O+HC1 chemistry models for hypervelocity collisions in DSMC

In earlier work,¹ atmospheric-jet interaction flows were simulated using direct simulation Monte Carlo (DSMC) calculations for altitudes of 80, 120 and 160 km. At high altitudes, the O(³P)+HC1( ¹Σ⁺) → OH(²II)+Cl(2P) reaction was found to contribute the most to OH production. However, the total collision energy (TCE) reaction probability was often found to be unphysically greater than unity. In this work, we examine in detail different reaction models such as the TCE model using the rate constants of Mahmud et al² and Xie et al.³ In addition to the TCE model, quasi-classical trajectory (QCT) calculations were performed to obtain the reaction probability for the O+HCl reaction with the new benchmark triplet A" and A' surfaces.⁴ Both reaction and total collision cross sections were calculated by the QCT method. The dynamic molecule collision model was used to calculate the viscosity cross section from which the VHS-equivalent collision cross sections were derived. The tabulated QCT reaction probabilities were then used in the DSMC calculations to model OH production at 120 km. It is found that the QCT-based collision cross sections are greater than the Bird VHS cross sections, and the QCT reaction probabilities for O+HCl are lower than the TCE probabilities using the rate of Mahmud et al. Using the QCT reaction probability, the maximum reaction probability for the O+HCl reaction was found to be lower than 0.4 in the atmospheric - jet interaction flows for an altitude of 120 km and a freestrearn velocity of 5 km/s.