(5.7)--Chapter 7 Convective heat transf传热学传热学传热学.ppt
《(5.7)--Chapter 7 Convective heat transf传热学传热学传热学.ppt》由会员分享,可在线阅读,更多相关《(5.7)--Chapter 7 Convective heat transf传热学传热学传热学.ppt(58页珍藏版)》请在得力文库 - 分享文档赚钱的网站上搜索。
1、Chapter7ConvectiveheattransferwithphasechangenCondensationheattransfermodenAnalyticalsolutionandcomputationalcorrelationoffilmcondensationnInfluencingfactorsandheattransferenhancementoffilmcondensationnBoilingheattransfermodenExperimentalcorrelationofboilingheattransferinpoolboilingnInfluencingfacto
2、rsandenhancementofboilingheattransfernIntroductionofheatpipeConvectiveheattransferwithphasechange,alsoknownasphase change heat transfer,iscurrentlyinvolvedinbothcondensation heat transferandboiling heat transfer.Characteristicsofphasechangeheattransfer:The fluid temperature remains essentially const
3、ant,achieving ahigher exothermic and endothermic effect at a relatively smalltemperaturedifferenceHeatexchangeismainlythelatentheatr.Becauserisrelativelygreat,sohisgreat.Inadditiontothefactorsaffectingsingle-phasefluid,otherfactorsarelatentheat,surfacetensionandbuoyancy.Therefore,atpresent,itcanonly
4、relyonempiricalformulasandexperimentalcorrelationsinengineering.nHeat transfer amount of condensation and boiling is still calculatedaccordingtoNewtonslawofcooling.7-1CondensationheattransfermodeKeypointsofcondensationheattransferCondensationmayoccurindifferentforms,film condensationanddropwise cond
5、ensation;Condensateisequivalenttoincreasingthermalresistanceoffurtherheattransfer;Experimental correlation oflaminar andturbulent filmcondensationheattransferFactorsaffectingfilmcondensationheattransferAnalysisofheattransferprocessofverticalandhorizontaltubes,andNusseltsfilmcondensationtheoryCondens
6、ation heat transfer examples:CondenserinsteamturbineIceflowersonwindowatcoldwinterManyotherindustrialapplicationprocessesImportant parameters in condensation heat transferDifferencebetweenvaporsaturationtemperatureandwalltemperature(tstw)LatentheatofvaporizationrCharacteristicscaleOtherthermophysica
7、lpropertiessuchasdynamicviscosity,thermalconductivity,specificheatcapacity,etc.CondensationprocessFilm condensationAfilmisformedalongtheentirewallsurfaceandflows under action of gravity.The latent heat ofvaporization released by condensation must passthrough liquid film.Therefore,the thickness ofliq
8、uidfilmdirectlyaffectstheheattransfer.Dropwise condensationWhencondensedliquiddoesnotwetthewallwell,manysmallliquidbeadsareformedonwallsurface,andatthistime,part of the wall surface is directly contact withvapor,soheatexchangerateismuchlargerthanthefilmcondensation(maybeseveraltimeslargerorevenonema
9、gnitude).ggAlthoughheattransferwithdropwisecondensationismuchlargerthanfilmcondensation,itisapitythatdropwisecondensationisdifficulttomaintain.Therefore,mostofthecondensationheattransferencountered in engineering belongs to film condensation.In1916,Nusseltssimplefilmcondensationheattransferanalysisw
10、asthebasisof modern film condensation theory and heat transfer analysis.Since 1916,variousrevisionsordevelopmentshavebeenmadeinresponsetotherestrictiveassumptions of Nusselts analysis,and various practical calculation methodshavebeendeveloped.Therefore,wemustfirstunderstandNusseltsanalysisofcondensa
11、tionheattransferofpuresaturatedvapor.Hypothesis:1)constantphysicalproperties;2)staticvapor;3)inertiaforceofliquidfilmisneglected;4)notemperaturedifferenceatgas-liquidinterface,i.e.liquidfilmtemperatureisequaltosaturationtemperature;5)lineardistributionoffilmtemperature,i.e.heattransferonlybyheatcond
12、uction;6)Thesubcoolingdegreeofliquidfilmisignored;7)Vapordensityisneglected;8)Thesurfaceofliquidfilmisflatwithoutfluctuation.7-2Analyticalsolutionandcomputationalcorrelationoffilmcondensationgt(y)u(y)ThermalboundarylayersVelocityboundarylayersmicro-control volumeBoundarylayerdifferentialequations:Su
13、bscriptlisliquidphase.xConsideringhypothesis(3):inertialforceoftheliquidfilmisignored:Consideringhypothesis(5):lineardistributionoffilmtemperature,heattransferonlybyheatconduction:Consideringhypothesis(7):vapordensityisignored:Thereareonlytwounknownsuandt,soaboveequationsaresimplifiedto:Boundarycond
