(5.6)--Chapter 6 Experimental correlati传热学传热学传热学.ppt
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1、Chapter6Experimentalcorrelationofsingle-phaseconvectiveheattransfernSimilarityprincipleanddimensionanalysisnApplicationofsimilarityprinciplenExperimentalcorrelationofinternalforcedconvectionheattransfernExperimentalcorrelationofexternalforcedconvectionheattransfer-flowacrossasingletube,sphereandtube
2、banksnExperimentalcorrelationoffreeconvectiveheattransferinlargespaceandfinitespace6-1ExperimentalcorrelationofinternalforcedconvectionheattransferI.Characteristicsofforcedconvectionflowandheattransferintubeandchannel1.LaminarflowandturbulentflowvLaminarzonevTransitivezonevTurbulentzone2.Entrancereg
3、ioneffectLaminarTurbulent Lengthofentranceregionforlaminar:Forturbulent:3.ImpactofboundaryconditionsBoundaryconditionsUniformwalltemperatureUniformheatfluxDiscussion:(1)Graduallydecreases,andthenapproachesacertainvalueGraduallyincreases,andthenapproachesacertainvalue(2)Turbulence:Inadditiontoliquidm
4、etal,thedifferencebetweenthetwoconditionscanbeignored.Laminarflow:Thedifferenceinheattransfercoefficientbetweenthetwoboundaryconditionsissignificant.4.Determinationofcharacteristicvelocityandqualitativetemperature Characteristic velocity:generallytakeaverageflowvelocityofsection.Qualitative temperat
5、ure:mostly the average temperature of fluid ofsection(oraveragetemperatureoftheinletandoutletsections)5.AveragetemperaturedifferenceinNewtonslawofcoolingForconstantheatflux,(tw-tf)canbetakenastm.Forconstantwalltemperature,thelocaltemperaturedifferenceoncrosssectionisconstantlychangingthroughoutheatt
6、ransfersurface.Thefollowingthermalbalanceshouldbeused:where:qmismassflowrate,kg/s;,areaveragetemperaturesofinletandoutletcrosssectionsrespectively;iscalculatedaslogarithmicaveragetemperaturedifference:ThemostwidelyusedistheDittus-Boelterformula:Heat fluid:Cool fluid:This formula is suitable for appl
7、ications where the fluid and wall have moderate temperature difference.where:qualitativetemperatureisaveragefluidtemperature,andcharacteristiclengthisinnerdiameterofthetube.Experimentalverificationrange:II.ExperimentalcorrelationofturbulentheattransferintubevInfact,thetemperatureonsectionisnotunifor
8、m,resultingindistortionofvelocitydistribution.Usuallyintroduceamultiplierortoconsidertheeffectofunevenphysicalfieldonheattransfer.1-isothermalflow;2-coolingliquidorheatinggas;3-heatingliquidorcoolinggasInthecaseoflargetemperaturedifference,thefollowingcorrectioncalculationscanbemade:Dittus-Boelterco
9、rrectionformulaHeatinggas:Coolinggas:LiquidPromotionoftheformula:(1)Non-circularcross-sectionchannelNote:Forflowareaswithsharpcornersonthesection,themethodofusingequivalentdiameterwillresultinalargeerror.where:isflowcross-sectionalareaofthechannel;isthewettedperimeter.(2)EntranceregionForsharpcorner
10、entrancesintypicalindustrialequipment,thefollowingentranceeffectcorrectionfactorsareavailable:Equivalentdiameter:(3)SpiraltubeSpiral tube enhances heat transfer.Therearespiraltubecorrectionfactorsforit:Gas:Liquid:SecondarycirculationVelocityDistributiononsectionUniformheatflowboundary:Forliquidmetal
11、swithasmallPrnumber,thelawofheattransferare completely different.The heat transfer criterion for fullydevelopedturbulentinasmoothcirculartubeisrecommendedas:Experimentalverificationrange:Uniformwalltemperatureboundary:Experimentalverificationrange:Characteristiclengthisinnerdiameterandqualitativetem
12、peratureisaveragefluidtemperature.Gnielinskiformula()Forliquid:Forgas:Darcyresistancecoefficient:III.LaminarheattransfercorrelationintubeResultsofconvectiveheattransferforfullydevelopedlaminarflowSectionshapeRegulartriangleSquareHexagonCircleRectangleUniformheatfluxUniformwalltemperatureSectionshape
