气泡动力学

ComputationalFluidDynamics(CFD)ModelingofBubbleDynamicsintheAluminumSmeltingProcessKaiyuZhang,‡,†YuqingFeng,*,†PhilSchwarz,†ZhaowenWang,‡andMarkCooksey§‡SchoolofMetallurgicalEngineering,NortheasternUniversity,Shenyang,China†CSIROMathematics,InformaticsandStatistics,Clayton,Victoria3169,Australia§CSIROProcessScienceandEngineering,Clayton,Victoria3169,AustraliaABSTRACT:Thispaperpresentsamicroscalemodelingapproachforinvestigationofbubbledynamicsinthealuminumsmeltingprocess.Themotionofasinglebubblehasbeenstudiedthroughacomputationalfluiddynamics(CFD)modelfacilitatedwiththevolume-of-fluid(VOF)methodtocapturethebubbleshapes.Usingatwo-dimensionalgeometryofpartofarealcellasthetestingbed,themotionofdifferentsizedbubbleshasbeensimulatedinanair−watersystemandaCO2−cryolitesystem.Comparisonsbetweenthetwosystemsareconductedthroughthethreeperiodsofbubblemotion:bubbleslidingundertheanode,bubblereleasingattheanodeedge,andbubblerisinginthesidechannel.Itwasfoundthatbothsystemsshowsimilartrendsinbubbledynamics,suchasanincreaseinthebubbleslidingvelocityasthebubblesizeincreasesandtheappearanceofathickheadatlargebubblesizes.Quantitatively,therearedifferencesbetweenthetwosystems,evidencedintermsofthedetailedbubbledynamicsateachperiodofbubblemotion,suchasthebubblemorphology,thebubbleslidingvelocity,thebubblelayerthickness,andthebubble-inducedliquidflow.Thedetailedmicroscalemodelingprovidesusefulinformationforthedevelopmentofamultiscalemodelingmethodologybybuildingconstitutivecorrelationstosupportthemacro/processscalemodeling.1.INTRODUCTIONTheHall−Héroultprocessistheonlycommercialprocessforproducingaluminumfromalumina.1Inanaluminumreductioncell,aluminaisfedto,anddissolvedin,amoltenbathofcryoliteatapproximately970°Cinwhichseveralcarbonanodesaresubmerged.Electriccurrentisfedbetweentheanodesandanunderlyingcathodetocauseelectrochemicalreductionofthealuminareactanttoaluminumwhichsettlesontoapoollyingoverthecathode.CO2gasbubblesaregeneratedbythereactionattheanode,whichcausesrecirculationflowsasaresultofthegasbubblesmovingupthroughthemoltencryolite(thebath)undertheinfluenceofbuoyancy.Becausecryolitewilldissolvemostpotentialwallmaterials,alayeroffrozencryolitemustbeformedonthewallsofthevesseltocontainthebath,andthisrequirestheachievementofadelicateheatbalanceinthecell,overwhichtherecirculatoryflowsinthebathhaveanimportantinfluence.Thegasesaregeneratedatthebottomsurfaceoftheanodesinacontinuousmanner.Thus,theanodebottomsurfacesarecoveredwithalayerofbubblesrightbeneaththeanodebottomsurface.Thebubbleareacoveragecanvaryfrom30to90%,2−4whichleadstoanextravoltagedrop.AccordingtoHaupin,5theextravoltagedropintheelectrolyteduetothepresenceofgasbubblesisintherange0.15−0.35V.Thebubblemotionbeneaththeanodesalsointroduceswavesintothebath−metalinterface,voltagefluctuations,andhighlocalcurrentdensity,andindirectlyresultsininstabilitiesofthemagneticfield.Moreover,theglobalscalebathflowandaluminamixingarecloselyrelatedtothebubblebehavior.Therefore,adetailedunderstandingofthebubbledynamicsandtheresultingbathflowisimportanttoquantitativelyassessitseffectoncellperformance.Thehostileenvironment(hightemperatureandcorrosivemoltensaltbath)restrictsdirectobservationofbubblebehaviorinindustrialcells.Studiesofbubblebehaviorinindustrialcells,laboratorycells,andphysicalmodelshavebeenreviewedbyCookseyetal.6Thereisgoodevidencethatthebubblelayerthicknessisatleast5mminindustrialcells,5andsimilarinlaboratorycells.6−8Inordertoobservethebubbledynamicbehavioratascaletypicalofindustrialcellgeometries,roomtemperaturelaboratorymodelshavebeenused.9−28Trans-parentmaterialssuchasPlexiglasareusedtoconstructthecell,andaroomtemperatureliquidisusedtoreplacethecryolitebath.AslistedinTable1,variousgas−liquidsystemshavebeenusedtorepresenttheCO2−cryolitesystem,suchasNaOHsolution,9CuSO4solution,10air−oil−water,11orsimplyair−water.12−28Sincethebubbleformationisquitecomplexandthemotioniscontrolledbymanyfactors,suchassurfacetension,contactangle,anodeshape,andeventheroughnessofthesurface,noneofthosesystemscancloselymatchallthefactorsoftherealsystem.Thestandpointforusingtheair−watersystemisthatthekinematicviscosityofwaterisverysimilartothatofcryolite(1.005×10−6m2s−1forwaterand1.43×10−6m2s−1forcryolite).Thiswillleadtoasimilarliquidflowdynamicsaslongasthesamevolumeofgasisusedbutmightnothaverelevanceonthesimilarityofbubbledynamics.SpecialIssue:MultiscaleStructuresandSystemsinProcessEngineeringReceived:December15,2012Revised:May7,2013Accepted:May7,2013Published:May7,2013Articlepubs.acs.org/IECR©2013AmericanChemicalSociety11378dx.doi.org/10.1021/ie303464a|Ind.Eng.Chem.Res.2013,52,11378−11390Accordingtoamathematicalsimulation,22,23thebubblemorphologymainlydependsontheliquid’sMortonnumber(Mo),adimensionlessnumberdefinedas((gμl4(ρl−ρg))/(ρl2σ3)),wheregisthegravitationalacceleration,μistheviscosity,ρisthedensity,andσisthesurfacetension.Thesubscriptsgandlstandforthebubblegasandthesurroundingliquid,respectively.AsshowninTable2,theMortonnumberfortheair−watersystemisverydifferentfromthatfortheCO2−cryolitesystem.TheEötvösnumb