CFD analysis of flow pattern and heat transfer in

ChemicalEngineeringScience61(2006)5204–5220flowpatternandheattransferindirectcontactsteamcondensationSagarS.Gulawania,JyeshtharajB.Joshia,∗,ManishS.Shahb,ChagantiS.RamaPrasadb,DayaS.ShuklabaInstituteofChemicalTechnology,Matunga,Mumbai400019,IndiabChemicalEngineeringandTechnologyGroup,BhabhaAtomicResearchCenter,Trombay,Mumbai400085,IndiaReceived1October2005;receivedinrevisedform14March2006;accepted14March2006Availableonline18March2006AbstractDirectcontactsteamcondensationusedinsteamjetinjectors,anddirectcontactfeedwaterheaters,hasbeenstudiedusingCFDsimulations.Fairlygoodagreementhasbeenobtained,withtheavailableexperimentaldataofplumelengthandtheprofilesofaxialandradialtemperature.Further,criticalanalyseshavebeencarriedoutforallthepublishedsemi-empiricalmodelsandcorrelations.Inaddition,CFDanalysishasbeenextendedtoexaminetheroleofnozzlediameterandgeometry,onheattransferphenomenonwhichgovernsthedirectcontactsteamcondensationphenomenon.Anewhydrodynamicmodelhasbeenformulatedwhichestimatestheinterfacialareaavailableforcondensation.Arationalcorrelationhasbeendevelopedfortheestimationofinterfacialarea,expressedintermsofNusseltnumber(Nu),Reynoldsnumber(Re),Prandtlnumber(Pr)andratioofviscosityofsteamandwater.
2006ElsevierLtd.Allrightsreserved.Keywords:Directcontactsteamcondensation;CFD;Hydrodynamics;Heattransfercoefficient;Phasechange;Interfacearea1.IntroductionDirectcontactsteamcondensationisusedinmanyengineer-ingapplicationssuchassteamjetinjectorsanddirectcontactfeedwaterheaters,etc.Inalltheseapplications,principalre-quirementforthedesignisapriorknowledgeofjetshapeanddimensions,whichprimarilydependuponthenozzletypeandsize,volumetricflowrateofsteam,pressureandtemperatureofsteamandwaterandtherateofcondensation.Alsoiftheequip-mentistobeprovidedwithadditionalheattransferelementsorfewotherinternals,thentheirdesigndimensionsdependuponthejetdimensions.Inthecaseofjetreactorswhichemploycondensingvapors,heattransfercoefficientdependsuponthelocalflowstructures.Further,theselectionandthedesignofimmersedheattransferareaarealsodictatedbytheflowpat-tern.Inviewoftheseimportantconsiderations,alargenumberoftheoreticalandexperimentalinvestigationshavebeenunder-takenduringthepast30years.∗Correspondingauthor.Tel.:[phone];fax:[phone].E-mailaddress:[e-mail](J.B.Joshi).0009-2509/$-seefrontmatter
2006ElsevierLtd.Allrightsreserved.doi:10.1016/j.ces.2006.03.032Thepublishedworkcanbebroadlyclassifiedintothreeob-jectives(a)evaluationoftheplumesizeandshape,(b)esti-mationofdirectcontactcondensationheattransfercoefficient,and(c)developmentofcondensationregimemap.Forthesethreeobjectives,practicallyallthetypesofflowvisualizationtechniqueshavebeenusedandtheresultsonjetdimensionhavebeenreportedintheformofsemi-empiricalcorrelations.Fortheestimationofflowpattern,alargenumberofanalyti-calaswellasnumericaltechniqueshavebeenreportedinthepublishedliterature.Inallthesecases,simplifyingassumptionshavebeenmadeanditisimportanttoassessthequantitativeroleoftheseassumptionsonthereliabilityofdesignprocedures.Furthernoneoftheauthorshavesystematicallyconsideredtherolesofnozzlediametervis-à-vistheinterfacialarea,whichplaysmajorroleonthedirectcontactsteamcondensation.Inthepresentpaper,aCFDsimulationhasbeenperformedandthepredictionshavebeencomparedwithallthepublishedexperimentaldata.Further,allthepublishedsemi-empiricalmodelshavebeencriticallyanalyzedbyundertakingtheCFDsimulationsoftherespectivecases.Asinterfacialareaisoneofthemostimportantparametersforheattransfercharacteristics,S.S.Gulawanietal./ChemicalEngineeringScience61(2006)5204–52205205CFDenablestoformulateanewhydrodynamicmodelwhichestimatestheinterfacialareaavailableforcondensation.Anat-tempthasbeenmadetodeveloparationalcorrelationforin-terfacialareabasedonNusseltnumber(Nu),Reynoldsnumber(Re),Prandtlnumber(Pr)andratioofviscosityofsteamandwater.Further,theCFDanalysishasbeenextendedtochecktheroleofnozzlediameterandgeometryonheattransferphe-nomenonindirectcontactcondensation.2.Previouswork2.1.DirectcontactsteamcondensationWheneversteamisinjectedwithveryhighvelocityinsub-cooledwater,themomentumandtheenergyofthesteamaretransferredtothesurroundingliquid,leadingtogenerationofflowpatternturbulentinnature.Theturbulentflowpatternen-hancestheheattransfercoefficientattheinterfaceofsteamandliquidaswellasattheimmersedsurfaces.Theinjectedsteamalsogeneratesasmallvaporcavity,calledplume,justnexttothenozzleexit.Whenthesteamisinjectedinitially,thetemperaturedifferencebetweentheinjectedsteamandthesur-roundingwaterisveryhigh,whichleadstofastcondensationofsteamatthenozzleexit.Transiently,thetemperatureofthebathincreasesgradually,decreasingthetemperaturedifferenceandthustherateofcondensation,leadingtoplumeexpansion.Themechanismofcondensationdependsuponavailableinter-facialarea(steam–waterinterface),amountofsteaminjectedandthebathtemperature.Thus,nozzlediameterhasbeenfoundtohaveastrongeffectonthecondensationphenomena(Younetal.,2003).Duringthepast30years,alargenumberofexper-imentalinvestigations