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木材浮压干燥过程的传热传质
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关键词 木材、真空干燥、浮压、水分移动 CharacteristicsofHeatandMassTransferinWoodDryingProcessunderFloatingPressureYiSonglin DirectedbyProf.ZhangBiguang&Prof.ChangJianming Abstract Wooddryingisgenerallyregardedasthekeyiteminwoodprocessingandutilization,thedryingprocessaccountsforapproximately40%to70%percentoftotalwood-productsmanufacturingenergyconsumption.Thereforeproperdryingisofimportancetoimprovethequalityofforestproductsandisplayinganessentialroleinusingandsavingwoodreasonably,anditscontributionstotheimprovedperformancetoproductsandcostreductionhavebeendrawinganevenincreasingattention.Usuallytheconventionaldryingmethodstakeadvantageofthetemperatureofair,relativehumidityandairvelocitytocontroldryingprocesswiththemediumofwet-air,however,thefactthattheroleofambientpressureindeterminingthemoisturetransferinwoodanditsevaporationintensityhasbeenneglected.Wooddryingunderfloatingpressureisaninnovativetechnology,whichconsiderablytakestheeffectsofambientpressureintoaccount.Comparedwiththetraditionaldryingandvacuumdryingmethods,itsambientpressurevariestoformfloatingpressurewiththedryingmediumofsuperheatedsteam.Asaresult,thefluctuationofambientpressureacceleratestherateofmoisturetransferwhilekeepswoodsurfacewet,whicheffectivelyprotecttheoccurrencesofdryingdefects.Withtheperformanceofshortdrying-cycleandgoodquality,wooddryingunderfloatingpressurehasbeenuniversallyrecognizedasanovelwooddryingmethodlightedwithcolorfuture.R&Doftheinnovativedryingprocessingandtechnologytoimprovethequalityofproductsandefficiency,especiallyinresponsetosolvingtheconflictsbetweensuppliesofwoodfailuretosatisfythedemandsofwood,hasanimportantstrategicmeaning.AlthoughdryingunderfloatingpressurehasalimitedapplicationtoindustriesinGermany,Danish,andFranceaswellasCanadarecently,anumberoffundamentalswithrespecttoprocessingarefarfromsatisfying.Atpresent,researchesonthetheoryofwooddryingunderfloatingpressurebothindomesticandoverseaarefew,don’tmentiontheheatandmasstransferofdryingunderfloatingpressure.Therefore,carryingoutthetheoreticalandexperimentalresearchondryingunderfloatingpressurehasanimportantimplicationbothintheoryandpractice.ThispaperissupportedbytheNaturalScienceFoundationofChina(GrantNo.59876005),itscontentiscloselyrelatedtotherequirementsofpractice,aimingattheproblemthatimportedhardwoodisdifficulttodryandhowtodryfastergrow-treewood.Basedonthebackgroundofdryingunderfloatingpressure,experimentalprogramiscarriedoutwhentheambientisunderanatmosphericpressurewhichisthemostuseful.Thispaperbeganwiththecharacteristicsofdryingmedium,andexploredthefundamentalsofheatandmasstransfer,thenamathematicalmodelbasedontheheatandmasstransferwasestablishedtooptimizedesignandcontrolthedryingprocess.Theresearchdevelopedaninnovativewayforwooddryingtechnologywithnewconception. Theconclusionsandoriginalitiesare:1. basedonthecharacteristicsofdryingmedium,theoreticalanalysisconcerningtheexistenceofinversiontemperaturebetweentheconditionofsuperheatedsteamundervacuum-dryingandairdryinghasbeencarriedout,developedthetheoreticalmodelofinversiontemperatureofsuperheatedsteamundervacuum,andforthefirsttimeithasbeenproventobetruethattheinversiontemperaturereallyexistundertheconditionofvacuumthroughcomparableexperienceofairandsuperheatsteam.Ifweconsiderthatthereisonlyconvection,whentheabsolutepressureofambientis0.02Mpa,thetheoreticalvalueofinversiontemperatureisapproximateto90℃,andtheexperimentalvalue,however,is80℃~85℃.Thereasonforthisdifferenceisthatweneglectedtheeffectofradiation.Thepreviousliteraturesindicatetheinversiontemperatureexistsundersuperheatedsteamdryingwithanatmosphericpressure,whenthetemperatureofsuperheatedsteamishigherthanthepointofinversiontemperature,thedryingrateofsuperheatedsteamisfasterthanthatofairdrying,butifthetemperatureofsuperheatedsteamislowerthanthepointofinversiontemperature,thedryingrateofsuperheatedsteamisslowerthanthatofairdrying.Owingtothefactthatthefloatingpressuredryinginindustriesisoperatedundervacuum,soitisofimportancetorevealwhethertheinversiontemperatureofsuperheatedsteamdryingexistsornotundervacuumisimportant,ifitdoes,wecanmakethetemperatureofdryingmediumisalwayshigherthantheinversiontemperature,thuswecanmakefulluseofthesecreditsofsuperheatedsteamdryingsuchasthebiggerconvectionheatcoefficient,lowerresistancetomasstransferandgoodquality,finallytooptimizewooddryingprocessingunderfloatingpressure.2. Forthefirsttimeanumberofexperiencesandtheoreticalanalysishavebeencarriedouttoexplorethefundamentalsofwooddryingunderfloatingpressure. l