第22讲气体的辐射传热、传质概要1

RADIATION EXCHANGE WITH EMITTING AND ABSORBING GASES（吸收性气体的辐射换热）Gases with asymmetric molecules such as H2O, CO2, CO, SO2, and hydrocarbons HnCm may participate in the radiation process by absorption at moderate temperatures, and by absorption and emission at high temperatures such as those encountered in combustion chambers.throughout its entire volume（射线的吸收概率）at a number of narrow wavelength bands（光带性）depends on the temperature, pressure, and composition of the gas mixture(确定了吸收性和发射性物质数量的多少 )气体辐射特征：1.Radiation Properties of a Participating MediumBeers lawk spectral absorption coefficientRadiation passing through a nonscattering (and thus nonreflecting) medium is either absorbed or transmitted.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.Emissivity and Absorptivity of Gases and Gas MixturesSpectral absorptivity ofCO2 at 830 K and 10 atmfor a path length of 38.8 cmnarrow wavelength bands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total pressure of 1 atmH2O gas总压力不等于1atm时的修正 1 m atm 3.28 ft atmMean beam length L for various gas volume shapes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total pressure of 1 atmCO2 gas1 m atm 3.28 ft atm总压力不等于1atm时的修正pressure correction factorWhat will happen if the CO 2 and H2O gases exist together in a mixture with nonparticipating gases？The emissivity of each participating gas can still be determined as explained above using its partial pressure.The effective emissivity of the mixture should be determined as:Unless the case of different gases emitted at different wavelengthsis the emissivity correction factor, which accounts for the overlap of emission bands.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：for surfaces that are nearly black with an emissivity 0.7取用折中的分析计算EXAMPLE Effective Emissivity of Combustion GasesA cylindrical furnace whose height and diameter are 5 m contains combustion gases at 1200 K and a total pressure of 2 atm. The composition of the combustion gases is determined by volumetric analysis to be 80 percent N2, 8 percent H2O, 7 percent O2, and 5 percent CO2. Determine the effective emissivity of the combustion gases . If the wall temperature is 600K then determine the heat transfer rate between the gas and the wall.SOLUTION：The temperature, pressure, and composition of a gas mixture isgiven. The emissivity of the mixture is to be determined.Assumptions 1 All the gases in the mixture are ideal gases. 2 The emissivity determined is the mean emissivity for radiation emitted to all surfaces of the cylindrical enclosure. 3 All interior surfaces of furnace walls are black.Analysis The volumetric analysis of a gas mixture gives the mole fractions yi of the components, which are equivalent to pressure fractions for an ideal gas mixture. Therefore, the partial pressures of CO2 and H2O are不变习题： 13.42 13.432.2 Mass Diffusion 传质Items we will discuss here:2.2.1 Ficks Law 2.2.2 Mass Diffusion Coefficient The flow of a commodity is always in the direction of decreasing concentration (diffusion process)Fouriers law of heat conduction, Ohms law of electrical conduction, and Newtons law of viscosityHere the proportionality constant kdiff is the diffusion coefficient of the medium, which is a measure of how fast a commodity diffuses in the medium, and the negative sign is to make the flow in the positive direction a positive quantity.Mass transfer can also occur in liquids and solids as well as in gases.diffusion process is strongly influenced by molecules collision process.unlike molecules collision influence the rate of diffusionThey may have different masses （ different momentums）The process be dominated by the heavier molecules.the temperature is a measure of the average velocity of gas moleculesDiffusion rates will be higher at higher temperatures.Molecular spacingThe larger the spacing, in general, the higher the diffusion rate.The diffusion rates are typically much higher in gases than they are in liquids and much higher in liquids than in solids.definition 1 Mass Basis2 Mole Basis2.2.1 Ficks LawUsing mass fractionUsing molar fractionBinary diffusion coefficient or Mass diffusivityStationary Medium Consisting of Two SpeciesThe unit of mass diffusivity is m2/s.The diffusion coefficients of some gases in air at 1 atm pressure2.2.2 Mass Diffusion CoefficientT DABThe Restriction conditions for above equations:1) Mass diffusion occurs as the result of a concentration gradient.2) The fluxes are measured relative to coordinates which move with some average velocity for the mixture.(temperature gradient, pressure gradient, external force may also make the mass diffusion phenomena.)(for a fixed coordinate system, if the mixture is combined by A and B, mass flux will make A species an absolut