暖通专业英语论文.docx

暖通专业英语论文.docx

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暖通专业英语论文

JournalofHeatTransfer,Vol.125,No.2,pp.349–355,April2003

©2003ASME.Allrightsreserved.

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TheEffectsofAirInfiltrationonaLargeFlatHeatPipeatHorizontalandVerticalOrientations

M.Cerza

B.Boughey

USNavalAcademy,MechanicalEngineeringDepartment,Annapolis,MD21402

Received:

July12,2001;revised:

May13,2002

Inthesatelliteorenergyconversionindustriesflatheatpipesmaybeutilizedtotransferheattothethermalsink.Inthisinvestigation,alargeflatheatpipe,1.22m×0.305m×0.0127m,fabricatedfrom50milMonel400metalsheetsandMonel400screenswasvideographedathorizontalandverticalorientationswithaninfraredvideocamera.Theheatpipeevaporatorsectionconsistedofa0.305m×0.305marea（oneheatedsideonly）whilethesideoppositetheheatedsectionwasinsulated.Theremainingareaoftheheatpipeservedasthecondenser.Inthehorizontalorientationtheheatedsectionwasonthebottom.Intheverticalorientationtheevaporatorwasalignedbelowthecondenser.Thesequenceofphotographsdepictsheatinputsrangingfrom200Wto800W,andtheeffectofairinfiltrationonheatpipeoperationforbothorientations.Forthehorizontalorientation,theairisseentorecedetowardsthesmallfillpipeastheheatinputisincreased.Fortheverticalorientation,theairandwatervaporexhibitabuoyantinteractionwiththeresultthattheairpresenceinhibitsheattransferbyrenderingsectionsofthecondensersurfaceineffective.Theeffectsdepictedinthispapersetthestageforfutureanalyticalandexperimentalworkinflatheatpipeoperationforbothnormalandvariableconductancemodes.

ContributedbytheHeatTransferDivisionforpublicationintheJOURNALOFHEATTRANSFER.ManuscriptreceivedbytheHeatTransferDivisionJuly12,2001;revisionreceivedMay13,2002.AssociateEditor:

G.P.Peterson.

Contents

∙Introduction

∙FlatHeatPipeFabrication

∙ExperimentalInvestigation

∙ExperimentalResultsandDiscussion

∙Infrared（IR）VideographicResults

oA.   HorizontalOrientation

oB.   VerticalOrientation

∙Conclusions

∙Acknowledgment

∙Nomenclature

∙REFERENCES

∙FIGURES

Introduction

Figure1depictsaconceptualthermophotovoltaic（TPV）energyconversionsystemutilizingflatheatpipes.Combustiongasesfromaheatsourcesuchasagasturbinecombustorflowthroughchannelsonwhichheatpipesaremounted.Thesehotsideheatpipesserveasemittersurfaces.Acrossfromthehotsideheatpipes,TPVcellscanbemountedtocoldsideheatpipeswhichareheatpipesincontactwiththethermalsink.AnisothermalemittingsurfaceisneededinTPVenergyconversionsystemsbecausethevoltageoutputsoftheTPVcellsareverysensitivetothewavelengthbandwidthoftheemittingsurface.Theemitter'swavelengthbandwidthisafunctionoftemperature.Onthecoldside,theTPVcellscouldutilizeaflatheatpipe,butthisislesscritical.Flatheatpipesarenotnewtotheindustry,severalcompanieshavedesignedthemforspaceorcomputerapplications[1][2][3][4].

Figure1.

Flatheatpipesaresimilartocylindricalheatpipes.Theonlyrealdifferencebetweenthetwoisgeometrical.Whilethismayseemaminordifference,itpresentsmanychallengesfromanengineeringstandpoint.Typically,heatpipesareusedtotransferquantitiesofheatacrossadistancewithonlyaslighttemperaturelossfromendtoend.Thecylindricaldesignworkswelltoservethispurpose.However,whendesigninganemitterforaTPVenergyconversionsystem,itisadvantageoustohavealargesurfaceareatovolumeratioinordertomaximizethepowerdensityofthesystem.Aflatheatpipewasconceivedforthispurpose.Flatheatpipesalsohavedifferentinternalflowandstructuraldesignconsiderationsthanthoseofcylindricalheatpipes.

Flowpropertiesincylindersaredifferentfromthoseinrectangulargeometriessuchasflatplatesand/orboxes.Theflowofathinfilmthroughaflatwick（suchasintheliquidreturnpathofaflatheatpipe）isnotthesameastheflowofacylindricalcircumferentialfilm.Also,vaporflowthroughacylindricalspacediffersfromvaporflowthrougharectangularcrosssection.Theflowgeometricaldifferencescanalterthesteady-statelimitationsinflatheatpipedesign.

Thelimitmostaffectedinthedesignofamoderatetemperature（100°C）flatheatpipeutilizingwaterastheworkingfluidisthecapillarylimit.Thecapillarylimitinvolvestheabilityofthewicktodevelopthenecessarypumpingheadtoovercomethevaporandliquidpressurelossesastheworkingfluidcirculatesthroughtheheatpipe.

