翻译原文.docx

翻译原文.docx

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翻译原文

Articalhistury

Received19February2014

Receivedinrevisedform11April2014

Accepted17April2014

Availableonline26April2014

Keywords:

SlugPorosityConveyingmechanisms；Horizontalpneumaticconveying；StressesPipeblockage

Movingslugsofplasticpelletswereinvestigatedin-situduringlowvelocitypneumaticconveyinginhorizontalpipelines.Slugcharacteri-

sticsincludingtheprofileofpressure,pressuregradient,particlevelocity,porosity,radialandwallshearstresses,aspectandbehaviourwerecombinedtoobtainacompletepictureofmovingslugs.Theobjectivewastogainuniqueknowledgeonthephysicalmechanismsinvolvedinslugformation,transport,anddecayandtheoccurrenceofpipeblockage.Slugsinbothstableandunstablestateswereanalysed.Astrongcorrelationbetweenparticlevelocityandwallstresseswasfound,whichsuggeststhatthestressesresponsibleforthehighpressurelosscharacterisingslugflowmayresultmostlyfromthetransferofparticleimpulsestothepipewall.Mostslugswerefoundtobedenserattherearwhereparticlevelocitywasthehighest,thusleadingtoslugshorteningovertime.ThisphenomenonwassuccessfullymodelledusingbothNewton's2ndlawandtheidealgaslawandpredictionofparticlevelocityshowedgoodagreementwithexperimentalvalues.Incontrast,otherslugswerefoundtoextendduetotheparticlesatthefrontmovingfasterthantheparticlesattherear.Pipeblockagewasfoundtoresultfrominsufficientpermeationoftheslugbytheconveyinggas,indicatingthatsufficientmaterialpermeabilityisaconditionforslugflowtooccur.

1.Introduction

Despiteincreasedinterestindensephasepneumaticconveyingsincetheseventiesandthedevelopmentofspecialdensephaseconveyingsystems,therealestablishmentofsuchsystemsforindustrialapplicationshasbeensomewhatslow.Theresistancetypicallycomesfrompractitionersforwhomtherandomperformancesoflowvelocitypneumaticconveyingsystemsresultintoohigherrisk.Infact,evenifoperatingapneumaticconveyingsystemconstitutesarelativelyeasytask,thedesignofsuchasystemisoftenproblematic.SincetheendofWorldWarII,researchteamsinbothindustryandacademiahavebeenworkingactivelytoassistdesignersofpneumaticconveyingsystemsbydevelopingdesignguidelinesfortheselectionofsystemparameterslikepressure,massflowrateandvelocityofboththegasandsolidphases.Thegoalistofurnishequipmentmanufacturersdesigntables,diagramsandequationstoaidinthedesignofnewconveyingsystems.Whilethishasbeensatisfactorilyachievedinthefieldofhighvelocitypneumaticconveyingbyintegratingfrictionfactorsasinthetransportofgasalone,thedesignoflowvelocitypneumaticconveyingsystemsandparticularlyslugflowconveyingsystemsstillremainsaproblem.Thisisbecausethecomplexphysicalmechanismsinvolvedinthetransportofhighparticleconcentrationsinagasphasehavestillnotbeenfullyunderstood.Infact,indensephase,theflowphenomenaoccurringinthepipelineareinfluencedbynotonlythegasvelocity,solidproperties,pipelinedirectionandconfigurationandsolidinthetransportofhighparticleconcentrationsinagasphasehave

stillnotbeenfullyunderstood.Infact,indensephase,theflowphenomenaoccurringinthepipelineareinfluencedbynotonlythegasvelocity,solidproperties,pipelinedirectionandconfigurationandsolidfeedingdevices,butalsotheparticle–particleandparticle–wallinteractionsthatareofgreatimportanceandshouldbetakenintoaccount.Inaddition,thetransportofsolidsbyagasstreamcancausesomeuniquephenomenathatoftenarenotencounteredingas–liquidflowsorsingle-phaseflows,liketheproductionofelectrostaticcharges,whichagainincreasesthecomplexityoftheflowphenomenaandtheirdescription.Afrequentapproachtodescribeandsystematiseslugflowpneumaticconveyingconsistsofextrapolatingthephysicalparameterscharacterisingtheconveyingprocess,suchasaveragegasvelocity,slugvelocityandpressurelosstothebehaviourofindividualslugs.However,sinceinformationsuchasnumberandlengthofslugsisusuallyunavailableandeachindividualslugisinaparticularstateofformation,stabilityordecay,generallylittleinformationcanbegained.Therefore,theconverseapproachinwhichmeasurementsperformedonindividualslugsareextrapolatedtotheentirepipelinehasalsobeenapplied.Thisapproachpresentsgreatadvantagesinthatifsufficientinformationisavailable,theactualphysicalmechanismsleadingtoslugflowandtheresultingpressurelosscanbeidentifiedandputintoequations,whichinturncanbeusedtopredictoverallslugflowdesignparameters.Someinthetransportofhighparticleconcentrationsinagasphasehave

stillnotbeenfullyunderstood.Infact,indensephase,theflowphenomenaoccurringinthepipelineareinfluencedbynotonlythegasvelocity,solidproperties,pipelinedirectionandconfigurationandsolidfeedingdevices,butalsotheparticle–particleandparticle–wallinteractionsthatareofgreatimportanceandshouldbetakenintoaccount.Inaddition,thetransportofsolidsbyagasstreamcancausesomeuniquephenomenathatoftenarenotencounteredingas–liquidflowsorsingle-phaseflows,liketheproductionofelectrostaticcharges,whichagainincreasesthecomplexityoftheflowphenomenaandtheirdescription.

