Chemical reaction modeling for hypervelocity collisions between O and HCl

Abstract

The sensitivity of a rarefied-to-transitional flow to the fidelity of the chemical reaction model is investigated for a new molecular dynamics/quasiclassical trajectory (MD/QCT)-derived model and compared with the widely used total collision energy (TCE) model of Bird. For hypervelocity collisions that occur in the space environment, it is not clear, a priori, that the TCE model will provide reasonable results for the required high energy range and, particularly, if strong favoring of the reaction among different forms of reactant energy occurs. In fact, in previous work, the TCE model, using available Arrhenius parameters, has been found, for these flow conditions, to give unphysical probabilities. A chemical reaction model, suitable for use in the direct simulation Monte Carlo (DSMC) method, is developed to simulate the hypervelocity collisions of O(P3)+HCl(Σ+1)→OH(Π2)+Cl(P2), an example of an important reaction in high-altitude atmospheric-jet interactions. The model utilizes the MD/QCT method with a new benchmark triplet A″ surface. Since the modeling of chemical reactions in DSMC simulations requires the use of a reaction probability, the adequacy of the overall collision cross section, usually modeled by the variable hard sphere (VHS) model, is also considered. To obtain an accurate collision cross section, the approach of Tokumasu and Matsumoto was used in the MD/QCT method with the aforementioned potential energy surface. Energy transfer between the target HCl translational and internal energy modes was investigated and it was found that the variation of the inelastic cross section has a negligible effect on the transport cross section. Therefore, a MD/QCT VHS equivalent collision cross section was obtained and along with the MD/QCT reaction cross sections were utilized in the full DSMC calculation of the flow field. It was found that for a low enthalpy reaction, in hypervelocity collisions, the TCE model with accurate Arrhenius rates appears to agree well with the rigorous MD/QCT calculations which shows that the reaction does not exhibit strong favoring.