Post-reaction internal energy distributions of quantum-kinetics model for simulating chemical reactions of polyatomic molecules

Chemical reactions significantly influence aerodynamic performance during spacecraft entry into the Martian atmosphere. Several chemical reaction models have been proposed in the direct simulation Monte Carlo simulation. The quantum-kinetics (Q-K) model has been applied in the case of diatomic molecules. Given that the Martian atmosphere consists primarily of CO2, it is crucial to find ways of implementing the Q-K model for polyatomic molecules. Although the chemical reaction rates involving CO2 have been investigated using the Q-K model, the problem of achieving detailed balance remains. Multiple vibrational modes exist for polyatomic molecules. Under the Q-K distribution, the average vibrational level of each mode is higher than that under the equilibrium distribution, and the total energy may be insufficient. Hence, its applicability to polyatomic molecules needs to be revealed. In this study, a comparison is made of the respective results obtained using the Larsen-Borgnakke (L-B) and Q-K distribution methods for the energy distribution of the reaction CO2+O⇋CO+O2, and detailed balance is achieved with the Q-K method but not the L-B method. Under the conditions assumed in this study, the vibrational energy distribution of CO consumed by the reverse reaction is not in good agreement with that generated by the forward reaction, leading to the failure of the L-B method. Finally, the results indicate that detailed balance is reached only when the collision temperature, based on the translational and vibrational energy, is employed to adjust the activation energy rather than the translational temperature generally adopted in the literature.