大气污染控制工程Lecture 82 The nature of Particulate Pollutants

Prof.JiakuanYangHuazhongUniversityofScienceandTechnologyAirPollutionControlEngineeringLecture8

ThenatureofParticulatePollutantsParticulatepollutants

(themostcommonairpollutants)VolatileOrganicCompounds(VOCs)SulfurOxidesNitrogenOxidesThefourmajorairpollutantsLecture8

ThenatureofParticulatePollutantsNonhomogeneouspollutantsParticulatepollutantsarenotchemicallyuniform,havedifferentsize,shape,andchemicalcompositions.Lecture8

ThenatureofParticulatePollutantsⅠDiameterofparticlesⅡPrimaryandsecondaryparticlesⅢForcesactingonparticlesⅣTerminalgravitationalsettlingvelocityⅤBehaviorofParticlesintheAtmosphereⅥSummaryⅣTerminalgravitationalsettlingvelocity(1)ForcesactingonparticlesinafluidAssumingasingleparticle,notheelectrostaticandVanderWaals.Forcesbetweentheparticleandotherparticlescanbeignored.buoyancygravityⅣTerminalgravitationalsettlingvelocitybuoyancyweightDragforcea(1)ForcesactingonparticlesinafluidParticlefallsdown,gettinganaccelerationv:v0Dragforcetakesplace.Initially,v=0ⅣTerminalgravitationalsettlingvelocity(1)ForcesactingonparticlesinafluidbuoyancygravityDragforceaμ

theviscosityofthefluidAccordingtoNewton'slawforviscosityⅣTerminalgravitationalsettlingvelocity(1)ForcesactingonparticlesinafluidThedragforceincreasesasthevelocityincreasesuntilitequalsthegravityminusthebuoyancy.Atthistime,thevelocityisterminalsettingvelocity,thesumoftheforcesactingiszero,sotheparticlecontinuestomoveataconstantvelocity.ⅣTerminalgravitationalsettlingvelocity(1)ForcesactingonparticlesinafluidWhydragforceisnotconsideredintheairparcelmodel?ⅣTerminalgravitationalsettlingvelocity(1)ForcesactingonparticlesinafluidStokes’lawSoρpartcanbeignored,buoyant0(2)theassumptionsofstokeslaw①Thefluidiscontinuous.④thetermsinvolvingvelocitiessquaredarenegligible.③Newton’slawofviscosityholds②Theflowislaminar.ⅣTerminalgravitationalsettlingvelocity(3)Whyareweinterestedinsettlingvelocity?ⅣTerminalgravitationalsettlingvelocityWhatistherelationshipbetweenvtanddiameterofparticles?Whatistherelationshipbetweenvtandairpollution?(3)Whyareweinterestedinsettlingvelocity?1000

μ

Vt=600cm/s1μVt=0.006cm/s10μVt=0.6cm/sP210,Figure8.1(3)Whyareweinterestedinsettlingvelocity?Vtu=VerticalwindvelocityForsmallerparticles,u>Vt

Suspendingintheatmosphereforalongtime.Forlargerparticles,u<Vt

Particleswillsettletotheground,notcausingairpollution.(4)ParticlestoolargeforStokes’LawⅣTerminalgravitationalsettlingvelocity①theflowoffluidaroundthespherebecomesturbulent.②velocitiessquared≠0Thentheprincipalassumptionsofstokes’lawbecomeinapplicable.(4)ParticlestoolargeforStokes’LawⅣTerminalgravitationalsettlingvelocity雷诺数ReynoldsnumberReynoldsnumberforaparticleisadimensionlessratiooftheinertialforceactingonamassoffluidtotheviscousforcesactingonthesamemassoffluidinthesameflow.(4)ParticlestoolargeforStokes’LawⅣTerminalgravitationalsettlingvelocityReynoldsnumberstokes’lawworkssatisfactorily.When,When,flowbecomesturbulent.Laminarflow(4)ParticlestoolargeforStokes’LawⅣTerminalgravitationalsettlingvelocityDragcoefficient阻力系数Ap=surfaceareaofparticle.(4)ParticlestoolargeforStokes’LawⅣTerminalgravitationalsettlingvelocityDragcoefficient(4)ParticlestoolargeforStokes’LawAttheterminalsettlingvelocity,ⅣTerminalgravitationalsettlingvelocitySettlingvelocity(4)ParticlestoolargeforStokes’LawⅣTerminalgravitationalsettlingvelocitySettlingvelocitywhenRp<0.3,Stokes’law(4)ParticlestoolargeforStokes’LawⅣTerminalgravitationalsettlingvelocitySettlingvelocityWhen0.3≤Rp≤1000(5)ParticlestooSmallforStokes’LawⅣTerminalgravitationalsettlingvelocityContinuousmediumassumptionbecomesinaccurate.Realgases,liquids,andsolidsarenottrulycontinuousbutaremadeupofatomsandmolecules.Whenaparticlebecomesassmallasorsmallerthantheaveragedistancebetweenmolecules,thenitsinteractionwithmoleculeschanges.(5)ParticlestooSmallforStokes’LawContinuousmediumassumptionbecomesinaccurate.λ=meanfreepathofgasmolecule,≈0.07μmA=anexperimentallydeterminedconstant,1.7281+Aλ/DCunninghamcorrectionfactor(5)ParticlestooSmallforStokes’LawⅣTerminalgravitationalsettlingvelocity(5)ParticlestooSmallforStokes’LawⅣTerminalgravitationalsettlingvelocityPage227,Table8.1CalculatedapplicabilityrangeofStokes’law,alone,andwithsimpleformoftheCunninghamcorrectionfactor.FormulaPermissionerror10%1%Stokes’lawStokes’lawwithC16<d<30μ1.6<D<70μ0.36<d<30μ0.1<D<70μ(6)Stokes’StoppingDistanceⅣTerminalgravitationalsettlingvelocityHowfardotheparticlestravelbeforeitisstoppedbyviscousfriction.V0FdHorizontalmotion(6)Stokes’StoppingDistanceⅣTerminalgravitationalsettlingvelocityinertialforceviscousforces(7)AerodynamicParticleDiameterStokes’lawStokes’lawwithCunninghamcorrectionfactorStokes’StoppingDistance(7)AerodynamicParticleDiameterMicrons,aerodynamicForexample,Page226(8)DiffusionofParticlesⅣTerminalgravitationalsettlingvelocity=Diffusivity,m2/sk=BoltzmannconstantC=CunninghamcorrectionfactorⅤBehaviorofParticlesintheAtmosphere0.0020.0112100.1100Particlediameter,μ

D(surface)/d(diameter)HotvaporcondensationagglomerationChemicalconversionRainoutandwashoutMechanicalgenerationGravitysettlingⅤBehaviorofParticlesintheAtmosphereProductionprocessSizerange0.05~0.1μ

0.1~1μ

2~100μ

condensationChemicalconversionMechanicalgenerationtypessecondaryprimarystateLiquidortarsecond