldhaveadditionaleffectonresultsAdditionalTestCase•AnadditionaltestcasebecameavailabletoMSI•Acomplexdoublesuctionpumpwasmadeavailablewithcompletedataregardingthecavitationdata•Thedataincludedfluidtemperature,sopreciserepresentationsoftheliquidandvapordensitycouldbeappliedintheCFDmodelDouble-SuctionPump–Drawing20Double-SuctionPump–TestData21Double-SuctionPump–Mesh22MeshStatistics23DomainVertexCountCellCountSuction4,898,6381,459,107Impeller6,945,7062,639,844Volute2,929,837911,935TOTAL14,774,1815,010,886Double-SuctionPump–Setup•SSTk-ωturbulencemodel•Segregatedflowsolver•2nd-orderconvectionscheme•Multi-phaseVolumeofFluid(VOF)model•Rayleigh-Plessetcavitationmodel•Boundaryconditions:-Variableinlettotalpressureviapressurereferencepoint-996rpmrotatingspeed-150.477kg/sinletandexitmassflow(1088.5m3/hr)•Transienttimestepof1.675e-4s(360perrev)•20iterationsperstep24VelocityFlowfield25