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1、7ComputationalFluidDynamicsWehaveexaminedsomeofthemostwidelyacclaimedandcitedcyclonemod-els.Thereisonemorewayofpredictingthe?owpattern,pressuredropandtheseparatione?ciencyincyclonesandswirltubes,however:byComputa-tionalFluidDynamics,or‘CFD’forshort.InCFD,theequationsgoverningthe?owofthegas:t
2、heNavier-Stokesequations,arewrittenina‘?nitedi?erence’form,andsolvedwiththeaidofacomputeronagridofpointsspanningthebodyoftheseparator.Theparticlescaneitherbetreatedasasortofsecond?uidinthecyclone,orasindividualparticles,whichcanbetrackedintheprecalculatedgas?ow?eld.Thegas?owintheseparatorist
3、urbulent,andthiscreatesoneproblemwhenusingCFD.Inprinciple,ifthecomputationalgridcouldbemade?neenough,CFDcouldbeusedtosolvetheNavier-Stokesequationsdirectly,andtheturbulencewouldautomaticallyariseinthesimulation.In‘directturbulencemodeling’suchCFDsimulationsarealreadybeingcarriedoutinsmall,si
4、mplegeometries,andthis?eldisadvancingfastasthecomputationalpowerincreases.However,inrealprocessingequipment,thisisnotpossibleyet.Thenumberofgridpointsandtimestepsrequiredistoohigh.Forthisreason‘turbulencemodels’arerequired.Thesearemeanttomimicthein?uenceontheturbulenceonthemeangas?owpattern.
5、Correctlymim-ickingthee?ectoftheturbulenceisespeciallydi?cultinswirling?ows,andwewill,amongotherthings,lookmorecloselyatthisissueinthischapter.Weshallturnnowfromthegas?owtothe?owoftheparticles.Theparticle?owcanbemodelledintwoways.Oneisthe‘Eulerian’approach,wheretheparticlesareconsideredaseco
6、nd‘?uid’interpenetratingthegasandinteractingwithit,inaccordancewiththeknowninteractionlaws,normallyStokes’draglaw.Theadvantageofthismethodisthat,notonlyisitrelativelyeasytoincorporatethee?ectofthegasonthe?owoftheparticles,butitisalsorelativelyeasytoincorporatethee?ectoftheparticlesonthe?owof
7、thegas(the‘coupling’betweenthetwophases).Theotherapproachis‘Lagrangian’particletracking,wherethemovementofasingleparticleisfollowed,solvingitsequationofmotionasitistrackedthroughthegas?ow1407ComputationalFluidDynamics?eld.Usingthistechnique,therefo
