New Approaches to Modeling Methane Flows with Vibrational Relaxation

This study presents a novel detailed mathematical model of vibrational relaxation in pure methane. Based on the critical analysis of the available experimental data, a new kinetic scheme of vibrational energy exchanges is constructed. A reduced five-process scheme for the bending modes is proposed for comparison with experiments. The state-to-state rate coefficients of vibrational–translational (VT) and vibrational–vibrational (VV) processes are calculated on the basis of the forced harmonic oscillator (FHO) model, which is for the first time applied to CH4–CH4 collisions in a unified two-oscillator formulation covering intermolecular and intramolecular energy exchanges. The model parameters are calibrated against experimental relaxation times in the temperature range 140–1100 K. The state-to-state, three-temperature, and two-temperature descriptions of the bending mode relaxation are assessed by solving the isothermal bath problem. It is shown that the three-temperature model yields excellent agreement with the state-resolved solution for the relaxation time. The two-temperature model is valid mainly for moderate and high temperatures. The roles of individual energy transitions in the relaxation are identified, the VT deactivation of the triply degenerate bending mode dominating the relaxation.