Formulation of Reduced Order Chemistry Models for Energetic Materials

A reduced order reactive chemistry model for the energetic material HMX has been formulated based on reactive molecular dynamics simulations using ReaxFF-lg. Simulations on 240 molecules were performed over a large range of initial densities and temperatures, and the results were analyzed by classifying each atom in the simulations according to its coordination environment. The time evolution of these geometries during a molecular dynamics simulation creates an array of data that can be transformed into a reduced chemistry model using the Non-negative Matrix Factorization technique. This captures the general reaction characteristics (e.g. reduction of nitrogen, oxidation of carbon) in a series of correlated chemical waves. From all of the simulations, a 7-component, pressure-dependent Arrhenius reaction rate model was formulated. In particular, separate high- and low-pressure reaction branches were identified. Large-scale 1D simulations (≥1M atoms) of both thermal ignition and void collapse were then performed and analyzed with this model. The flame structure found in both cases mapped quite well onto the derived chemistry model and reflected the expected pressure dependence. At higher pressures, the flame was quite thin (~10 nm) and propagated at ~100 m/s.