03-heat-exchanger

Tutorial:TurbulentFlowinaCompactHeatExchangerIntroductionThepurposeofthistutorialistoevaluatethepressuredropandtheheattransfercharacteristicsforliquidammoniaflowingthroughthespecifiedinterruptedfinheatexchangeratagivenmassflowrate.Attheendofthesession,youcancomparethecomputedvalueswiththeexperimentaldata.PrerequisitesThistutorialassumesthatyouhavesuccessfullycompletedtheintroductorytrainingfortheANSYSFLUENTsolver.ProblemDescriptionThecompactheatexchangerforthisproblemconsistsofthestaggeredfinsthatenhancetheheattransferrelativetoacontinuous-finheatexchangerbypromotingtheturbulentmixinginthewakeregionbehindeachfinandtakingadvantageoftherelativelyhighheattransfercoefficientsastheboundarylayerscontinuouslyre-formontheinterruptedfins.Asketchofthegeometry,withdimensionsinmm,isshowninFigure1.Thegeometrycontainssymmetryboundaryconditionsatthetopandbottomplanes.Figure1:HeatExchangerGeometryAssumethatpressurizedliquidammoniaat240Kistobeheatedintheheatexchangerandthatthefinwallsareataconstanttemperatureof350K.Consideringtheviscosityoftheliquidammonia,0.000152kg/m-s,andthecharacteristiclengthscaleof5.84mmforthehalf-widthbetweenthepins,theReynoldsnumberisaround9000andthustheflowisturbulent.Periodicboundarieswillbeusedatthebeginningandendingplanesofthedomainandaperiodicmassflowrateof1.385kg/swillbeused.Theperformanceofthisheatexchangerwillbeexpressedintermsofadragcoefficient,CD,andaStantonnumber,St.CDisbasedonthedragforceD,thefinsurfaceareaA,thedensityofthefluidρ,andthemeanvelocity,U:
12 Thedragcoefficient,
,representsboththeviscousandthepressuredragcomponents.TheStantonnumberdependsupontheaverageheattransfercoefficientandthethermalcapacitanceoftheflow:Preparation1.Developastrategyformodelingthisturbulentflow.Thestrategywillincludedecisionsonwhichturbulencemodeltouse,whattypeofnear-walltreatmenttouse,whattypeofmeshtouse(quad,tri,orhybrid),andhowtodistributethenodes.Besuretoestimatethefrictionvelocity,uτ,sothatyouwillbeabletospecifythepropernodespacinginthedirectionnormaltothefinwalls.SetupandSolutionStep1:Mesh1.Readthemeshfile,htx.msh.,intoa2DdoubleprecisionversionofANSYSFLUENT.Note:Thereareperiodicboundariesintheexistingmesh;however,insomecasesthesourcemeshmightnothavetheperiodicboundariesassigned.ShouldyoueverhaveasituationlikethatthenyoucanmakethecorrespondingboundariesasperiodicbyusingsomeTUIcommands:ThereferencestozonenamesandIDnumbersabovemaydifferfromyours,sinceyourboundaryzonesmightbenamedandnumbereddifferently.2.Checkthemesh.Mesh→Check3.Scalethemeshfrommicronstometers.Mesh→ScaleOneneedstousethescalingfactorsof1.e-6inbothx&ydirections.Pleasepressthe“Scale”button,checkthe“DomainExtents”fieldinthesamepanelandoncesatisfiedwiththeobtainedvaluesyoucanclosethe“ScaleMesh”panel4.DisplaytheMesh.Display→MeshThecomputationaldomainandmeshwillbecomeextremelysmallandinvisibleafterdownscalingbyafactorof1.e+6.Pleasepress“FittoWindow”buttontoseethemesh.Figure2:GraphicsDisplayoftheMeshStep2:Models1.Retainthedefaultsolversettings.2.EnabletheEnergyEquation3.Choosethek-epsilon(2-equation)turbulencemodelunderDefine→Models→Viscousa.SelectRNGk-epsilonModelfromthelistofthreeavailablek-epsilonmodels.b.ChoosetheEnhancedWallTreatmentasaNear-WallTreatment.c.Retainthedefaultvaluesfortheotherparameters.Step3:MaterialsMaterials1.Copyammonia-liquid(nh3l)fromthedatabase.Step4:OperatingConditionsDefine→OperatingConditions1.Retainthedefaultoperatingconditions.Step5:BoundaryandPeriodicConditions1.SpecifytheCellZoneConditions.CellZoneConditionsSelectammonia-liquidinthe“MaterialName”fieldforthefluidcellzoneandretainthedefaultvaluesfortheotherparameters.2.SpecifytheBoundaryConditions.BoundaryConditionsa.SettheTemperatureforwall-bottomandwall-topzoneto350K.b.Pressthe“Edit”buttonfortheperiodiczoneandensurethatatranslationalperiodictypehasbeenselected.3.SpecifythePeriodicConditionsBoundaryConditions→Press“PeriodicConditions”tab.a.Underthe“Type”,select“SpecifyMassFlow”b.SettheMassFlowRateandUpstreamBulkTemperatureto1.385kg/sand240K,respectively.Step6:Solution1.Initializetheflowfield.SolutionInitialization→Initialize...Youcanassigntheinitialvaluesasfollows:InitializedQuantityValueEvaluationMethodStreamwiseVelocity,U0.39m/sFrommassflowrate:⁄1.385/610·5.84·10!0.39m/sTurbulentKineticEnergy,k1.6×10-3m2/s2Assumingtheinletturbulenceintensityof10%:#$· 0.1·0.39 !1.52·10m2/s2TurbulentDissipationRate,ε8.3×10-3m2/s3Assumingturbulentviscosityratioof100forRe=9000:%&100%
'()*+,-./()011'!8.3·10m2/s3Temperature,T240KConstanttemperatureofoncomingflowNote:Theaboveevaluationsarenotstrictlynecessary,butthemorerealisticinitializationcanspeeduptheconvergenceratesorevenpreventthedivergenceatearlierstagesofthecalculation.2.Enabletheplottingofresidualsduringthecalculation.Solution→Monitors→Residuals…Pressthe“Edit”buttonordoubleclickinthe“Residuals–Print,Plot”fieldandthe“ResidualMonitors”panelshouldopen.Pleasemakesurethatplottingandprintingoptionsfortheresidualsareswitchedon.Inaddition,itisrecommendedtoreducetheconvergencecriteriavaluesfortheso