Thermal Diffusion in Polyatomic Gases: A Generalized Stefan—Maxwell Diffusion Equation

Abstract

A study is made of the influence of inelastic processes on thermal diffusion in polyatomic gases, based on the Wang-Chang—Uhlenbeck—deBoer treatment of the Boltzmann equation, in which it is assumed that the distribution function for molecular spins is isotropic. A generalized Stefan—Maxwell diffusion equation is obtained for multicomponent mixtures of polyatomic gases, carried to the second Chapman—Enskog approximation. The external form of this equation is the same as the well-known result for monatomic gases, but contains higher-order corrections for the binary diffusion coefficients (i.e., the composition dependence of these coefficients), and correction terms for the effect of inelastic collisions on the diffusion and thermal-diffusion coefficients. Numerical calculations for several selected systems show that the effects of inelastic collisions on the thermal-diffusion factor are not negligible and must be considered in any attempt to derive information on intermolecular forces from thermal-diffusion measurements. However, inelastic effects are not capable by themselves of explaining the known anomalies in systems such as Ar–HCl and D2—HT.