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, Zrot, for CH4 relaxing in
collisions with N2 and N2 relaxing in collisions with CH4. Both collision numbers
increase with temperature, with the former being consistently about twice the value
of the latter.