Radiation from Gases and Suspended Particulate Matter

RADIATION FROM GASES AND SUSPENDED PARTICULATE MATTER 

Introduction Flame radiation originates as a result of emission from water vapor and carbon dioxide in the hot gaseous combustion 

Gas Emissivities Radiant transfer in a gaseous medium is characterized by three quantities the gas emissivity, gas absorptivity, and gas transmissivity. Gas emissivity refers to radiation originating within a gas volume which is incident on some reference surface. Gas absorptivity and transmissivity, however, refer to the absorption and transmission of radiation from some external surface radiation source characterized by some radiation temperature 7. The sum of the gas absorptivity and transmissivity must, by definition, be unity. Gas absorptivity may be calculated from an appropriate gas emissivity. The gas emissivity is a function only of the gas temperature Tg while the absorptivity and transmissivity are functions ofboth Tg and Tt. 

The standard hemispherical monochromatic gas emissivity is defined as the direct volume-to-surface exchange area for a hemispherical gas volume to an infinitesimal area element located at the center of the planar base. Consider monochromatic transfer in a black hemispherical enclosure of radius ft that confines an isothermal volume of gas at temperature Tg. The temperature of the bounding surfaces is T. Let A2 denote the area of the finite hemispherical surface and dAi denote an infinitesimal element of area located at the center of the planar base. The (dimensionless) monochromatic direct exchange area for exchange between the finite hemispherical surface A2 and d then follows from direct integration of Eq. (5-116a) as 

Note that Eq. (5-136b) is identical to the expression for the gas emissivity for a column of path length ft. In Eqs. (5-136) the gas absorption coefficient is a function of gas temperature, composition, and wavelength, that is, Kb = Kb(T,X). The net monochromatic radiant flux density at dAi due to irradiation from the gas volume is then given by 

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Radiation from Gases and Suspended Particulate Matter. 5-30... 

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