Reactive Molecular Dynamics Simulation of Methane-Oxygen Autoignition at High-Pressure Conditions

Abstract

An investigation of autoignition using molecular dynamics simulations and ReaxFF force fields is presented. The study is motivated by the fact that combustion at rocket engine conditions of high pressures can involve real gas behavior that is not captured by chemical kinetic models and kinetic solvers based on ideal gas assumptions. Also, the mechanistic reaction pathways at these conditions may not be well known. Molecular dynamics simulations based on reactive force fields can be used to gain insight into combustion under these conditions. However, for such molecular dynamics simulations to yield useful and trustworthy results, they must be able to simulate thermodynamic ensembles that are relevant to practical combustion, such as constant volume adiabatic reactors. They must also be able to reproduce known features from combustion simulations using continuum and statistical chemical kinetic models. These aspects can be verified for small molecular fuel systems, such as methane. In this work, the autoignition of methane-oxygen mixtures at pressures of 200 atm is simulated using non-equilibrium molecular dynamics with the ReaxFF force fields and the LAMMPS software package. To account for difficulties associated with maintaining the internal energy constant, a combination of NVT and NVE ensembles is used to capture the rapid temperature rise associated with autoignition. The evolution of key chemical species is examined and a characteristic ignition delay time is defined for each temperature. The results are contextualized by comparing them to the predictions of two continuum and statistical chemical kinetic models and the Chemkin Pro solver. ReaxFF simulations are found to reproduce the chemical structure of autoigniting reactors. The ignition delay times obtained from the ReaxFF are comparable to those obtained from continuum kinetic models, although the ReaxFF results are characterized by a higher global activation energy. With respect to the final products of the ignition process, ReaxFF predicts CO and OH levels that are comparable with continuum kinetic and equilibrium models. Generally, ReaxFF under predicts the formation of triatomic molecules. This study advances the use of molecular dynamics simulation to study standard combustion problems, such as constant-volume autoignition.