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1、CH-03-8-1Two-PhaseFlowPattern,HeatTransfer,andPressureDropinMicrochannelVaporizationofCO2JosteinPettersen,Ph.D.MemberASHRAEABSTRACT(Pettersenetal.1998).Geometriesusingflatextrudedmicro-channeltubeswithflowthroughanumberofparallelsmall-Theheat-transfercoefficientandpressure
2、dropwerediameterchannelsgivecompactandlightweightheatmeasuredforflowvaporizationofCO2inanextrudedmicro-exchangers,evenwithahigh-pressurefluidsuchasCO2.channeltubewith25channelsthatwere0.81mmindiameterand0.5minlength.ThetestsectionwasheatedbyawaterStudiesontwo-phaseflowandh
3、eattransferinmicro-jacket,andaregressionmethodwasusedtoderivetheinternalchannelgeometrieshavemostlybeenfocusedonlow-pressureheat-transfercoefficientbasedonthemeasuredoverallheat-refrigerantsandair/waterflowatatmosphericpressure.Eventransfercoefficient.Two-phaseflowpatterns
4、werevisualizedforconventionalfluids,theknowledgeofmicrochanneltwo-inaseparatetestrig,usinga0.98mmheatedglasstubeandaphaseflowandheattransferislimited,andveryfewresultshigh-speeddigitalcamera.Wavyannularflowwithentrain-havebeenpublishedonCO2.Owingtothelowcriticaltemper-ment
5、dominatedathighmassflux.Flowpatternobservationsature(31°C),CO2vaporizationtakesplaceatnear-criticaldidnotfitgeneralizedmaps.Heat-transfermeasurementswerepressure.At0°C,forinstance,reducedpressurewithCO2isconductedatvaryingmeanvaporfractionsattemperaturesof0.47,whileforamor
6、econventionalfluidsuchasHFC-134a,0°Cto25°C,massfluxof190to570kgm-2s-1,andheatfluxofitis0.07.Theresulting“unusual”propertiesofcarbondioxide10and20kWm-2.Heat-transferresultsshowsignificantinflu-giveheat-transferandtwo-phaseflowcharacteristicsthatareenceofdryout,particularlya
7、thighmassfluxandhightemper-verydifferentfromthoseofconventionalrefrigerants.Exam-ature.Nucleateboilingdominatespriortodryout.Frictionalplesofsuchdifferencesareamuchhigherpressure,givingpressuredropdatawerecorrelatedwithameandeviationofhighvapordensity,andlowsurfacetensiona
8、ndlowliquid16%usinganexistingmodeldevelopedforverticalflow.Heatviscosity.Two-phaseflowpat
