Dissociation Kinetics of Oxygen Molecules under High-Temperature Nonequilibrium Conditions

In hypersonic and other thermochemical nonequilibrium aerodynamic environments, conventional two-temperature models typically assume translational-rotational equilibrium and assign a fixed weighting to the vibrational temperature to describe nonequilibrium dissociation rates, but such empirical treatments lack rigorous validation at the microscopic kinetic level. In this work, the O + O₂ collision induced dissociation system is investigated to quantitatively elucidate the effects of rotational and vibrational nonequilibrium on dissociation mechanisms and their temperature dependence from the state-to-state perspective. A state-resolved kinetic database is constructed using the quasi-classical trajectory method in combination with a hybrid Gaussian process regression-neural network (GPR-NN) model. Based on this database, nonequilibrium effects are analyzed in terms of microscopic energy distribution and flux contribution structures to reveal the underlying nonequilibrium kinetic mechanisms. The results show that, assuming the vibrational temperature equals the translational temperature, the sensitivity of the dissociation rate to rotational temperature deviation decreases significantly with increasing temperature. Furthermore, vibrational nonequilibrium is found to induce a nonlinear amplification of the dissociation flux by enhancing the population of highly excited vibrational states, leading to a non-monotonic contribution structure governed by the competition between state specific reaction rates and population probabilities. Meanwhile, the dissociation mechanism gradually shifts from a vibration-dominated regime to a translation-dominated regime with increasing temperature, accompanied by a non-continuous transition of the dominant vibrational level. In the investigated high-temperature range from 4000 to 20000 K, rotational nonequilibrium can be reasonably neglected, whereas vibrational nonequilibrium exerts a significant and nonlinear influence on dissociation kinetics, and conventional two-temperature models are inadequate to capture such mechanism transition. Conventional two-temperature models with fixed weighting factors are inadequate to capture such transitions and may overestimate vibrational effects under strong nonequilibrium conditions. The present study provides a useful reference for the development and improvement of nonequilibrium chemical kinetic models for high-temperature dissociation processes.