Table Of Content

DISPERSIONOFFLOCCULATEDPARTICLES INSIMPLESHEARANDELONGATIONALFLOWS By XUELIANGZHANG ADISSERTATIONPRESENTEDTOTHEGRADUATESCHOOL OFTHEUNIVERSITYOFFLORIDAINPARTIALFULFILLMENT OFTHEREQUIREMENTSFORTHEDEGREEOF DOCTOROFPHILOSOPHY UNIVERSITYOFFLORIDA 1998 ACKNOWLEDGMENTS Iwouldliketoexpressmysincereappreciationtothefollowingpeoplefortheirhelp andsupportthroughoutmystudyattheUniversityofFlorida: Dr. Renwei Mei, my advisor, constantly gave me his trust, advice, guidance and patience. Without his commitmenttotheproject,thisdissertationcouldn'thavebeencompleted.Dr.Roger Tran-Son-Tay, my co-advisor, offered me a great deal ofhelpful advice and expert guidance in my experimental work. Dr. James F. Klausner, as a member ofmy supervisorycommittee,gavemealotofguidanceandadvice,especiallyinmyearly experimental work. Dr. Wei Shyy and Dr. Corin Segal served as members on my supervisory committee, reviewed my proposal and dissertation, and made valuable comments.Dr.BrijM.MoudgilandDr.HassanEl-Shallgavemetheirguidancesand helps.Ms. EmmanuelleDemayandMr. PhilippeVigneronaidedmeinsomeofmy experiments.Mr.RonBrownhelpedmeinthesetupofexperimentalapparatuses.Dr.A. Zamam,Mr.J.Adler,Dr.J.S.Zhu,andDr.S.Mathurassistedmeinmakingfloesand usesomeinstruments. TheEngineeringResearchCenter(ERC)forParticleScience&Technologyatthe UniversityofFlorida,theNationalScienceFoundation(Grantnumber:EEC-9402989), andtheindustrialpartnersoftheERCprovidedthefinancialresourcesfortheproject. Inaddition,mycolleaguesandfriendsattheUniversityofFlorida,Dr.JianLiu,Dr. GuobaoGuo,Dr.HongOyang,MsHongShang,andMr.CunkoHu,providedmewith varioushelpsinmystudyandlifeinGainesville,Florida.Iamalsogratefultothestaffat theAeMESdepartmentalofficefortheirhelps. SpecialthanksaregiventoMr.DarrellD.WilliamsatBristol,Englandforhishelp andencouragementduringthepasttenyears. Lastbutmostimportant,Iamdeeplyindebtedtomywife,Jie,forherunderstanding, encouragement,patience,andlove. TABLEOFCONTENTS Pages ACKNOWLEDGMENTS ii ABSTRACT vi CHAPTERS 1. INTRODUCTION 1 1.1Background 1 1.2LiteratureReview 2 1.3ObjectivesandScope 6 2. VISUALIZATIONOFFINEFLOCBREAKUPPROCESS 9 2.1ExperimentalDevices 9 2.1.1Cone-plateDeviceandFlowDescription 9 2.1.2HyperbolicFlowDeviceandFlowCharacteristics 13 2.1.3ContractileFlowChamber 19 2.2ExperimentalDevicesandMaterials 20 2.2.1Floes 20 2.2.2SuspendingFluids 22 2.3ResultsandDiscussions 23 2.3.1FloeBreakupinCone-plateShearFlow 23 2.3.2FloeDeformationandBreakupinContractileFlow 26 2.3.3FloeBreakupinHyperbolicFlow 33 2.4Summary 37 3. FLOCBREAKUPINSIMPLESHEARFLOWANDFLOCSTRENGTH 38 3.1Introduction 3g 3.2ExperimentalProcedureandDataProcessing 39 iii 1 3.3ResultsandDiscussions 42 3.3.1VariationofFloeMasswithTimeunderaConstantShearing 42 3.3.2VariationofFloeSizewithTimeatConstantShearRates 50 3.3.3VariationofFloeSizeandSizeDistributionwithShearStress 54 3.3.4ChangeofFloeShapewithTimeandStress 58 3.4Summary 62 FLOFCLBORWECAHKAURPAICNTOERRIIFZIACTEIFOLNOW-PART1 63 4.1Introduction 63 4.2Formulation 65 4.2.1GoverningEquationandBoundaryConditions 65 4.2.2GridArrangementandNumericalSchemes 68 4.2.3ValidationoftheNumericalMethod 72 4.3ResultsandDiscussions 75 4.3.1BasicFeaturesofOrificeFlowField 75 4.3.2StrainRateCharacteristicsofOrificeFlow 79 4.3.3MaximumCenterlineVelocityGradient 81 4.3.4ComparisonBetweenaxisymmetricFlowandTwo-dimensional Flow g7 4.4Summary gy FLOCBREAKUPINORIRICEFLOW-PART2 MEASUREMENTS 90 5.1Introduction 91 5.2ExperimentalApparatusandProcedure .92 5.2.1OrificeSetupandProcedure 92 5.2.2CouetteShearDevice 94 5.2.3ParticleSizeAnalyzer 9g 5.2.4EstimateofReflocculationinCouetteFlowandOrificeFlow 102 5.3ResultsAndDiscussions 104 5.3.1EffectofFlowConditiononFloeSizeDistribution 107 5.3.2DependenceofMeanFloeSizeandMaximumFloeSize onFlowRate 11 5.3.3ComparisonwiththeResultfromUniformCone-plate SimpleShearFlow jj5 iv 5.3.4ComparisonofFloeDispersionbetweenOrificeFlow andCylindricalCouetteFlow 119 5.3.5Re-ExaminationofSonntag'sExperimentalData 123 5.3.6FloeStrengthAssessment 125 5.4Summary 129 SUMMARY 6. 