Thermal diffusion in binary mixtures containing molecular gases. I. Formal expressions in the total-energy-flux and two-flux representations

Abstract

Expressions are developed for the thermal diffusion factor, αT, for a binary mixture of molecular gases. Two different representations have been considered, namely, the so-called ‘total-energy-flux’ and ‘two-flux’ representations that have been used for the description of the thermal conductivity. Two approximations have been considered for binary mixtures in which both chemical species are molecular, one being for the minimal basis set, in which only the leading expansion term for each species is considered in addition to the microscopic diffusion flux, the other being for the complete lowest-order basis set, which has five elements. Explicit expressions are also given for the thermal diffusion factor for binary mixtures in which one chemical species is atomic, the other molecular. The thermal diffusion factor is written as the sum of two pieces, one of which is referred to as the ‘isotropic’ contribution, αiso T, the other as the ‘polarization’ contribution, αpol T. When both species in the binary mixtures are noble gases, αiso T reduces, as it should do, to the standard noble gas expression for αT found in the literature, and αpol T vanishes.