流體力學(xué)往屆課件-09chapter

華南理工大學(xué)化工學(xué)院，伍欽，88370073,4.6.RADIATIONHEATTRANSFER

Radiation,whichmaybeconsideredtobeenergystreamingthroughspaceatthespeedoflight,mayoriginateinvariousways.Sometypesofmaterialwillemitradiationwhentheyaretreatedbyexternalagencies.華南理工大學(xué)化工學(xué)院，伍欽，88370073,Allsubstancesattemperaturesaboveabsolutezeroemitradiationthatisindependentofexternalagencies.Radiationthatistheresultoftemperatureonlyiscalled

thermalradiation.華南理工大學(xué)化工學(xué)院，伍欽，88370073,FundamentalfactsconcerningradiationRadiationmovesthroughspaceinstraightlines,orbeams,andonlysubstancesinsightofaradiatingbodycaninterceptradiationfromthatbody.華南理工大學(xué)化工學(xué)院，伍欽，88370073,Radiationassuchisnotheat,andwhentransformedintoheatonabsorption,itisnolongerradiation.華南理工大學(xué)化工學(xué)院，伍欽，88370073,Thefractionthatisabsorbediscalledabsorptivityα.Thefractionthatistransmittediscalled

transmissivity

τ.Thefractionoftheradiationfallingonabodythatisreflectediscalled

reflectivity

ρ.華南理工大學(xué)化工學(xué)院，伍欽，88370073,Themaximumpossibleabsorptivityisunity.Abodywhichabsorbsallincidentradiationiscalledablackbody.Thesumofthesefractionsmustbeunity,orρ+α+τ=14.6-1華南理工大學(xué)化工學(xué)院，伍欽，88370073,EmissionofradiationTheradiationemittedbyanygivenmassofsubstanceisindependentofothermaterialinsightof,orincontactwith,themass.Thenetenergygainedorlostbyabodyisthedifferencebetweentheenergyemittedbythebodyandthatabsorbedbyitfromtheradiationreachingitfromotherbodies.華南理工大學(xué)化工學(xué)院，伍欽，88370073,Whenbodiesatdifferenttemperaturesareplacedinsightofoneanotherinsideanenclosure,thehotterbodieslossenergybyemissionofradiationfasterthantheyreceiveenergybyabsorptionofradiationfromthecoolerbodies.Temperaturesofhotterbodiesdecrease.華南理工大學(xué)化工學(xué)院，伍欽，88370073,WavelengthofradiationKnownelectromagneticradiationscoveranenormousrangeofwavelengths,fromtheshortcosmicraystolongwavebroadcastingwave.華南理工大學(xué)化工學(xué)院，伍欽，88370073,Althoughradiationofanywavelengthis,inprinciple,convertibleintoheatonabsorptionbymatter,theportionoftheelectromagneticspectrumthatisofimportanceinheatflowliesinthewavelengthrangebetween0.5and50μm.華南理工大學(xué)化工學(xué)院，伍欽，88370073,Visiblelightcoversawavelengthrangeofabout0.38to0.78μmAttemperatureaboveabout5000oC，heatradiationinthevisiblespectrumesignificant.Thehigherthetemperatureoftheradiatingbody,theshorterthepredominantwavelengthofthethermalradiationemittedbyit.華南理工大學(xué)化工學(xué)院，伍欽，88370073,EmissivepowerThemonochromaticenergyemittedbyaradiatingsurfacedependsonthetemperatureofthesurfaceandonthewavelengthoftheradiation.華南理工大學(xué)化工學(xué)院，伍欽，88370073,Atconstantsurfacetemperature,acurvecanbeplottedshowingtherateofenergyemissionasafunctionofthewavelength.華南理工大學(xué)化工學(xué)院，伍欽，88370073,Themonochromaticradiationemittedinthismannerfromunitareainunittime,dividedbythewavelength,iscalledthemonochromaticradiatingpowerWλ.華南理工大學(xué)化工學(xué)院，伍欽，88370073,Fortheentirespectrumoftheradiationfromasurface,thetotalradiatingpowerWisthesumofallthemonochromaticradiationsfromthesurface,or,mathematically,4.6-2

華南理工大學(xué)化工學(xué)院，伍欽，88370073,BlackbodyradiationAblackbodyhasthemaximumattainableemissivepoweratanygiventemperature.