Inthisinvestigation,itwasdesiredtoqualitativelyexaminetheeffectsofwhatwouldhappenifairinfiltratedahermeticallysealedflatheatpipecontainingonlywater.Inorderforaflatheatpipetowithstandpressuredifferencesacrossitsflatsurfaces,theflatsurfacestructureneedstobesupported.Monelpinswereusedassupportstructuresinthisflatheatpipedesign.Thesepinswereweldedtothesheetmetalsurfaces,andthewelds,shouldtheycrack,wouldbeasourceforairinfiltrationforaheatpipecontainingwaterastheworkingfluidandoperatingbelow100°Cinanatmosphericenvironment.

Itshouldbepointedoutthatthisflatheatpipewasnotdesignedasavariableconductanceorgasloadedheatpipe.Therewasnononcondensablegasreservoiratthecondenserend,however,therewasashort5cminlength,18mmindiameterfillpipeattachedtothecondenserend.Inagasloadedvariableconductanceheatpipe（VCHP）,Fig.2,areservoirwhichcontainsaamountofanon-condensablegasisaddedtotheheatpipecondenserend.Marcus[5],andMarcusandFleischman[6]giveanexcellentreviewofasimplifiedVCHP.AprimarygoalforaVCHPoperatingwithaconstantheatsinktemperatureistoachieveasteadyinternaloperatingtemperatureatvaryingheatinputconditions.Thisisaccomplishedforincreasingevaporatorheatinputbytheworkingfluidvaporcompressingthenoncondensablegastowardsthereservoir,thus,lengtheningtheactivecondenserlength.Thecondenserlengththatcontainsthegasessentiallyprohibitsheatrejectionfromthatportionoftheheatpipecondenser.Withproperdesign,thisincreaseinheatpipecondenserareawithincreasingheatinputcanachieveanearlyisothermalvaporoperatingcondition.Generally,thiscallsforthegasreservoirvolumetobemuchlargerthanthecondenservolume.Inthisinvestigation,theratioofthefillpipevolumetothecondenservolumewas0.002.Soifairinfiltratestheheatpipe,itwillnotbehaveasatraditionalVCHP,i.e.,foranincreasingheatinput,ariseinthisheatpipe'soperatingtemperatureisexpected.

Figure2.

SeveralinvestigatorshaveexaminedtheeffectsofnoncondensablegaslevelsonVCHPoperation.ThesehavebeenpredominatelyforcylindricalVCHPs.Kobayashietal.[7]haveconductedanexperimentalandanalyticalstudytoexaminetheflowfieldbehaviorofthevapor/non-condensablegasmixture.Theydeterminedthatgravityandnoncondensablegaslevelhadastrongeffectonthelocationandprofileofthegas/vaporinterfacelayer.PetersonandTien[8]examinedthemixeddoublediffusiveconvectioningasloadedheatpipesandtwo-phasethermosyphons.Theyshowedthattemperatureandconcentrationgradientscanredistributethegaswithinthecondenser.Thisredistribution,however,didnotgreatlyaltertheoverallcondenserheattransfer.Petersonetal.[9]alsoshowedthatdoublediffusiveconvectionchangesthenon-condensablegasflowstructureastheRayleighnumberisincreased.

Theheatpipeemployedinthisstudywasaverylargeflatheatpipesinceintheenergyconversionindustrylargesurfaceareasarerequiredtocoollargepowerproducingdevices.Initially,asmallamountofairwasloadedintotheheatpipe.Anattemptwasmadetocomparetheperformanceforthisairloadedheatpipetoonewithoutair,butunfortunately,airwasbelievedtohaveinfiltratedthesecondcase.Thisinvestigationalsopresentstheuseofinfraredvideographyasadiagnosticmeasurementtooltorecordtheexternalsurfacetemperaturesoftheheatpipe'scondenserregionandtoinferwhatwasinternallyhappeningbetweentheairandwatervaporinthecondenserend.Theprimaryfocusofthispaperisonthequalitativeeffectsofairinfiltrationinalarge,flatheatpipe.

FlatHeatPipeFabrication

Aflatheatpipe,1.22m×0.305m×0.0127m,wasfabricatedfrom50milMonelR400metalsheetsandMonelR400screens,[10][11].Theheatpipewasdesignedtoutilizewaterastheworkingfluidinanoperationaltemperaturerangeof25°Cto130°C.TwolayersofMonelscreenswereused,40meshand120mesh.Thepurposeofthetwodifferentscreensizeswastodesignawickofvaryingpermeability.

Longcopperbarswithfineradiusedgeswereutilizedtofacilitatebendingtheheatpipetotherequireddimensions.Thetwolayersofscreenwerethenplacedontopofthevessel.The120meshscreenwasthenplacedonthetopofthe40meshscreentoaidinthedevelopmentofthecapillarypumpingheadofthisscreenwick.Thescreenwasthentack-weldedtothevesselwallinregularintervalsbetweenthepinspacerlocations.TheMonelsheetsandscreenswerethenpunched,makingholesinthelocationswherethesupportpinsweretobeTIGwelded.The6.35mmdiameterpinswerethencuttotheproperlength,milled,anddeburredtofitintothenecessaryspace.Figure3depictsasectionoftheMonelsheets,screensandpins（Boughey,1999）.ThegapsizebetweentheMonelsheetswasappr