A.C.Hoffmann-5Cyclone Separation Efficiency.pdf

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1、5CycloneSeparationEfficiencyInthepreviouschapterwehaveexaminedvariousmodelsforpredictingthevelocitydistributionsincyclones.Inthischapterwewishtofirstpresentabriefdiscussionoftheissues,andtothenpresentsomeoftheliteraturemodelsavailableforcomputingcycloneseparationefficiency.5.1DiscussionThere

2、arebasicallytwomodelingconceptsforcycloneseparationefficiencyintheliterature:the‘equilibrium-orbit’modelsand‘time-of-flight’models.Figure5.1.1aillustratestheconceptbehindtheequilibrium-orbitmodels.TheyconsidertheimaginedcylindricalsurfaceCSthatisformedbycontinu-ingthevortexfinderwalltothebo

3、ttomofthecyclone.ThesemodelsarebasedonaforcebalanceonaparticlethatisrotatinginCSatradiusR=1D.Inx2xthisbalancetheoutwardlydirectedcentrifugalforce1isbalancedagainsttheinwarddragcausedbythegasflowingthrough,andnormalto,surfaceCSandintotheinnerpartofthevortex.Thecentrifugalforceisproportion

4、altotheparticlemassandthereforetox3whilethe(Stokesian)dragispropor-,tionaltox(Chap.2).Largeparticlesaretherefore“centrifuged”outtothecyclonewall,andsmallparticlesaredraggedinandescapeoutthevortextube.Theparticlesizeforwhichthetwoforcesbalance(thesizethat‘orbitsinequilibrium’inCS)istaken

5、asthecyclone’sx50orcutsize.Assuch,itistheparticlesizethatstandsa50-50chanceofbeingcaptured.Thisparticlesizeisoffundamentalimportanceandisameasureoftheintrinsicseparationcapabilityofthecyclone.Asdiscussedinthepreviouschapter,allthegasvelocitycomponentsareassumedconstantoverCSforthecomput

6、ationoftheequilibrium-orbitsize.Figure5.1.1billustratestheothermodelingapproach:time-of-flightmod-eling.Here,theparticle’smigrationtothewallisconsidered,neglectingthe1Seediscussionpertainingtotheuseoftheterm“centrifugalforce”inSect.2.1905CycloneSeparationEfficiencyinwardgasvelocity.Intheor

7、iginaltime-of-flightmodel,thequestionwasposedwhetheraparticle,injectedatsomelateralpositionintheinlet,hadthetimetoreachthecyclonewallandbecollectedbeforereachingthecyclonebottom.Somelatermodelshavebeenhybridsbetweenthesetwoconcepts,consid-eringbothaninterchangeofparticlesacrossC