Hydrogen combustion: mixture rules and rate constants. A case study on the multicomponent pressure dependence of H+O 2 +M=HO 2 +M

Abstract Hydrogen combustion has recently been the subject of considerable interest because of its perspective use as energy vector. It has though been shown that the mixture rules used to determine rate constants of termolecular reactions currently implemented in most kinetic simulation software are physically inconsistent when used in the fall-off regime. Also, there is considerable uncertainty in the dependence on the bath gas of the rate constants of some key termolecular reactions. In this context, we developed an approach to study hydrogen reactivity whose final aim is the full a priori revision of the elementary kinetics of hydrogen combustion chemistry. Specifically, we present an efficient and accurate implementation of a method for determining the rate constants of termolecular reactions, we apply it to the study of the H+O₂+M→HO₂+M reaction both for single and multiple colliders, and we investigate its impact on kinetic simulations. First, the contribution of intermolecular energy transfer to the reactive process is determined using a 1D master equation model with the collisional kernel described using the single exponential energy transfer model and recombination fluxes computed using Variable Reaction Coordinate Transition State Theory. The energy transfer parameters of the collisional model are then fitted for several colliders (Ar, He, N₂, H₂, CO₂, H₂O) through regression over a large set of experimental rate constants, by interfacing the master equation simulator MESS to a non-linear regression software. Finally, a physically consistent mixture model is implemented for the first time in a kinetic simulator, the OpenSmoke software, and kinetic simulations are performed to study model performances. The simulation results show that the impact of the new sets of rate constants so determined can be significant in some combustion environments.