sourcescaleup

Scale-upDesignofUltrasoundIrradiatorforAdvancedOxidationProcess(AOP)UsingCOMSOLSimulationZongsuWei*11TheOhioStateUniversity,Columbus,OH,USA*Correspondingauthor:HI470,2070NeilAvenue,Columbus,OH,43210,USA;Phone:(614)906-8511;Fax:(614)292-3780;E-mail:wei.
187@osu.
eduAbstract:Inthispaper,COMSOLMultiphysicswasusedasatooltodesignandcharacterizeanultrasoundirradiatorwithamulti-steppedconfiguration,whichaimstoovercomedisadvantagesoftypicalirradiatorsandtoenhancecontaminantremovalinlarge-scalewatertreatments.
Inthesimulation,threedifferentphysicswerecoupledtogetherforeachcomponentofthedesignedultrasonicsystem:piezoelectricmaterialmodelfortransducer,linearelasticmaterialmodelforirradiator,andpressureacousticsmodelforreactor.
TheCOMSOLadequatelysimulatedtheacousticwavegenerationinthepiezoelectrictransducerandpropagationthroughtheirradiator.
Thesimulatedacousticpressurelevelshowsthemulti-steppedirradiatorsuccessfullyintroducedmultiplehighpressureregionsandthusmorereactivezones.
Acousticsimulationsinthewatertanksuggestedthedesignedirradiatorhasagreatcapacityforlarge-scaleAOPs.
ThesecompatiblesimulationresultstoexperimentalmeasurementsindicateCOMSOLisareliabletoolinthedesignandcharacterizationofascaled-upultrasoundirradiator.
Keywords:Ultrasound,Irradiator,Piezoelectric,Cavitation,AdvancedOxidationProcess(AOP)1.
IntroductionUltrasoundhasbeenconsideredapromisinggreentechnologyfortheadvancedoxidationprocess(AOP)sinceitaddsnochemicalstothetreatedwater.
Ithasbeenshowntoeffectivelydestroyvariousorganicandinorganiccontaminantsinwater[1].
Ultrasoundinducescavitationbubblesintheaqueoussolution,andcollapseofthosebubblesgenerateslocalized"hotspots"wheretemperatureandpressureareashighas5000Kand1000atm,respectively[2].
Inthisextremecondition,thermolysisandOH(fromwatermoleculedissociationbyheat)oxidationaretwomechanismsforthecontaminantdegradation[1,2].
AlthoughultrasoundtechnologyshowsgreatpotentialintheAOP,thecommonly-usedultrasoundirradiator(e.
g.
,horntypeinFigure1a)generatesalocalizedcavitationandnon-uniformcavitationfieldintreatmentreactors.
Theinhomogeneoustreatmentmakesitverychallengingtoscale-uptheAOPwiththetypicalirradiator[3].
Therefore,anovelconfigurationdesignofultrasoundirradiatorisnecessarytoenhanceandmaximizethecavitation-inducedchemicaleffectsforlarge-scaleAOP.
Inthedesignprocess,computationalsimulationwascommonlyusedasreferences.
Whenexpectingefficiencyandeconomicsinthedesignofanexpensivelarge-scalesystemforAOP,thecomputationaltoolseemsmoreattractivesinceitcaneasilyinvestigatedifferentreactorgeometries,irradiatorconfigurations,andultrasoundfrequenciestooptimizethedesign.
Ofthosecomputationaltools,COMSOLMultiphysicshavebeenappliedtosimulateacousticfieldandsonochemistryinreactors[4-6],whichprovidedcompatibleresultstolaboratorymeasurements.
Thedesignandcharacterizationbecomemuchsimpleandstraightforwardwiththeaidofcomputationalsimulations.
Inthisstudy,COMSOLsimulationwascarriedouttoassistanultrasoundirradiatordesignandcharacterization.
Amulti-steppedconfiguration(Figure1b)wasintroducedtobringmoreenergy-emittingsurfaceandlargecavitationvolume.
This"proof-of-concept"studywithCOMSOLsimulationstartedwiththesimplestscenario,inwhichitwasassumedthatmaterialsassignedincludingwaterandstainlesssteelwerelinearmedia.
Inaddition,anotherassumptionwasmadethatacousticwavesweretime-harmonicsincesinusoidalalternatingcurrent(AC)wasthepowersource.
Figure1.
Configurationsofatypicalirradiator(a)andthedesignedirradiator(b).
2.
MethodologyThesimulationwasestablishedto2Dsymmetricdimensionduetothesymmetricconfigurationofthescaled-upultrasoundsystemwithapiezoelectrictransducer,anewlydesignedirradiator(20kHz,2638mmindiameter,and28.
