第22讲气体的辐射传热、传质

RADIATIONEXCHANGEWITHEMITTINGANDABSORBINGGASES（吸收性气体的辐射换热）GaseswithasymmetricmoleculessuchasH2O,CO2,CO,SO2,andhydrocarbonsHnCm

mayparticipateintheradiationprocessbyabsorptionatmoderatetemperatures,andbyabsorptionandemissionathightemperaturessuchasthoseencounteredincombustionchambers.throughoutitsentirevolume（射线的吸收概率）atanumberofnarrowwavelengthbands（光带性）dependsonthetemperature,pressure,andcompositionofthegasmixture(确定了吸收性和发射性物质数量的多少)气体辐射特征：1.RadiationPropertiesofaParticipatingMediumBeer’slawkλ

spectralabsorptioncoefficientRadiationpassingthroughanonscattering(andthusnonreflecting)mediumiseitherabsorbedortransmitted.贝尔定律假设此系数为常数，有此系数称气体的单色吸收系数假设无反射假设基尔霍夫定律可用2.EmissivityandAbsorptivityofGasesandGasMixturesSpectralabsorptivityofCO2at830Kand10atmforapathlengthof38.8cmnarrowwavelengthbands在工程计算时，采用平均发射率，其值取决于气体分子数量和气体的温度。下面的数据是H2O、CO2与不参与辐射气体混合物数据totalpressureof1atmH2Ogas总压力不等于1atm时的修正1m·atm＝3.28ft·atmMeanbeamlengthLforvariousgasvolumeshapes射线平均行程概念：半球空间内气体对球心的辐射这里理解为最大的平均距离其它几何形状：可以理解为其中气体的发射率为相当于某半径下半球空间气体的发射率之值。其中：容积为气体体积；面积为气体与包壳接触面积。理念上，比较模糊重点是以用为主totalpressureof1atmCO2gas1m·atm＝3.28ft·atm总压力不等于1atm时的修正pressurecorrectionfactorWhatwillhappeniftheCO2andH2Ogasesexisttogetherinamixturewithnonparticipatinggases？Theemissivityofeachparticipatinggascanstillbedeterminedasexplainedaboveusingitspartialpressure.Theeffectiveemissivityofthemixtureshouldbedeterminedas:Unlessthecaseofdifferentgasesemittedatdifferentwavelengthsistheemissivitycorrectionfactor,whichaccountsfortheoverlapofemissionbands.需要注意到此修正是与气体的温度相关的。由于气体的辐射特性与波长有关系，因此它们的吸收率与发射率不相同！但吸收率同样需要考虑到重叠部分，有：其中，H2O和CO2的吸收率分别按下计算：确定时，注意：与发射率的修正因子相同。的确定所使用的温度要以发射源的温度为准。H2OCO2同时，不再以为参数，而是以替代之。重叠部分修正系数：它与温度是没有关系的。辐射性气体与其包壳之间的换热（已知气体的发射率和吸收率）换热模型本质：两个物体组成的封闭系统。单位面积气体（实际上是与包壳接触的面积）的辐射力：假定包壳的温度是，包壳对气体的辐射力为：考虑各自的多次吸收，反射及透射等因素，换热量计算将是复杂的。简单的情况：包壳是“黑体”，那么它的反射量为0，对气体而言，单位面积上离开的能量和吸收的能量代数和为：换热量：forsurfacesthatarenearlyblackwithanemissivity>0.7取用折中的分析计算EXAMPLEEffectiveEmissivityofCombustionGasesAcylindricalfurnacewhoseheightanddiameterare5mcontainscombustiongasesat1200Kandatotalpressureof2atm.Thecompositionofthecombustiongasesisdeterminedbyvolumetricanalysistobe80percentN2,8percentH2O,7percentO2,and5percentCO2.Determinetheeffectiveemissivityofthecombustiongases.Ifthewalltemperatureis600Kthendeterminetheheattransferratebetweenthegasandthewall.SOLUTION：Thetemperature,pressure,andcompositionofagasmixtureisgiven.Theemissivityofthemixtureistobedetermined.Assumptions1Allthegasesinthemixtureareidealgases.2Theemissivitydeterminedisthemeanemissivityforradiationemittedtoallsurfacesofthecylindricalenclosure.3Allinteriorsurfacesoffurnacewallsareblack.AnalysisThevolumetricanalysisofagasmixturegivesthemolefractionsyiofthecomponents,whichareequivalenttopressurefractionsforanidealgasmixture.Therefore,thepartialpressuresofCO2andH2Oare不变习题：13.4213.432.2MassDiffusion传质Itemswewilldiscusshere:2.2.1Fick’sLaw2.2.2MassDiffusionCoefficientTheflowofacommodityisalwaysinthedirectionofdecreasingconcentration(diffusionprocess)Fourier’slawofheatconduction,Ohm’slawofelectricalconduction,andNewton’slawofviscosityHeretheproportionalityconstantkdiffisthediffusioncoefficientofthemedium,whichisameasureofhowfastacommoditydiffusesinthemedium,andthenegativesignistomaketheflowinthepositivedirectionapositivequantity.Masstransfercanalsooccurinliquidsandsolidsaswellasingases.diffusionprocessisstronglyinfluencedbymoleculescollisionprocess.unlikemoleculescollision——influencetherateofdiffusionTheymayhavedifferentmasses（differentmomentums）Theprocessbedominatedbytheheaviermolecules.thetemperatureisameasureoftheaveragevelocityofgasmoleculesDiffusionrateswillbehigherathighertemperatures.MolecularspacingThelargerthespacing,ingeneral,thehigherthediffusionrate.Thediffusionratesaretypicallymuchhigheringasesthantheyareinliquidsandmuchhigherinliquidsthaninsolids.definition1MassBasis2MoleBasis2.2.1Fick’sLawUsingmassfractionUsingmolarfractionBinarydiffusioncoefficientorMassdiffusivityStationaryMediumConsistingofTwoSpeciesTheunitofmassdiffusivityism2/s.Thediffusioncoefficientsofsomegasesinairat1atmpressure2.2.2MassDiffusionCoefficientTDABTheRestrictionconditionsforaboveequations:1)Massdiffusionoccursastheresultofaconcentrationgradient.2)Thefluxesaremeasuredrelativetocoordinateswhichmovewithsomeaveragevelocityforthemixture.(temperaturegradient,pressuregradient,externalforcemayalsomakethemassdiffusionphenomena.)(forafixedcoordinatesystem,ifthemixtureiscombinedbyAandB,massfluxwillmakeAspeciesanabsolutevelocity