Afrequentapproachtodescribeandsystematiseslugflowpneumaticconveyingconsistsofextrapolatingthephysicalparameterscharacterisingtheconveyingprocess,suchasaveragegasvelocity,slugvelocityandpressurelosstothebehaviourofindividualslugs.However,sinceinformationsuchasnumberandlengthofslugsisusuallyunavailableandeachindividualslugisinaparticularstateofformation,stabilityordecay,generallylittleinformationcanbegained.Therefore,theconverseapproachinwhichmeasurementsperformedonindividualslugsareextrapolatedtotheentirepipelinehasalsobeenapplied.Thisapproachpresentsgreatadvantagesinthatifsufficientinformationisavailable,theactualphysicalmechanismsleadingtoslugflowandtheresultingpressurelosscanbeidentifiedandputintoequations,whichinturncanbeusedtopredictoverallslugflowdesignparameters.

SomeworkersincludingRamachandranKonrad[2],Mi[3],Krull[4],Mason[5],andDaoud[6]focusedonobservingslug

flowandinvestigatingexperimentallysomeoftheslugcharacteristics,

usuallytheslugvelocityandpressuregradient.Whilesomegeneral

knowledgeonslugflowcouldbegained,becauseusuallyonlyoneor

twocharacteristicswereinvestigated,themechanicsofslugformation,

stabilityanddecaycouldnotbeidentified.Oneoftheissuesisthatbecauseslugflowisadynamicprocess,significantparameterssuchasslug

porosityandinternalstressstatesareparticularlydifficulttodetermineexperimentally,inparticularbecausethemeasurementsmustbenonintrusivetoavoidflowdisturbance.Asaresult,complexmeasuring

devicesmustbespecificallydesignedoradaptedtothisapplication.In

addition,thevariationsofporosityandstressesalongaslugorbetween

slugsaregenerallylowandtheirrecordingnecessitateshighlyaccurate

measuringdevices,whichwerenoteasilyavailableuntilthelastdecade.TheexperimentalstudiesofNiederreiter[7],Pahk[8],Lecreps[9]andNied[10]inthisareaareparticularlyrelevant.

Technologicaladvancesalsogaverisetothedevelopmentofnumericalanalysis,whichareincreasinglyappliedandpermitslugflowanalysiswithouttherequirementofpneumaticconveyingtrials.Tsuji[11],Kuang[12],Wensrich[13]andLevy[14]areamongtheauthorswhotookupthechallengeofsimulatingslugflowpneumaticconveyingusingdiverseapproachessuchasDiscreteElementMethods,ComputationalFluidDynamicsoracombinationofthetwo.Numericalsimulationshavetheadvantageofidentifyingsomeprecisephysicalphenomenathatphysicalmeasuringdeviceswouldonlypickupwithdifficulty.Inaddition,theycandeliveracompletepictureofamovingslugwithdataonlocalstresses,porosity,velocity,pressureandalsoshape.However,computersimulationsrelyonmanyassumptionsandrequirepre-calibrationofthemodels,whichusuallynecessitateexperimentaldata.Inaddition,mostsimulationworksconsidersteady-stateslugflowandashortlengthofpipe.Thus,thoseresultsareoftenmorequalitativethanquantitative.Onlyinrecentyears,simulationresultsforunstableslugflowbecameavailable[15].

Thispaperdealswithdetailedin-situinvestigationsofmovingslugs

duringpneumaticconveyinginhorizontalpipelines.Itaddressesboth

experimentalandtheoreticalinvestigationsperformedwiththeaimof

identifyingthemainphysicalmechanismsplayingaroleintheformation,transportanddecayofslugsandtheoccurrenceofpipeblockage.Inparticular,focushasbeenonthemechanismsdrivingtheflowinstabilitiesandleadingtopipeblockagethroughestablishmentofrelationshipsbetweenprofilesofpressure,porosity,particlevelocityandwallstressesalongmovingslugs.Bycombiningallthosecharacteristics,auniqueinsightintothephysicalmechanismsinvolvedinthetransportofslugsinhorizontalpipeswasobtained.Slugsinbothsteadyandunsteadystatesaswellasoccurrenceofpipeblockagewereanalysed.

2.Thirtyfiveyearsofresearchtounderstandslugflow

2.1.Flowobservation

Ramachandranwaspossiblythefirsttostudytheflowofsolid–gas

mixturesusinglongtransparentpipestoenableflowobservationin

largepipediameters[1].Henoticedthattheeaseofmove