0cminlength)andawatertank(610mm*610mm*450mmindimensionand167.
5Linvolume).
Theultrasonicsysteminvolvesdifferentphysicalphenomena[3,7,8].
Thepiezoelectricmaterialinthetransducerconvertselectricalenergytomechanicalvibrationwhichpassesthroughtheultrasoundirradiatorandisintensifiedattheendoftheirradiator.
Theirradiatoremitsthoseamplifiedmechanicalwaves(ultrasoundwaves)towater,andthosewavesthenpropagateinthewatertankradially.
Therefore,threedifferentmodelswereselectedtosimulatetheultrasonicsystem:piezoelectricmaterialmodelfortransducer,linearelasticmaterialmodelforirradiator,andpressureacousticsmodelforwater.
Eachmodelisgovernedbyitsownequations.
2.
1PiezoelectricMaterialModelThepiezoelectriceffectisaphenomenonthatanappliedstressonpiezoelectricmaterialsinduceselectricpolarizationoranappliedelectricfieldinducesdimensionchangeforpiezoelectricmaterials[3,8-10].
Inthetransducer,thesyntheticceramicsofPZT(leadzirconatetitanate)providesanelectricalfieldandamechanicalfieldatthesametime.
TheelectromechanicalbehaviorsoftheisotropicPZTcanbeexpressedbytwolinearizedconstitutiveequations[7,9-11]:{{whereTisstressvector(6*1matrix),Sisstrainvector(6*1matrix),Eiselectricfieldintensityvector(3*1matrix),Diselectricfluxdensityvector(3*1matrix),cEiselasticcoefficients(6*6matrix)atconstantelectricfieldstrength,eTisdielectricpermittivitymatrix(6*3),eisdielectricpermittivity(3*6matrix),εSisdielectricmatrix(3*3)atconstantmechanicalstrain,sEiselasticcompliance(6*6matrix)inaconstantelectricfield,dTispiezoelectricstrainconstantmatrix(6*3),dispiezoelectricstrainconstant(3*6matrix),εTisdielectricmatrix(3*3)atconstantmechanicalstress.
2.
2LinearElasticMaterialModelTheparticledisplacementsgeneratedinthepiezoelectrictransduceraretransmittedtotheirradiatorsincetheyareconnectedtoeachother[7,8].
BothPZTandstainlesssteelareisotropicandelasticmaterials.
Therefore,theirlinearelasticbehaviorisgovernedbytheNewton'sSecondLaw[11,12]:whereuisparticledisplacement,σisstress,FVisforcepervolume,andeiφindicatestheAC.
2.
3PressureAcousticsModelThepressureacousticsmodelhasbeenusedtosimulatetheultrasoundpropagationinthewater.
Theacousticwaveequationisgivenasfollows[7,8,10-12]:Table1:InitialinputforthreedomainsLiquiddomainMaterialWaterρ1000kg/m3cS1418m/sIrradiatordomainMaterial1000kg/m3ρ7850kg/m3E(Young'smodulus)205E09Paν(Poisson'sratio)0.
28TransducerdomainMaterialPZT-5Hρ7500kg/m3cE(6*6matrix)[]eT(6*3matrix)[]εS(3*3matrix)[]sE(6*6matrix)[]dT(6*3matrix)[]εT(3*3matrix)[](())wherep(Pa)isacousticpressure,ρ(kg/m3)isdensityofwater,andc(m/s)isspeedofultrasoundpropagationinthewater.
Thedipolesourceq(N/m3)andthemonopolesourceQ(1/s2)arebothoptional.
Thecombinationρc2iscalledtheadiabaticbulkmodulus(Pa).
Sinceultrasoundislongitudinalwaves[13],thereisnopolarization(q=0andQ=0)[14].
Waterisassumedasanidealliquid(ρ=constantandη=0).
Therefore,thewaveequationfortheacousticpressurecanbesimplifiedto[7]:Thisequationdescribestheacousticpressureatanygivenpoint(x,y,z)andtimet.
2.
4BoundaryConditionandInitialInputThesettingofboundaryconditionsreferstoCOMSOLModelingGuideandprevioussimulationstudies[7,8,12,15-17].
Astructure-acousticboundarywassettotheinterfacebetweenirradiatorandwater[8,12].
Sinceultrasoundwavesarelongitudinalwaves,thehornsidewassetassoundhardboundaryatwhichthenormalcomponentoftheaccelerationiszero(thereisnoparticlemovementsinthedirectionperpendiculartohornaxis)[7]:(())Displacementsattheinterfacebetweenwaterandwallofthetankwasalsoconsideredaszero(u=0orP=0)assumingthetankmaterialwithalargeacousticimpedancesufficientlyabsorbedthosecomingacousticwaves.
