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1、3HowCyclonesWorkAsmentionedinChap.1,cyclonesworkasaresultofthecentrifugalforcesactingontheparticlessuspendedintheswirlinggasstream.Thiscausestheparticles,whicharedenserthanthegas,tomoveoutwardtothecyclonewall,alongwhichtheyaretransporteddownwardtothedustexit.Thecleanedgasleavesneart
2、hecenterline,inareverse-flowcyclonethroughtheroof.Ina‘once-through’or‘flow-through’cyclone,thecleanedgasexitsoutthebottom1.Inthischapterwetakeafirstlookattheflowofgasandparticlesincyclones.Wealsointroducethetwokeyperformanceindicatorsforcyclones:theirseparationefficiencyandtheirpressuredr
3、op.3.1FlowinCyclonesThegasflowpatternincyclonesisfairlywellknownfromexperimentalevi-dencecollectedoverdecades.Forparticletrajectories,ontheotherhand,verylittleexperimentaldataareavailable,soforthisweshallresorttocomputa-tionalfluiddynamics(CFD)simulations.3.1.1GasFlowPatternThevelocit
4、yfieldincycloneshasbeenmeasuredusinghot-wireanemometers,pitottubesand,recently,laser-Doppleranemometry(LDA).SeeChap.10formoreinformationaboutthesetechniques.1Insomeconfigurations,cyclonesmaybeorientedatanangletotrueverticaland,inthelimit,maybeorientedhorizontally.Suchconfigurationsdono
5、tviolatetherulesandequationsgoverningverticalcyclonesexceptincertain‘highlyloaded’cyclones,whereintheforceofgravitycompeteswiththeradially-directedcen-trifugalforce,whichtheparticlesexperience.Insuchlatterconfigurationsthedesignermustbeespeciallycarefulinscaling-up.Weshallreturntothi
6、stopicinalaterchapter.463HowCyclonesWorkFigure3.1.1showsasketchofastandardreverse-flowcylinder-on-conegascyclonewithatangentialentry.Theglobalflowpatternisindicated.Aswirlingmotioniscreatedintheseparationspacebythetangentialinjectionofthegas.Thegasflowsdownwardintheouterpartoftheswirl(
7、the‘outervortex’)andupwardinthecenter(the‘innervortex’).Thedownwardflowintheouterpartofthecycloneiscriticallyimportantasit,andnotgravity,isthedominantmechanismfortransportingcollectedsolids(thoseatthewall)outthebottomofthecyclone.Inverticallyorientedcyclones,gravitywillassistbutitsin
8、fluenceisimportanton
