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Heat Transfer.pdf

Heat Transfer.pdf

上传者: freedom-xu 2013-03-29 评分 5 0 122 17 556 暂无简介 简介 举报

简介:本文档为《Heat Transferpdf》,可适用于高等教育领域,主题内容包含Chris Long Naser Sayma Heat Transfer Download free books at ChrisLongNaser符等。

Chris Long Naser Sayma Heat Transfer Download free books at ChrisLongNaserSaymaHeatTransferHeatTransferChrisLong,NaserSaymaVentusPublishingApSISBNHeatTransferContentsIntroductionHeatTransferModesSystemofUnitsConductionConvectionRadiationSummaryMultipleChoiceassessmentConductionTheGeneralConductionEquationOneDimensionalSteadyStateConductioninRadialGeometries:FinsandExtendedSurfacesSummaryMultipleChoiceAssessmentConvectionTheconvectionequationFlowequationsandboundarylayerDimensionalanalysisContentsStandoutfromthecrowdDesignedforgraduateswithlessthanoneyearoffulltimepostgraduateworkexperience,LondonBusinessSchool’sMastersinManagementwillexpandyourthinkingandprovideyouwiththefoundationsforasuccessfulcareerinbusinessTheprogrammeisdevelopedinconsultationwithrecruiterstoprovideyouwiththekeyskillsthattopemployersdemandThroughmonthsoffulltimestudy,youwillgainthebusinessknowledgeandcapabilitiestoincreaseyourcareerchoicesandstandoutfromthecrowdApplicationsarenowopenforentryinSeptemberFormoreinformationvisitwwwlondonedumimemailmimlondoneduorcall()MastersinManagementLondonBusinessSchoolRegent’sParkLondonNWSAUnitedKingdomTel()EmailmimlondoneduwwwlondonedumimFasttrackyourcareerPleaseclicktheadvertHeatTransferForcedConvectionrelationsNaturalconvectionSummaryMultipleChoiceAssessmentRadiationIntroductionRadiativePropertiesKirchhoff’slawofradiationViewfactorsandviewfactoralgebraRadiativeExchangeBetweenaNumberofGreySurfacesRadiationExchangeBetweenTwoGreyBodiesSummaryMultipleChoiceAssessmentHeatExchangersIntroductionClassificationofHeatExchangersAnalysisofHeatExchangersSummaryMultipleChoiceAssessmentReferencesContentsUBSAllrightsreservedwwwubscomgraduatesLookingforacareerwhereyourideascouldreallymakeadifferenceUBS’sGraduateProgrammeandinternshipsareachanceforyoutoexperienceforyourselfwhatit’sliketobepartofaglobalteamthatrewardsyourinputandbelievesinsucceedingtogetherWhereveryouareinyouracademiccareer,makeyourfutureapartofoursbyvisitingwwwubscomgraduatesYou’refullofenergyandideasAndthat’sjustwhatwearelookingforPleaseclicktheadvertHeatTransferIntroductionIntroductionEnergyisdefinedasthecapacityofasubstancetodoworkItisapropertyofthesubstanceanditcanbetransferredbyinteractionofasystemanditssurroundingsThestudentwouldhaveencounteredtheseinteractionsduringthestudyofThermodynamicsHowever,ThermodynamicsdealswiththeendstatesoftheprocessesandprovidesnoinformationonthephysicalmechanismsthatcausedtheprocesstotakeplaceHeatTransferisanexampleofsuchaprocessAconvenientdefinitionofheattransferisenergyintransitionduetotemperaturedifferencesHeattransferextendstheThermodynamicanalysisbystudyingthefundamentalprocessesandmodesofheattransferthroughthedevelopmentofrelationsusedtocalculateitsrateTheaimofthischapteristoconsoleexistingunderstandingandtofamiliarisethestudentwiththestandardofnotationandterminologyusedinthisbookItwillalsointroducethenecessaryunitsHeatTransferModesThedifferenttypesofheattransferareusuallyreferredtoas‘modesofheattransfer’Therearethreeofthese:conduction,convectionandradiationxConduction:Thisoccursatmolecularlevelwhenatemperaturegradientexistsinamedium,whichcanbesolidorfluidHeatistransferredalongthattemperaturegradientbyconductionxConvection:Happensinfluidsinoneoftwomechanisms:randommolecularmotionwhichistermeddiffusionorthebulkmotionofafluidcarriesenergyfromplacetoplaceConvectioncanbeeitherforcedthroughforexamplepushingtheflowalongthesurfaceornaturalasthatwhichhappensduetobuoyancyforcesxRadiation:Occurswhereheatenergyistransferredbyelectromagneticphenomenon,ofwhichthesunisaparticularlyimportantsourceIthappensbetweensurfacesatdifferenttemperaturesevenifthereisnomediumbetweenthemaslongastheyfaceeachotherInmanypracticalproblems,thesethreemechanismscombinetogeneratethetotalenergyflow,butitisconvenienttoconsiderthemseparatelyatthisintroductorystageWeneedtodescribeeachprocesssymbolicallyinanequationofreasonablysimpleform,whichwillprovidethebasisforsubsequentcalculationsWemustalsoidentifythepropertiesofmaterials,andothersystemcharacteristics,thatinfluencethetransferofheatHeatTransferIntroductionSystemofUnitsBeforelookingatthethreedistinctmodesoftransfer,itisappropriatetointroducesometermsandunitsthatapplytoallofthemItisworthmentioningthatwewillbeusingtheSIunitsthroughoutthisbook:xTherateofheatflowwillbedenotedbythesymbolQItismeasuredinWatts(W)andmultiplessuchas(kW)and(MW)xItisoftenconvenienttospecifytheflowofenergyastheheatflowperunitareawhichisalsoknownasheatfluxThisisdenotedbyqNotethat,AQqwhereAistheareathroughwhichtheheatflows,andthattheunitsofheatfluxare(Wm)xNaturally,temperaturesplayamajorpartinthestudyofheattransferThesymbolTwillbeusedfortemperatureInSIunits,temperatureismeasuredinKelvinorCelsius:(K)and(qC)Sometimesthesymboltisusedfortemperature,butthisisnotappropriateinthecontextoftransientheattransfer,whereitisconvenienttousethatsymbolfortimeTemperaturedifferenceisdenotedinKelvin(K)ThefollowingthreesubsectionsdescribetheabovementionedthreemodesofheatflowinmoredetailFurtherdetailsofconduction,convectionandradiationwillbepresentedinChapters,andrespectivelyChaptergivesabriefoverviewofHeatExchangerstheoryandapplicationwhichdrawsontheworkfromthepreviousChaptersConductionTheconductivetransferisofimmediateinterestthroughsolidmaterialsHowever,conductionwithinfluidsisalsoimportantasitisoneofthemechanismsbywhichheatreachesandleavesthesurfaceofasolidMoreover,thetinyvoidswithinsomesolidmaterialscontaingasesthatconductheat,albeitnotveryeffectivelyunlesstheyarereplacedbyliquids,aneventwhichisnotuncommonProvidedthatafluidisstillorveryslowlymoving,thefollowinganalysisforsolidsisalsoapplicabletoconductiveheatflowthroughafluidHeatTransferIntroductionFigureshows,inschematicform,aprocessofconductiveheattransferandidentifiesthekeyquantitiestobeconsidered:Q:theheatflowbyconductioninthexdirection(W)A:theareathroughwhichtheheatflows,normaltothexdirection(m)Figure:OnedimensionalconductioneveryonedeservesgooddesigneduceeuseecycleRWORKWITHUSInterIKEASystemsBVInterIKEASystemsBVIKEASESTUDENTIt’sonlyanopportunityifyouactonitmePleaseclicktheadvertHeatTransferIntroductiondxdT:thetemperaturegradientinthexdirection(Km)ThesequantitiesarerelatedbyFourier'sLaw,amodelproposedasearlyas:dxdTkqdxdTAk=Q=or()AsignificantfeatureofthisequationisthenegativesignThisrecognisesthatthenaturaldirectionfortheflowofheatisfromhightemperaturetolowtemperature,andhencedownthetemperaturegradientTheadditionalquantitythatappearsinthisrelationshipisk,thethermalconductivity(WmK)ofthematerialthroughwhichtheheatflowsThisisapropertyoftheparticularheatconductingsubstanceand,likeotherproperties,dependsonthestateofthematerial,whichisusuallyspecifiedbyitstemperatureandpressureThedependenceontemperatureisofparticularimportanceMoreover,somematerialssuchasthoseusedinbuildingconstructionarecapableofabsorbingwater,eitherinfiniteporesoratthemolecularlevel,andthemoisturecontentalsoinfluencesthethermalconductivityTheunitsofthermalconductivityhavebeendeterminedfromtherequirementthatFourier'slawmustbedimensionallyconsistentConsideringthefiniteslabofmaterialshowninFigure,weseethatforonedimensionalconductionthetemperaturegradientis:LTT=dxdTHenceforthissituationthetransferlawcanalsobewrittenLTTkqLTTAk=Q=or()=CkUD()Tablegivesthevaluesofthermalconductivityofsomerepresentativesolidmaterials,forconditionsofnormaltemperatureandpressureAlsoshownarevaluesofanotherpropertycharacterisingtheflowofheatthroughmaterials,thermaldiffusivity,whichisrelatedtotheconductivityby:WhereUisthedensityinmkgofthematerialandCitsspecificheatcapacityinKkgJHeatTransferIntroductionThethermaldiffusivityindicatestheabilityofamaterialtotransferthermalenergyrelativetoitsabilitytostoreitThediffusivityplaysanimportantroleinunsteadyconduction,whichwillbeconsideredinChapterAswasnotedabove,thevalueofthermalconductivityvariessignificantlywithtemperature,evenovertherangeofclimaticconditionsfoundaroundtheworld,letaloneinthemoreextremeconditionsofcoldstorageplants,spaceflightandcombustionForsolids,thisisillustratedbythecaseofmineralwool,forwhichthethermalconductivitymightchangefromtoWmKacrosstherangetoqCTableThermalconductivityanddiffusivityfortypicalsolidmaterialsatroomtemperatureMaterialkWmKĮmmsMaterialkWmKĮmmsCopperAluminiumMildsteelPolyethyleneFaceBrickMediumconcreteblockDenseplasterStainlesssteelNylon,RubberAeratedconcreteGlassFireclaybrickDenseconcreteCommonbrickWood,PlywoodWoodwoolslabMineralwoolexpandedExpandedpolystyreneForgasesthethermalconductivitiescanvarysignificantlywithbothpressureandtemperatureForliquids,theconductivityismoreorlessinsensitivetopressureTableshowsthethermalconductivitiesfortypicalgasesandliquidsatsomegivenconditionsTableThermalconductivityfortypicalgasesandliquidsMaterialkWmKGasesArgon(atKandbar)Air(atKandbar)Air(atKandbar)Hydrogen(atKandbar)Freon(atKbar)LiquidsEngineoil(atoC)Engineoil(atoC)Water(atoC)Water(atoC)HeatTransferIntroductionMercury(atoC)NotetheverywiderangeofconductivitiesencounteredinthematerialslistedinTablesandSomepartofthevariabilitycanbeascribedtothedensityofthematerials,butthisisnotthewholestory(Steelismoredensethanaluminium,brickismoredensethanwater)Metalsareexcellentconductorsofheataswellaselectricity,asaconsequenceofthefreeelectronswithintheiratomiclatticesGasesarepoorconductors,althoughtheirconductivityriseswithtemperature(themoleculesthenmoveaboutmorevigorously)andwithpressure(thereisthenahigherdensityofenergycarryingmolecules)Liquids,andnotablywater,haveconductivitiesofintermediatemagnitude,notverydifferentfromthoseforplasticsThelowconductivityofmanyinsulatingmaterialscanbeattributedtothetrappingofsmallpocketsofagas,oftenair,withinasolidmaterialwhichisitselfaratherpoorconductorExampleCalculatetheheatconductedthroughamthickindustrialfurnacewallmadeoffireclaybrickMeasurementsmadeduringsteadystateoperationshowedthatthewalltemperaturesinsideandoutsidethefurnaceareandKrespectivelyThelengthofthewallismandtheheightismYOURCHANCETOCHANGETHEWORLDShapingtomorrow’sworld–todayOurbusinessisattheheartofaconnectedworld–aworldwherecommunicationisempoweringpeople,businessandsocietyOurnetworks,telecomservicesandmultimediasolutionsareshapingtomorrowAndthismightjustbeyourchancetoshapeyourownfutureIt’sapeoplethingWearelookingforhighcaliberpeoplewhocanseetheopportunities,peoplewhocanbringknowledge,energyandvisiontoourorganizationInreturnweofferthechancetoworkwithcuttingedgetechnology,personalandprofessionaldevelopment,andtheopportunitytomakeadifferenceinatrulyglobalcompanyWearecurrentlyrecruitingbothnewgraduatesandexperiencedprofessionalsinfourareas:Software,Hardware,SystemsandIntegrationVerificationAreyoureadytoshapeyourfutureBeginbyexploringacareerwithEricssonVisitwwwericssoncomjoinericssonPleaseclicktheadvertHeatTransferIntroductionSolutionWefirstneedtomakeanassumptionthattheheatconductionthroughthewallisonedimensionalThenwecanuseEquation:LTTAkQThethermalconductivityforfireclaybrickobtainedfromTableisWmKTheareaofthewallmuAThus:WmKKmKWmuuQComment:NotethatthedirectionofheatflowisfromthehighertemperatureinsidetothelowertemperatureoutsideConvectionConvectionheattransferoccursbothduetomolecularmotionandbulkfluidmotionConvectiveheattransfermaybecategorisedintotwoformsaccordingtothenatureoftheflow:naturalConvectionandforcedconvectionInnaturalof‘free’convection,thefluidmotionisdrivenbydensitydifferencesassociatedwithtemperaturechangesgeneratedbyheatingorcoolingInotherwords,fluidflowisinducedbybuoyancyforcesThustheheattransferitselfgeneratestheflowwhichconveysenergyawayfromthepointatwhichthetransferoccursInforcedconvection,thefluidmotionisdrivenbysomeexternalinfluenceExamplesaretheflowsofairinducedbyafan,bythewind,orbythemotionofavehicle,andtheflowsofwaterwithinheating,cooling,supplyanddrainagesystemsInalloftheseprocessesthemovingfluidconveysenergy,whetherbydesignorinadvertentlyHeatTransferIntroductionFloorCeilingRadiatorFreeconvectioncellSolidsurfaceNaturalconvectionForcedconvectionFigure:IllustrationoftheprocessofconvectiveheattransferTheleftofFigureillustratestheprocessofnaturalconvectiveheattransferHeatflowsfromthe‘radiator’totheadjacentair,whichthenrises,beinglighterthanthegeneralbodyofairintheroomThisairisreplacedbycooler,somewhatdenserairdrawnalongthefloortowardstheradiatorTherisingairflowsalongtheceiling,towhichitcantransferheat,andthenbacktothelowerpartoftheroomtoberecirculatedthroughthebuoyancydriven‘cell’ofnaturalconvectionTheword‘radiator’hasbeenwrittenaboveinthatwaybecausetheheattransferfromsuchdevicesisnotpredominantlythroughradiationconvectionisimportantaswellInfact,inatypicalcentralheatingradiatorapproximatelyhalftheheattransferisby(free)convectionTherightpartofFigureillustratesaprocessofforcedconvectionAirisforcedbyafancarryingwithitheatfromthewallifthewalltemperatureislowerorgivingheattothewallifthewalltemperatureislowerthantheairtemperatureIfTisthetemperatureofthesurfacereceivingorgivingheat,andfTistheaveragetemperatureofthestreamoffluidadjacenttothesurface,thentheconvectiveheattransferQisgovernedbyNewton’slaw:)(=orTThq)TT(Ah=Qcc()AnotherempiricalquantityhasbeenintroducedtocharacterisetheconvectivetransfermechanismThisishc,theconvectiveheattransfercoefficient,whichhasunitsWmKThisquantityisalsoknownastheconvectiveconductanceandasthefilmcoefficientThetermfilmcoefficientarisesfromasimple,butnotentirelyunrealistic,pictureoftheprocessofconvectiveheattransferatasurfaceHeatisimaginedtobeconductedthroughathinstagnantfilmoffluidatthesurface,andthentobeconvectedawaybythemovingfluidbeyondSincetheHeatTransferIntroductionfluidrightagainstthewallmustactuallybeatrest,thisisafairlyreasonablemodel,anditexplainswhyconvectivecoefficientsoftendependquitestronglyontheconductivityofthefluidTableRepresentativerangeofconvectiveheattransfercoefficientNatureofFlowFluidhcWmKSurfacesinbuildingsAirSurfacesoutsidebuildingsAirAcrosstubesGasLiquidIntubesGasOrganicliquidWaterLiquidmetalNaturalconvectionGasLiquidCondensingLiquidfilmLiquiddropsBoilingLiquidvapourwhat‘smissinginthisequationmaeRsKinteRnationaLteChnoLogYsCienCePRogRammeYoucouldbeoneofourfuturetalentsAreyouabouttograduateasanengineerorgeoscientistOrhaveyoualreadygraduatedIfso,theremaybeanexcitingfutureforyouwithAPMollerMaerskwwwmaerskcommitasPleaseclicktheadvertHeatTransferIntroductionThefilmcoefficientisnotapropertyofthefluid,althoughitdoesdependonanumberoffluidproperties:thermalconductivity,density,specificheatandviscosityThissinglequantitysubsumesavarietyoffeaturesoftheflow,aswellascharacteristicsoftheconvectingfluidObviously,thevelocityoftheflowpastthewallissignificant,asisthefundamentalnatureofthemotion,thatistosay,whetheritisturbulentorlaminarGenerallyspeaking,theconvectivecoefficientincreasesasthevelocityincreasesAgreatdealofworkhasbeendoneinmeasuringandpredictingconvectiveheattransfercoefficientsNevertheless,forallbutthesimplestsituationswemustrelyuponempiricaldata,althoughnumericalmethodsbasedoncomputationalfluiddynamics(CFD)arebecomingincreasinglyusedtocomputetheheattransfercoeffic

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