Thecomparableexperimentsfromthepreheatingprocessoffloatingpressuredryingandconventionaldryingindicatedthatheattransferincreasessharply.Whentheabsolutepressureis0.02Mpa,thetemperaturesofdryingmediumsare60℃and80℃respectively,theaverageratesofrisingtemperaturewiththedryingmediumofsuperheatedsteamis1.55and1.64timesthanthatofusingairfordryingmedium.Therationoftherateofwoodaveragetemperatureincrementwiththedryingmediumofsuperheatedsteamtothatofairillustratesthatcoupledwiththedryingmediumtemperaturerising,therisingrateoftemperatureofsuperheatedissuperiortothatofairduringthestageofpreheating;meanwhilethemoisturecontentofwoodtreatedwithsuperheatedsteamishigherthatofitsinitialmoisturecontent,whichcausedbythecondensationofwater.Thusthisphenomenonisfavorableinthewholedryingprocess.l Theeffectsofsamplesizeincludingthicknessandlengthonthedryingrateunderfloatingpressurehavebeenanalyzedthroughexperimentsandtheoreticalanalysis.Theresultsshowedsamplethicknesswithinacertainrangemakesnodifferenceonthedryingrate,butthelengthdo.Thefactillustratesthatmoisturetransfermainlyalonglongitudinaldirection.However,onethingneedtomentionistherateofmoisturemovementalonglongitudinaldirectionreducessharplywiththeincreasingoflength;simultaneouslymoisturemovementalonglateraldirectionwillbedominantincontributiontothedryingrate.Therefore,wecandrawaconclusionthatacriticalsamplesizeexists.l Resultsfromanalyzingtheeffectsofdryingmediumunderfloatingpressureonmoisturemovementsrateshow:thedryingrateoffloatingpressuredryingincreaseswiththeincreasingofdryingmediumtemperature;thereducingofabsolutepressureandtheincreasingoffluctuatingfrequency.Theircontributionstothedryingratelistasfollowingssequence:thefirstistemperature,followedbyfluctuatingpressurefrequency,andthelastisabsolutepressure.3. Thedrivingforceandthecharacteristicsoffreewaterandabsorbedwatermovementsunderfloatingpressurehavebeenrevealedforthefirsttime,andtheirmathematicequationshavealsobeenestablished.l Throughtheresearchofboththedryingrateandthechangesoftemperatureplateauandmoistureplateauunderfloatingpressure,resultsdemonstratethatwhenthetemperatureofdryingmediumis80℃,thefluctuatingfrequencyare0.75timesperhourand0.375timeperhourrespectively, thedryingratesofwooddryingunderfloatingpressureare3.24timesand2.69timethanthatofairdrying,thefactshowthattherangeof pressurefluctuationanditsfrequencyplayimportantrolesindeterminingthedryingrate.Whenthetemperatureisfixed,theexpandingrangeoffluctuatingpressureandfasterfluctuationfrequencyincreasesthechangeoftemperatureandmoistureplateau,asaresult,thetimerequiredisshortened.Howeverthesurfacemoisturecontentisapproximatetothecoremoisturecontent,thatistosaymoisturecontentisnotthedominantforceunderfloatingpressure.l Themovementoffreewatercomprisestwopartsduringtheprocessofdryingunderfloatingpressure,oneisthemasstransferdrivenbycapillarypressure,andtheotherissteamtransferduetotheevaporationandboilingoffreewatercausedbythepressurefluctuationunderpressuregradient.Furthermore,thelaterdominatestheremovingoffreewater.Essentiallytheevaporationorboilingoffreewaterincapillarycanberegardedastheheatandmasstransferwithmicro-dimension.theminimizationofdimensionmaketheheatandmasstransferaffectedbytheflow,sizeofpipeline,anditsshapeaswellassurfacestructuremoreeasily.Whenthetemperatureofwoodis80℃,theabsolutepressureofambientis0.02Mpa,theequilibriumradiusofbubblecalculatedtheoreticallyis4.68μm,however,thetracheidofsoftwoodinternaldiameteris20~30μm,obviouslythereissufficientspacetoformbubbleinliquid.Whenthewaterinbiggercapillaryreachesitsboilingpoint,itispossiblethattheintensiveevaporationincellcavityoccurs.Theconceptionofvaporspaceandboilingcanbealsousedinwooddrying.Wecanimagethatboilingwillbeginwhentheliquidwaterliesintheentrancetopits.l Themoisturediffusionbelowfibersaturationpointcanbedividedintotwoparts:thetransferscausedbysteamgradientandthepressurefluctuationrespectively.l Measuringrespectivelythemoisturediffusioncoefficientandfloatingpressurecoefficientbythemeansofdiffusiblecupundervariouspressureandanatmosphericpressure(0.1Mpa).Theircorrespondingmathematicequationshavebeendevelopedtodescribefactorsaffectingthemoisturediffusioncoefficient.Thecorrectionofrelationshipbetweenbothmoisturecontentandtemperatureandmoisturetransfercoefficients,coupledwiththeseequationsmentionedabove,wecangetthecalculatingequationsofmoisturetransfermass.Conclusionsbasedontheanalysisofregressingtheexperimentaldadashow:diffusioncoefficientdecreasesastheincreasingofabsolutepressureinlinear;increasesinawayoflogarithmwiththevaryingrateofpressure.4. Basedonthemathematicalmodelofwoodlumberdryingunderanatmosphericpressure,mathematicalmodelofheatandmasstransferunderfloatingpressurehasbeendeveloped.l Comparableresultsofthesimulationandexperimentsshowthattheymatchwellbesideshighermoisturecontent(themax.derivationvalueofmoisturecontentis8.12%),especiallywhenthescopeofmoisturecontentrangesfrom90%to12%,thedryingratecalculatedbythesimulationis2.95%perhour,andtheexperimentalvalueis3.04%perhour,thetoleranceisnomorethan5%.Mathematicalmodelcomprisesthedynamicmechanismofwooddrying,experimentshasproventhatthismodelcanpreciselypredicttheaveragemoisturecontentofMassionPine(Pinusmassoniana)underfluctuatingpressure.
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