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1、4449TheoryofHeat/MassTransferAdditivesinAbsorptionChillersSatheeshKulankara,Ph.D.KeithE.Herold,Ph.D.,P.E.MemberASHRAEMemberASHRAEAnewtheoryispresented,termedthevaporsurfactanttheory,whichexplainstheheatandmasstransferenhancementmechanismofadditivesinabsorptionchillers.Thet
2、heoryholdsthattheenhancementadditivesaresurfactantsdeliveredtothesurfaceofthesolutionfilmfromthevapor,andthattheyproduceenhancementbygeneratingintensesecondaryflowsonthefilm.Thesecondaryflows,drivenbysurfacetensiongradients(Marangoniconvection),augmentthetransferprocessbyp
3、rovidingmixingofthefilm.Theprimaryvariableintheenhancementisthesurfaceconcentrationdistributionofthesurfactantadditivebecausethesurfacetensiongradi-entsariseduetonon-uniformitiesinthesurfaceconcentration.Thevaporsurfactanttheoryisfoundtoexplainallobservationsfromseveraltyp
4、esofexperiments.Thenewtheoryisasignifi-cantdeparturefrompasttheoriesinitsemphasisontheprimaryroleofthevaporintheprocess.INTRODUCTIONInalithiumbromide(LiBr)absorptionchiller,watervaporisabsorbedintoanddesorbedfromaqueousLiBrsolution.Insuchachiller,theabsorberistypicallythel
5、argestcomponentbecauseoftherelativelypoorheattransfercharacteristicsandthehighspecificvolumeofsteamatabsorberconditions.Inatypicalabsorber,aqueousLiBrsolutionflowsasafilmontheoutersurfaceofinternallycooledtubesandabsorbswatervapor.Theabsorptionisastronglycoupledheatandmass
6、transferprocessandislimitedbymassdiffusionintheliquidphase.Onecommonmethodusedtoenhancethemassdiffusionand,thus,augmenttheoverallabsorptionprocessistheuseofamasstransferenhancementadditivesuchas2-ethyl-1-hexanolor1-octanol.Suchadditives,withliquidconcentrationsintherangeof
7、100mg/kg,increasethefilmsideheattrans-fercoefficientofthecoupledabsorptionprocessuptoavaluetwotothreetimesthevaluewith-outtheadditive.ThoughenhancementadditiveshavebeencommonlyusedinLiBrabsorptionmachinessincethe1950s,themechanismbywhichtheyprovideenhancementhasnotbeenclea
8、rlyunderstood.Neitherthelimittotheenhancementprovidedbyadditivesnorthebestadditiveforthea
