外文翻译--钢包浇注过程的实验与数值分析 英文版.pdf

EXPERIMENTALANDNUMERICALANALYSISOFLADLETEEMINGPROCESSGastónM.MazzaferroA,1,MarceloPivaB,2,SergioP.FerroA,3,PabloBissioB,4,MarinaIglesiasB,AdrianaCalvoB,5,MarcelaB.GoldschmitA,6.ACenterforIndustrialResearch,FUDETEC.Dr.Simini250,2804Campana,ARGENTINA.BPorousMediaGroup,EngineeringSchool,UniversidaddeBuenosAires,PaseoColón850,1063,BuenosAires,ARGENTINA.1Emailsidgsmsiderca.com2Emailmpivafi.uba.ar3Emailsidfessiderca.com4Emailpablobissiociudad.com.ar5Emailacalvofi.uba.ar6Emailsidgldsiderca.comABSTRACTInthecontinuouscasting,themoltensteelispouredfromtheladletothetundishthroughanozzlelocatedatthebottomoftheladle.Thisprocess,however,mustbestoppedbeforetheladleiscompletelyemptiedtoavoidslagcarryovertothetundish.Theamountofsteelthatremainsunteemedintheladleisusuallysignificantsosteelplantsarehighlyinterestedinstudyingdifferentwaystoimprovetheprocess.Inthepresentwork,experimentalstudiesonwatermodelsandnumericalsimulationsareusedtoanalyzetheconditionsneededforvortexformationandtheinfluenceofgeometricalandflowparametersintheamountofwastedsteel.Bothexperimentalandnumericalresultsleadtotheconclusionthatnovortexformationisexpectedtotakeplaceduringladledrainageunderindustrialconditions.1.INTRODUCTIONDuringthecontinuouscastingprocess,moltensteelispouredfromladletotundishthroughanozzlelocatedinaneccentricpositionofladlefloor.Aladleisavesselofgenerallycylindricalshapewithadiameterof23metersandaheightofapproximately3meters.Itcontainsbetween100and200tonsofliquidsteelwhicharedrainedthrougha5/10cmdiameternozzle.Moltensteelinladleiscoveredbyaslaglayerwhichthicknessvariesbetween5and20cmwhichpreventsitsoxidationbyaircontact.Asthedrainingprocessprogresses,theinterfacethatseparatesthesteelfromtheslag,eventuallydeflectstowardsthedrainagenozzleandadoptstheformofafunnel.Funnelformationleadstoslagcarryoverfromladletotundish.Thedrainageisstoppedwhenfirsttracesofslagaredetectedinthenozzle,leavingasignificantamountofsteelupto3tonsunteemedintheladle.Funnelformationeffecthasbeenanalyzedbyseveralstudies17,generallybasedonexperimentsperformedonphysicalmodelswherewaterisusedinsteadofsteel.Intheseexperimentswaterdrainagefromcylindricalorsquareshapedvesselswithflathorizontalfloorsisstudiedfordifferentnozzlediameters0.54cmto8cmandeccentricitiesupto0.7.Thesubjacentfluiddynamicsofthephenomenonhasalsobeenanalyzedbyseveralauthors814.Accordingtobibliography18twodifferentmechanismscanleadtothedeflectionofthesteelsurfacevortexsinkordrainsink.VortexsinkischaracterizedbyhightangentialvelocitiesintheneighborhoodofthenozzleseeleftschemeonFigures1and2andcanbedevelopedevenwithahighcolumnofsteelintheladle.Vortexformationcantakeplaceinvesselswherethefluidhasimportantinitialtangentialvelocitiesespeciallyifthedischargenozzleiscentered.Theamountofliquidinladlewhenvortexsinkreachesthenozzledependsoninitialrotationalvelocityandonnozzleeccentricity.Ontheotherhand,thedrainsinkischaracterizedbyradialflowrightschemeonFigures1and2anddevelopsinthelaststageoftheteemingprocess,whenlessliquidsteelisleftintheladle.Drainsinkisalwayspresentattheendoftheprocessanddoesnotdependonthepreviousformationofavortexsink.Theheightoftheliquidcolumnleftunteemedinladlewhendrainsinkirruptsinthenozzle,isapproximatelyequaltothediameterofthenozzle.Drainsinkirruptioninthenozzle,unlikevortexsinkirruption,leadstosignificantproportionofslagcarryover1,4,6.Takingintoaccounttheseconsiderationsfoundinliterature,itseemsthatduetonozzleeccentricityandtheabsenceofpreferredrotationsense,vortexsinkisnotexpectedtotakeplaceundergeneralplantconditions.Theamountofsteelusuallydowngradedinplantsduetoslagcarryoverisconsistentwiththishypothesis.Inthisworkwefirstpresentwatermodelexperiments–Section2–carriedouttoconfirmthelastassumptionbyestimatingtheprobabilityofvortexformationinactualcastingpractice.InSection3wepresentnumericalsimulationsofindustrialladledrainageprocess,withfocusondrainsinkformationandpossibleinfluenceofladlefloorgeometry.AnanalysisofexperimentalandnumericalresultsispresentedinSection4andconclusionsarepresentedinSection5.2.WATERMODELEXPERIMENTSWatermodelexperimentswerecarriedoutbythePorousMediaGroup,attheEngineeringSchoolofBuenosAiresUniversity.TheexperimentalsetupusedtoanalyzetheflowofwaterduringthedrainingprocessispresentedinFigure3.AcylindricalcontainerofdiameterD20cmispartiallyfilledwithwateruptoaconstantheightH0,throughtwotangentialpipeslocatedatoppositesidesofthecontainerbottomandat1cmofthewall.Thiswayoffillingprovidesthefluidthetangentialvelocitynecessarytoinducevortexformation.Waterisdrainedthroughanozzleofdiameterdn0.5cmandlengthhn10cmlocatedatthebottomofthevesselandatadistancerεfromitscenter.Oncethefillingprocesshasconcludedthevelocityfielddecayswithtimeuntilthefluidremainsatrest.So,wecanselecttheinitialvelocityfieldbythewaitingtime,ti,betweentheendofthefillingandthebeginningofthedrainage.AssumingthattheflowfieldisessentiallytangentialandaxisymmetricwecharacterizetheinitialflowconditionthroughVθ,definedasthemaximuminitialtangentialvelocitymeasuredatthefreesurface.Oncethenozzleisopenthemassofwaterleavingtheladle,m,iscollectedinasecondvesselwhereitsweightismonitoredasafunctionoftimewithaprecisionbalance.Inthiswaytheflowrateatthenozzleoutput,Qdm/dt,canbeobtained.ThecriticalheightHc,definedasthelevelofwaterintheladlewhenairirruptioninthenozzletakesplace,wasanalyzedasfunctionofthenozzleeccentricityε2rε/D,andinitialtangentialvelocity,Vθ.InFigure4aanexampleshowingthetimeevolutionoftheflowrateQthroughthenozzleispresented.ThefigurecorrespondstothedrainageofaH011cmwatercolumnthroughacenterednozzleε0andlargeinitialtangentialvelocity,Vθ≈2.5cm/s.Itcanbeseenthattheinitialflowrate,Qi≈32g/s,smoothlydecreaseswithtimeuntilt28s.Inthisperiodthefreesurfaceofthefluidremainsalmostflatexceptinthevicinityofthenozzleaxiswheretheformationofasmallfreesurfacedepressionordimplewasobservedafterfewseconds.ThetheoreticalflowratecalculatedfromBernoullilawwasfoundtofitquitewelltheexperimentaldataindicatingtheinviscidnatureofthedrainageprocessinthisstage.Theabruptdropintheflowrateattc≈28sindicatesthebeginningofthevortexsink.ThecriticalheightisHc8.6cmwhilethedrainedpercentageisabout22.Simultaneouslythefreesurfacedimpleevolvesdownwardtoformalongvortexfunnelwhichgoesthroughtheoutputproducingtheirruptionofairinthenozzle.Afterthiscriticaleventtheflowratemeanvaluedecreasessmoothlywithfluctuationsduetothepresenceofairinthenozzle.Inthefinalpartofthedrainage,t≈190s,itisobservedthatthefunneldisappearstheremainingcirculationisnotenoughtosustainthevortexsinkandthefreesurfaceofthefluidbecomesflat.ThisphenomenonoccursatH≈dn.Thenozzleisagainfilledwithwaterandtheflowrateincreasessuddenlyduringafewsecondsproducingasignificantdrainage.Then,thedrainsinkoccursatt198s.InFigure4banewexampleisconsideredwithidenticalconditionsasthepreviouscasebutwithamuchlowerinitialtangentialvelocitymuchlongerwaitingtime.Inthiscasetheinitialcirculationisnotlargeenoughtoproduceavortex,theladledrainswiththefreesurfacealmostflatuntilt109swhenthedrainsinkoccurs.Thedrainedpercentageisabout85.ToanalyzetheinfluenceofnozzleeccentricityweperformedexperimentsinsimilarconditionsasinFigure4aH011cmandlargeinitialtangentialvelocitybutwithtwodifferentvaluesofthenozzleeccentricityε0.5andε0.9.InFigure4ctheresultsforε0.5areshown,itcanbeseenthatavortexdevelopsatt96s.ThecriticalheightisHc1.6cmandthedrainedpercentageis86.Forε0.9theflowrateevolutionissimilarasthatshowninFigure4cthatis,novortexisobservedinspiteoftheconsiderableamountofinitialcirculation.Again,thedrainsinkoccursattheendofthedrainagewhenH≈dn.AcompletesetofresultsarepresentedinFigure5wherethecriticalheightHcisplottedasafunctionoftheinitialtangentialvelocityVθforε0circles,ε0.5downtrianglesandε0.9squares.Inagreementwithpreviousstudies18wefoundthattherollplayedbynozzleeccentricityistodelaythevortexformationandeventoinhibititforthelargestvalueofε.Thus,forthelargesteccentricityε0.9novortexwasobservedinanyoftheconsideredvaluesofVθ.Amoredetailedanalysisoftheseexperimentscanbefoundelsewhere15.InordertorelatetheresultsjustshownwithatypicalfullscalemoltensteeldrainageonecanusethedimensionalanalysisproposedbyGuthrieetal8.TheyconcludethatthedimensionlesscriticalheightHc/H0dependsonlyontwohydrodynamicsparametersaReynoldsnumberdefinedasReVoutH0/νVoutistheinitialoutputvelocity,νthekineticalfluidviscosityandthevortexnumberdefinedasKvVθR/Voutdn/2.Itwasargued6,8thattheextremelylargeReynoldsnumbervaluesreachedbothinphysicalmodellingandinactualcastingpracticemakethisparameterirrelevantinvortexformation.Thus,thevortexnumberbecomesthekeyparametertoestablishsimilaritycriterionsInFigure5itisobservedthatforε0.9novortexformationoccursevenforvaluesofVθaslargeas10cm/sKv≈3.Therefore,foraladlewithasimilarnozzleeccentricity,D200cmH0300cmanddn10cm,aninitialtangentialvelocitylargerthan60cm/sshouldbenecessaryforavortexsinktoappear.Assumingthatthefillingprocessinactualpracticeexcludesthepossibilityofhavingsuchlargeinitialtangentialvelocitieswecanconcludethatnovortexsinkispresentinatypicalfullscalemoltensteeldrainage.Consideringthepreviousdiscussion,inthefollowingsectionwepresentanumericalsimulationofafullscaledrainageprocessfocusedinthedrainsinkphenomenon.