In our discussion of radiation gasdynamics in the previous chapters, we assume implicitly that the mean free path of the gas particles is small so that the gas may be considered as a continuum in the analysis of flow problems. In many flow problems, especially those connected with space sciences, the mean free paths of the gas particles are not small and we have to consider the discrete properties of the gas. We have to use the kinetic theory of gases to study the flow problems of rarefied gases. In the kinetic theory of radiating gas, we should use the corpuscular picture to represent radiation instead of the wave picture as we discussed in chapter II. In other words, we should consider radiation as a stream of photons. The gas should be considered as a mixture of various types of particles, material particles as well as photons. However, it is usually not possible to investigate the detailed motion of all the particles in a gas flow because of the large number of particles considered. We have to use the statistical average of the motions of the particles of a gas. In such a kinetic theory, we may use a molecular distribution function for each species of the mixture of the gas. In the most successful kinetic theory of gases, one particle distribution function (2, 3, 5, 8) is used to describe the microscopic behavior of the system. We shall review briefly some of the essential features of the one particle molecular distribution function in section 2.
KeywordsBoltzmann Equation Kinetic Theory Knudsen Number Radiative Heat Flux Collision Term
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