13、UniformheatfluxUniformwalltemperatureRatioofinnerandouterdiameterInnerwallOuterwallOnesidewallisinsulated,andtheothersidewallisisothermalExperimentalverificationrange:Inactualengineeringheatexchangeequipment,heattransferinlaminarflowisoftenintherangeofentranceregion.ThefollowingSieder-Tateformulasca
14、nbeused.Qualitative temperature is average fluid temperature ,and innerdiameterofthetubeischaracteristiclength.Thetubeisatuniformwalltemperature.6-2ExperimentalcorrelationofexternalforcedconvectionheattransferExternal flow:Flow boundary layer and thermal boundary layer onheat exchange wall can devel
15、op freely and are not limited by theexistenceofadjacentwalls.Acrosssingletube:fluidflowsthroughsurface of tube in adirectionperpendiculartoaxisofthetube.Theflow has a boundarylayer and wake willoccur.I.ExperimentalcorrelationofheattransfercrosssingletubeSeparation phenomenon of flow boundarylayeraro
16、undasingletubeExperimentsshowthatifRe5,boundarylayerwilldetachwhenfluidflowsaroundthecylinder,formingavortex.Thisisduetochangesinflowrateandpressureastheviscousfluidflowsthroughthecylinder.Featuresofdisconnectedpoint:Boundarylayerislaminar,andseparationpointisBoundarylayerfirstchangesfromlaminartotu
17、rbulent,andtheseparationpointisat.FlowstatewhenfluidflowsacrossasingletubenotseparateStartseparate,vortexappearsinwakeSeparate,wakeformsalaminarvortexstreetboundarylayermaintainslaminarflowbeforeseparating,turbulentvortexstreetBoundarylayertransitsfromlaminartoturbulentandthenseparates,andwaketurbul
18、encenarrows.Alsoaturbulentvortexstreet,butnarrowerthanthefourthVariationcurveoflocalNusseltnumberNuwithangle .Growthandseparationofboundarylayer determines the characteristicsof external heat transfer of followacrosssingletubeAlthough variation of local surface heat transfer coefficient iscomplicate
19、d,thevariationofaveragesurfaceheattransfercoefficienthasaobviousgradualregularity.Characteristiclengthisouterdiameterofthetube;characteristicvelocityofReisinflowvelocity.Usethefollowingpiecewisepowercorrections:where:values of C and n are shown in table below;qualitativetemperatureis0.440.9890.33044
20、00.9110.3854040000.6830.4664000400000.1930.618400004000000.02660.805Experimentalverificationrange:,第四章 对流换热 26 Convectiveheattransferforgasflowsacrossacylinderortubewithnon-circularcrosssectioncanalsousetheaboveformula.SquareHexagonVerticalplateTheaboveformulagenerallyrequiressegmentationforexperime
21、ntaldata.where:qualitativetemperatureisApplicabletothecaseof.ChurchillandPensdenproposedthefollowingcriterionforexternalflowacrosssingletube:II.experimentcorrelationofheattransferforflowacrosstubebanksInadditiontoReandPr,thefactorsaffectingheattransferoftubebanksarecrossandsequential,tubespacingandn
22、umberoftuberows.vFlow across tube banks ismostcommoninheatexchangers.vUsually,tube banks havetwo kindsofarrangement,crossandsequential.Crossarrangement has strongerheat exchange and greaterresistancelossandisdifficulttoclean.CrossarrangementSequentialarrangementExperimentalcorrelationofgasflowacross
23、10rowsoftubebanksis:Thereartubeisaffectedbythedisturbanceofthefronttubewaketoaveragesurfaceheattransfercoefficientuntiltherowsoftubereaches10ormore.Inthiscase,thecorrelationthatdoesnotconsidertheinfluenceofnumberofrowsisgivenfirst,andthenusethefactorofnumberofrowsoftubesascorrectionfactor.where:qual
24、itativetemperatureis;characteristiclengthisouterdiameteroftube.Flowvelocityistakenastheoneatthenarrowestsectionoftheentiretubebanks.Experimentalverificationrange:SequentialarrangementCrossarrangementForbundleswithfewerthan10rows,theaveragesurfaceheattransfercoefficientcanbemultipliedbytubecorrection
25、factorbasedontheaboveequation.Thevaluesofarelistedinthefollowingtable:SequentialarrangementCrossarrangementNumberofrows summed up a set of more convenient formulas for heattransferofflowacrosstubebanksoverawiderangeofPr.where:qualitativetemperatureisaveragetemperatureofinletandoutletfluids;Prwisdete
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