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1、2BasicIdeasInordertounderstandthepracticalworkingofcyclones,itisnecessarytomasteranumberoftopics,whichspanarangeofdifferentdisciplines.Fluidmechanics,particularlyrelatingtoswirlingflows,particlemotioninafluid,anddifferentaspectsofparticleproperties,sucha
2、ssizeandsizedistribution,shape,anddensity,arealltopicsrelevanttothelaterchapters.Thischaptercontainsaseriesofshortdiscussionsofthesetopics.Duetothenatureofthesubjectmatter,itcanbedifficulttorecognizea‘redthread’inthischapter,buttheaccountofeachparticul
3、artopicshouldbesufficientforappreciatingthematerialinthesubsequentchapters.Literaturereferencesaregivenforthereaderwishingtostudythedisciplinesmorebroadly.2.1GasFlowThissectiondiscussessomeaspectsoffluidmechanicsthatareparticularlyrelevanttocyclonesands
4、wirltubes.2.1.1SwirlingFlowSwirlingflow,orvortexflow,occursindifferenttypesofequipment,suchascyclones,hydrocyclones,spraydryersandvortexburners.Swirlingflowalsoplaysacentralroleinthedevelopingfieldsoffluidicsandprocessintensifi-cation.Itisalsothebasisforthe
5、operationoffoam-breakingor‘defoaming’separatorsthathavereceivedsignificantindustrialattentioninrecentyears.Wederivetheequationsforthetangentialvelocitydistributionintwotypesofidealswirlingflows:1.forcedvortexflow,whichisswirlingflowwiththesametangentialv
6、elocitydistributionasarotatingsolidbody,and2.freevortexflow,whichisthewayafrictionlessfluidwouldswirl.Thetangentialvelocityinsuchaswirlissuchthatthemoment-of-momentumoffluidelementsisthesameatallradii.242BasicIdeasThetangentialvelocitydistributioninreal
7、swirlingflowsisintermediatebetweenthesetwoextremes.Inordertoderivetheseequations,westartbyconsideringtheforcesactingonafluidelementinaswirlingflow,asshowninFig.2.1.1.Weuseacylindricalcoordinatesystem(r,θ,z)(Weisstein,1999)fixedinspacewiththez-axis,theaxi
8、sofrotation,pointingoutofthepaper.Astheelementrotates,itacceleratestowardthecenter.Ifitdidnotaccel-erateitwouldcontinueinalinearpathtangenttotheorbittowardtheaxisofrotation.Thisaccelerationisthe‘centripetalacceleration’.Ifweobservetheelementfromacoor
