Calculation of the thermal conductivity of low-density CH₄-N₂ gas mixtures using an improved kinetic theory approach

Abstract

The thermal conductivity of low-density CH₄-N₂ gas mixtures has been calculated by means of the classical trajectory method using state-of-the-art intermolecular potential energy surfaces for the CH₄-CH₄, N₂-N₂, and CH₄-N₂ interactions. Results are reported in the temperature range from 70 K to 1200 K. Since the thermal conductivity is influenced by the vibrational degrees of freedom of the molecules, which are not included in the rigid-rotor classical trajectory computations, a new correction scheme to account for vibrational degrees of freedom in a dilute gas mixture is presented. The calculations show that the vibrational contribution at the highest temperature studied amounts to 46% of the total thermal conductivity of an equimolar mixture compared to 13% for pure nitrogen and 58% for pure methane. The agreement with the available experimental thermal conductivity data at room temperature is good, within ±1.4%, whereas at higher temperatures, larger deviations up to 4.5% are observed, which can be tentatively attributed to deteriorating performance of the measuring technique employed. Results are also reported for the magnitude and temperature dependence of the rotational collision number, Z(rot), for CH₄ relaxing in collisions with N₂ and for N₂ relaxing in collisions with CH₄. Both collision numbers increase with temperature, with the former being consistently about twice the value of the latter.