Theparticledisplacementattheinterfaceoftransducerandirradiatorwassettobeequal[15-17].
Boundaryconditionsforsurfacescontactingairweresettofree(P=0)[12].
TheInitialvalueofelectricpotentialwassetto110V,anddefaulttemperaturewas293.
15K.
Theliquid,transducer,andirradiatordomainswereassignedtolinearwatermedia,piezoelectricmaterial(PZT-5H),andstainlesssteelmaterial(AISI4340),respectively.
TheinputinformationofthosethreematerialsissummarizedinTable1.
3.
ResultsandDiscussionFirst,inordertoevaluateandcomparetheperformanceofourirradiator,theacousticpressurelevelwascalculatedinCOMSOLforbothtypical(Figure2)anddesignedirradiators(Figure3).
Thetypicalirradiatoronlydeliversonehighacousticpressureareabelowitstip,whereasourmulti-steppeddesignbringsmultiplehighacousticpressureregionsaroundthe"edges".
Figure2.
Scatteredsoundpressurelevelsurroundingthetypicalirradiator(UnitforcolorlabelisdB).
Figure3.
Scatteredsoundpressurelevelsurroundingthedesignedirradiator(UnitforcolorlabelisdB).
Figure4.
Deformationoftransducerandirradiator(Unitforcolorlabelisμm).
Thesimulationresultsareconsistentwithlaboratoryhydrophonemeasurementsandsonochemiluminescenceimaging[18].
Sinceahighacousticpressureistheprerequisiteforcavitationresponsibleforcontaminantoxidation,thesimulationresultsinFigure3demonstratethatthedesignedirradiatorintroducedmoreenergy-emittingsurfacesandthereforemultiplereactivezones.
Fortheothertwodomainsbesideswater,theparticledisplacement(u)forthepiezoelectrictransducerandstainlesssteelirradiatoris1.
24μmatmaximumundertheappliedelectricalandmechanicalfield,showninFigure4.
Next,theacousticpressuredistributioninthewatertankwassimulatedtoevaluatethelarge-scaleapplicationwithdesignedirradiator,asshowninFigure5(2D)andFigure6(3D).
Inthesimulatedacousticfield,theredoryellowcoloralongirradiatorneckandbelowitstipalsoindicatesahighacousticpressureinthoseregions.
Atfurtherregions,ultrasoundwavespropagateinthewaterformingrippleshapes.
Acousticattenuationisalsoobservedbycolorchangingfromredtoyellow,thentolightyellow.
Themappingofacousticpressureinthewatertankindicatesthedesignedultrasoundirradiatorwithalargeradiationradius(>20cm)showsagreatcapacityforlarge-scaleAOP.
4.
ConclusionThecomputedresultshaveshowedthattheultrasoundirradiatordesignwithamulti-steppedFigure5.
Simulationofacousticpressuredistributioninwatertankin2D(UnitforcolorlabelisPa).
Figure6.
Simulationofacousticpressuredistributioninwatertankin3D(UnitforcolorlabelisPa).
configurationimprovedcavitationeffectsascomparedtotypicalirradiatorsgeneratinglocalizedcavitation.
TheCOMSOLprovidingcompatibleresultstoexperimentaldataseemstobeareliableandconvenienttoolforsuchscale-updesignofultrasoundirradiatorforAOP.
Thissimulationworkappliedtheidealconditionforallphysicalmodels.
Forexample,thesimulationresultsmayoverestimatetheparticledisplacementsforbothpiezoelectricmaterialandstainlesssteelirradiatorsinceitisassumedthereisnoenergylossforpiezoelectriceffectsandtransmittingofmechanicalenergyfromtransducertoirradiator.
Theacousticpressuredistributioninthetankissymmetricandlinearlydecreasingfromcentertoedgesduetothelinearityofwatermedia.
Actually,thehydrophonemeasurementsinthelaboratoryillustrateasymmetricanddiscretedistributionofacousticpressureduetotheacousticcavitation,wavecollision,andwatermovementbyultrasoundirradiation.
Therefore,waterviscosity,heatproduction,cavitationbubble,andmodelmodification[8,10,19]willbeaddedonebyonetocurrentsimulationtoobtainmorereliabledatainthefuturestudy.
Eventhoughthissimplestsimulationisnotanaccuratereflectionoftherealsystem,itisaworthystartingplatformandvaluablereferenceforfuturesimulationdesignwhichcanrepresenttherealsystemsetup.
5.
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