131 6.1SummaryandConclusions 131 6.2SuggestionsforFutureStudies 134 REFERENCE 136 BIOGRAPHICALSKETCH 140 V AbstractofDissertationPresentedtotheGraduateSchool oftheUniversityofFloridainPartialFulfillmentofthe RequirementsfortheDegreeofDoctorofPhilosophy DISPERSIONOFFLOCCULATEDPARTICLES INSIMPLESHEARANDELONGATIONALFLOWS By XueliangZhang May1998 Chairman:Dr.RenweiMei Co-Chairman:Dr.RogerTran-Son-Tay MajorDepartment:AerospaceEngineering,MechanicsandEngineeringScience Experimentalstudiesonthedispersionprocessoffineflocculatedparticlesindifferent flowsarecarriedoutthroughvisualimageanalysesandparticlesizemeasurements.The flowsinvestigatedincludeacone-plateshearflow,acylindricalCouetteflow,anorifice contractileflow,andahyperbolicflow. Visualstudiesonthemechanismsoffloebreakupindifferentflowsarefirstconducted throughavideoimageacquisitionandanalysissystem.Avarietyofdynamicprocessesof thedeformationandbreakupoffinefloesofsizefrom3mmto30mminthecontractile flow,hyperbolicflow,andsimpleshearflowarevisualized.Thebreakupanderosion processoffloessubjectedtoaconstantshearstressinthecone-plateflowisanalyzed basedonthechangesoffloemass,size,andshapewithshearstressandshearingtime throughtheimageanalysis.Asignificantportionofthebreakup,orsizereduction,ofthe vi finefloestakesplaceupontheapplicationoftheshearstress. Floesizecontinuesto decreasethrough erosionmechanism. Theerosionrate depends on the applied shear stress,thefloesize,andthefloeshape. Anorificeflowisappliedtobreakfloesanddeterminefloestrength.Theflowfield beforeanorificeofhigharearatioisfirstnumericallysimulatedandanalyzedinorderto characterize the flow and stress field. The dependence ofthe maximum centerline velocitygradientonorificearearatioandReynoldsnumberisobtainedanditsasymptotic behaviorinhighReynoldsnumberregimeisanalyzed. The dispersion offloes in the orifice flow is analyzed based on the floe size distributionmeasuredusingaparticlesizeanalyzer.Duetotherapidriseoftheaxial velocity gradient near the orifice entrance, the floe breakup in the orifice flow is instantaneousandthefloeerosionmechanismcanbeexcluded.Thecenterlinemaximum shearstressintheorificeflowthusgivesthefloestrengthoftheresultingfloeswhose averagesizeissubsequentlymeasured.Thefloestrengthdeterminedfromtheshort-time shearinginacylindricalCouetteflowatlowershearstressesfollowsessentiallythesame powerlawdependenceonthefloe sizeasdeterminedintheorificeflow. Thus, floe strengthmeasuredindifferentflowscanbeunifiedusingthemaximumshearstressofthe flow. vii CHAPTER 1 INTRODUCTION 1.1Background Many modern advanced materials, such as electronic, magnetic, optic, and fine ceramic materials, are produced from suspensions of colloidal particles. Floes or aggregates are loose, irregular, three-dimensional clusters of particles in such suspensions.Thewords,floeandaggregateusuallyarebothusedtorefertothewet powderstructureinliquids.Highperformanceofmaterialsrequiressufficientdispersion ofthe floes in suspensions, that is, sufficientbreakup offloes into smaller floes or constituentparticles.Althoughthisdispersionprocessisactuallytheresultofanumber ofdifferentstepsincludingmilling,mixing,stirring,andsoon,hydrodynamicshearing playsanimportantroleincontrollingthestabilityanduniformityofthesuspensionsince thedispersionprocessisusuallycarriedoutinahydrodynamic environmentwithor