14、itions:Solvetheaboveequationcanobtain:(1)LiquidfilmthicknessQualitativetemperature:Note:r is determined by ts(2)LocalconvectiveheattransfercoefficientAveragesurfaceheattransfercoefficientoftheentireverticalwall:Qualitativetemperature:(3)Correction:experiment shows that condensation heat transfer iss
15、trengthened due to fluctuation of liquid film surface,soexperimentalvalueisabout20%higherthantheoreticalvalueabove.Aftercorrection:Forinclinedwalls,usegsininsteadofgintheaboveformula.Inaddition,inadditiontothecorrectionoffluctuations,otherhypothesishavebeenmadeinrelatedresearch,suchaswhenPr1and,thee
16、ffectsofinertialforcetermandsupercoolingofliquidfilmcanbeignored.(4)HorizontalcirculartubeNusseltstheoreticalanalysiscanbeextendedtolaminarfilmcondensationonhorizontaltubesandsphericalsurfaces.where:subscript“H”meanshorizontaltube,“S”meanssphere;disdiameterofhorizontaltubeorsphere.Qualitativetempera
17、tureisthesameasthepreviousformulaRatioofconvectiveheattransfercoefficientofhorizontaltubetoverticaltube:FlowregimeinsideboundarylayerNon-fluctuationlaminarflowFluctuatinglaminarflowTurbulentThe condensed liquid flow also includeslaminarandturbulent,anditsjudgmentbasisisstillRe:where:ul isaverageflow
18、velocityofliquidfilmlayeratx=l;deisequivalentdiameterofliquidfilmlayeratcrosssection.Asshown,Bythermalbalance:Therefore:Forhorizontaltube,useprinsteadoflinaboveformula.Andhorizontaltubeisgenerallyinalaminarstate.TurbulentfilmcondensationheattransferThecriticalReynoldsnumberofliquidfilmfromlaminartot
19、urbulentflowcanbesetat1600.Duetosmalldiameterofhorizontaltube,itispracticallyinlaminarflowrange.Forturbulentliquidfilm,inadditiontolaminarbottomlayerclosetowallstillrelies on heat conduction to transfer heat,and turbulent flow is mainlytransmittedoutsidelaminarflowbottomlayer,andtheheattransferisgre
20、atlyenhanced.Forturbulentcondensationheattransferofverticalwall,theaveragesurfaceheattransfercoefficientalongentirewallsurfaceiscalculatedas:where:hlisheattransfercoefficientoflaminarsection;htisheattransfercoefficientofturbulentsection;Xcisheightofturningpointwhenlaminarflowisconvertedtoturbulent;l
21、isthetotalheightofverticalwall.Experimentalcorrelationsobtainedusingtheaboveidea:inwhich:.Exceptiscalculatedby wall temperature ,the qualitative temperature of theremainingphysicalquantitiesis.7-3InfluencingfactorsandheattransferenhancementoffilmcondensationFilm condensation processes in engineering
22、 tends to be morecomplex,affectedbyvariousfactors.1.Non-condensing gasNon-condensinggasincreasesresistanceoftransferprocess,whilelowering saturation temperature,reducing driving force ofcondensationt.2.Vapor flow rateAthigherflowrate,vaporstreamcreatessignificantviscousstressonsurfaceofliquidfilm.If
23、vaporflowisinthesamedirectionasthedownwardflowofliquidfilm,theliquidfilmisthinnedandhisincreased;otherwise,hisdecreased.5.Number of rows of tubesGeometricarrangementandfluidpropertiesofthetubebankscan affect condensation heat transfer.The previously derivedformula for horizonal tube condensation hea
24、t transfer is onlyapplicabletoasinglehorizonaltube.3.Superheated steamConsiderenthalpydifferencebetweensuperheatedsteamandsaturatedliquid.4.Non-linearity of subcooling and temperature distribution of liquid film Considering actual situation of liquid film and temperaturedistribution,the following fo
25、rmula instead of r in the calculationformulashouldbeused.Forthiscase,heatexchangehasagreatrelationshipwithvaporflowrate.At low flow rate,condensateisprimarilyatthebottomofthetubeandthevaporislocatedintheupperhalfofthetube.At high flow rate,anannularflowforms,andcondensateisevenlydistributedaroundthe
- 配套讲稿:
如PPT文件的首页显示word图标,表示该PPT已包含配套word讲稿。双击word图标可打开word文档。
- 特殊限制:
部分文档作品中含有的国旗、国徽等图片,仅作为作品整体效果示例展示,禁止商用。设计者仅对作品中独创性部分享有著作权。
- 关 键 词:
- 5.7-Chapter Convective heat transf传热学传热学传热学 5.7 Chapter transf 传热学
链接地址:https://www.deliwenku.com/p-96451069.html
限制150内