Direct simulation Monte Carlo modeling of H2-O2 deflagration waves

Combustion at extreme conditions such as high-speed and microscale involve nonequilibrium transport and chemical reactions that require atomistic treatment of molecular processes. We present a framework for applying the direct simulation Monte Carlo method (DSMC) to model combustion at the molecular scale. We show that the standard DSMC approach employing Total Collision Energy (TCE) chemistry and Larsen-Borgnakke (LB) energy exchange models is not applicable for combustion simulations which are dominated by exchange and recombination reactions. A methodology for modifying the TCE-LB approach is developed to ensure detailed balance and relaxation towards thermal equilibrium regardless of the internal energy relaxation rates. A simplified 6-species and 7-reversible reaction mechanism with rates modified to account for discrete vibrational levels in DSMC is used for the benchmark flame study. The laminar flame structure of H2-O2 premixed systems and the corresponding deflagration wave speeds by DSMC are compared with PREMIX results. The DSMC simulations based on the extended TCE-LB framework correctly reproduces the 1-D flame structure and its propagation speed is consistent with continuum modeling predictions for the same reaction mechanism and similar flow conditions. The DSMC approach presents opportunities to study combustion phenomena at the molecular scale including state-to-state processes in conditions far from thermal equilibrium for improved combustion diagnostics and control.