華南理工大學(xué)化工學(xué)院，伍欽，88370073,TheratioofthetotalemissivepowerWofabodytothatofablackbodyWBisbydefinitiontheemissivityεofthebody,thus4.6-3

華南理工大學(xué)化工學(xué)院，伍欽，88370073,EmissivitiesofsolidsEmissivityusuallyincreaseswithtemperature.Emissivitiesofpolishedmetalsarelow,intherange0.03to0.08.Emissivitiesofmostoxidizedmetalsrangefrom0.6to0.85,thoseofnonmetalsfrom0.65to0.95.華南理工大學(xué)化工學(xué)院，伍欽，88370073,Noactualsubstanceisablackbody,althoughsomematerials,suchascertaingradesofcarbonblack,doapproachblackness.華南理工大學(xué)化工學(xué)院，伍欽，88370073,LawsofblackbodyradiationAbasicrelationshipforblackbodyradiationistheStefan-Boltzmannlaw,whichstatesthatthetotalemissivepowerofablackbodyisproportionaltothefourthpoweroftheabsolutetemperature,or

Whereσisauniversalconstant4.6-5

WB=σT4華南理工大學(xué)化工學(xué)院，伍欽，88370073,Thedistributionofenergyinthespectrumofablackbodyisknownaccurately.ItisgivenbyPlanck’slaw4.6-6

Planck’slawcanbeshowntobeconsistentwiththeStefan-BoltzmannlawbysubstitutingWb,λfromEq4.6-6intoEq4.6-2andintegrating.華南理工大學(xué)化工學(xué)院，伍欽，88370073,AbsorptionofradiationbyopaquesolidsKirchhoff’slawstates,attemperatureequilibrium,theratioofthetotalradiatingpowerofanybodytotheabsorptivityofthatbodydependsonlyuponthetemperatureofthebody.華南理工大學(xué)化工學(xué)院，伍欽，88370073,Thus,consideranytwobodiesintemperatureequilibriumwithcommonsurroundings.Kirchhoff’slawstatesthat4.6-9

華南理工大學(xué)化工學(xué)院，伍欽，88370073,ThusIfthefirstbodyisblackbody,α1=1,and4.6-10

華南理工大學(xué)化工學(xué)院，伍欽，88370073,Bydefinition,theemissivityofthesecondbodyε2is4.6-11

華南理工大學(xué)化工學(xué)院，伍欽，88370073,Thus,whenanybodyisattemperatureequilibriumwithitssurroundings,itsemissivityandabsorptivityareequal.Kirchholff’lawapplieswhetherornotthetwosurfacesareatsametemperature.華南理工大學(xué)化工學(xué)院，伍欽，88370073,RadiationbetweensurfacesThetotalradiationfromaunitareaofanopaquebodyofareaA1,emissivityε1,andabsolutetemperatureT1is4.6-13華南理工大學(xué)化工學(xué)院，伍欽，88370073,Qualitatively,theinterceptionofradiationfromanareaelementofasurfacebyanothersurfaceoffinitesizecanbevisualizedintermsoftheangleofvision.華南理工大學(xué)化工學(xué)院，伍欽，88370073,ThefactorFiscalledtheviewfactororanglefactor;itdependsuponthegeometryofthetwosurface4.6-14Theequationfortwobodiesradiatingeachothercanbewrittenintheform華南理工大學(xué)化工學(xué)院，伍欽，88370073,IfsurfaceA1ischosenforA,Eq(4.6-14)canbewritten4.6-154.6-16IfsurfaceA2ischosen華南理工大學(xué)化工學(xué)院，伍欽，88370073,Ingeneral,forgraysurfaces,Eq(4.6-15)andEq(4.6-16)canbewritten(4.6-26)F12andF21aretheoverallinterchangefactorandarefunctionsofε1andε2.華南理工大學(xué)化工學(xué)院，伍欽，88370073,Twolargeparallelplanes4.6-274.6-28Onegraysurfacecompletelysurroundedbyanother華南理工大學(xué)化工學(xué)院，伍欽，88370073,problem華南理工大學(xué)化工學(xué)院，伍欽，88370073,Achamberforheat-curinglargealuminumsheets,lacqueredblackonbothsides,operatesbypassingthesheetsverticallybetweentwosteelplates150mmapart.Oneoftheplatesisat300°C,andtheother,exposedtotheatmosphere,isat25°C.(a)Whatisthetemperatureofthelacqueredsheet?(b)Whatistheheattransferredbetweenthewallswhenequilibriumhasbeenreached?Neglectconvectioneffects.Emissivityofsteelis0.56;emissivityoflacqueredsheetsis1.0.Solution(a)Letsubscript1refertohotplate,2tolacqueredsheets,and3tocoldplate:華南理工大學(xué)化工學(xué)院，伍欽，88370073,ε1,ε3=0.56ε2=1.0T1=573KT3=298KFromEq.(4.6-27)華南理工大學(xué)化工學(xué)院，伍欽，88370073,SinceA1=A2,T2=490.4K=217.4?C華南理工大學(xué)化工學(xué)院，伍欽，88370073,(b)FromEq.(4.6-26)theheatfluxis=5.672×0.56(5.734?4.9044)=1587W/m2Check:=5.672×0.56(4.9044?2.984)=1587W/m2Note:Ifthelacqu