Non-equilibrium CO2 kinetics: Assessment of advanced multi-temperature models of vibrational–chemical relaxation

Coupled vibrational–chemical relaxation in gas mixtures containing carbon dioxide is studied using a hybrid multi-temperature (HMT) approach. Reliable kinetic schemes are considered taking into account multiple vibrational energy relaxation mechanisms in CO2, such as intramode, intermode, and intermolecular energy exchange coupled to chemical reactions. The reliable forced harmonic oscillator (FHO) model is generalized for polyatomic gases and implemented for calculating the rate coefficients of all vibrational energy transitions. The model is assessed by solving the isothermal bath relaxation problem accounting for diverse vibrational exchanges and allowing for the comparison with independent experimental data on the overall relaxation time. It is shown that the proposed approach provides a good agreement with experimental vibrational relaxation times in CO2, CO2/Ar, and CO2/He mixtures in a wide temperature range, whereas the Landau–Teller model and the Schwartz–Slawsky–Herzfeld models yield significantly overestimated relaxation rates. Recommendations on the steric factors in the FHO model are given. The model is further assessed in adiabatic bath relaxation problems corresponding to heating, cooling regimes, and strongly non-equilibrium conditions typical for high–enthalpy ground test facilities. The role of intermolecular transitions, chemical–vibrational coupling is discussed, and effects of various kinetic schemes and different models for the vibrational energy production rates in polyatomic gas mixtures are analyzed. The combination of the HMT approach with the FHO model represents a robust and numerically efficient tool for simulating strongly non-equilibrium carbon dioxide flows with numerous relaxation channels, providing good accuracy and natural coupling between vibrational relaxation and chemical kinetics.