withouttheaidofdispersants.Theflocculation(particlesizeenlargement)ofparticles and redispersion (particle size reduction) of flocculated particles take place simultaneouslyandconstantlyintheflowenvironmentofthesolid-liquidsuspension. Animportantcharacteristicoffloesistheirbindingforce,thatis,theabilityofthe aggregate structureto resist deaggregation. As ameasureofthisbinding force, floe strengthcanbedefinedasresistancetobreakupbyshearforcesinducedbyfluidvelocity gradients. Thequantitativeevaluationoffloestrengthisimportanttobothdispersion l 2 processandflocculationprocess.However,itisunderstoodthatthestrengthoffloesina suspensioncannotbemeasureddirectlyduetoitsspatiallyirregularstructureandthe randomcharacteristicinitsformationbutmustbededucedfromtheevaluationofother measurableparameters.Becausetheconceptof"strength"forfloesisalwaysassociated withtheirbreakupwhichinvolvesdifferentmechanisms,thestudyonthefloestrength shouldincludethemechanismsoffloebreakupandtheforcewhichcausesthisbreakup. 1.2LiteratureReview Thomas(1964)gavethefirstanalysisonthemechanismsoffloebreakupandfloe strength.Heproposedthatlargefloesinaturbulentflowfieldbreakintheformsofbulgy deformationandrupture.Heassumedthatthepressuredifferenceontheoppositesidesof afloecausesitsbulgydeformationandeventualruptureandthatthepressuredifference isduetotherandomvelocityfluctuationsofturbulentflow.Hisworkformedthebasis foranumberofexperimentalinvestigationstodeterminefloestrengthsincethen. BasedonThomas'modelsforfloerapturemechanismandisotropicturbulencetheory, severalexperimentalstudiesoffloebreakupinturbulentflowshavebeenconductedto determinethefloestrengthbyrelatingthefloe sizetotheturbulentflowconditions. TamboandHozumi(1979)devisedaspecialflocculatorexperimenttostudyfloestrength bymeasuringthemaximumfloediameterunderaweakagitation. Matsuo andUnno (1981)usedaturbulentpipeflowtoevaluatefloestrength.BacheandAl-Ani(1989)used averticalpulsatingwatercolumndrivenbyanoscillatingplungertorelatethefloesizeto the turbulence energy dissipation. Moudgil, Springgate, and Vasudevan (1989) experimentallystudiedthestrengthofkaolinite,dolomite,andA1203floesinastirred tank. Theresultsforfloestrengthobtainedbytheapplicationofisotropicturbulence theoryprovide some qualitative understandings offloe characteristics. However, the shearfieldisspatiallynonuniforminastirringtankandonlytheoverallmeanenergy dissipationratecanbeestimatedforflowdescriptionbasedonthepowerinput.Floe breakup andreflocculation are usuallypresent simultaneously. Therefore, the results obtained from such experiments do not suffice forthepurposes ofdetermining floe strength. Parker,Kaufman,andJenkins(1972)derivedamodelforthebreakupofcomplex activatedsludgefloesandinorganicchemicalfloesbasedonthebreakupmodeofsurface erosionsuggestedbyArgamanandKaufman(1970).Theyproposedthattheprimary particlesarestrippedfromthesurfaceofafloebyfluidshearataratethatisproportional tothefloesurfaceareaandthesurfaceshearingstress. KaoandMason(1975)andPowellandMason(1982)usedafour-rollerdeviceintheir experimentsoffloedeformationandbreakupinanelongationalflow.Thismaybethe first systematic visual work to study aggregate dispersion in fluid flows. Couette apparatushadalsobeenusedintheirstudyforthecaseofsimpleshear. Quigleyand Spielman(1977,seeLuandSpielman, 1985)conductedsimilarexperimentsforferric hydroxide agglomerates in a four-rollerdevice. It is important to note that in these experimentsthesizeofprimaryparticlesfromwhichthefloesoraggregatesaregenerated rangesfrom20umto400umandthesizeoffloesoraggregatesisabout3mm~5mm. Sonntag and Russel (1986, 1987) investigated experimentally the structure and propertiesofflocculatedsuspensionsinasimpleshearflowofcylindricalCouetteflow