Automated discovery of influential chemically termolecular reactions in energetic material combustion: A case study for RDX

Recent studies have indicated that chemically termolecular reactions, mediated by ephemeral collision complexes, have significant impact on the global kinetics of certain combustion systems. This discovery has since prompted the question of which systems are significantly influenced by chemically termolecular reactions and therefore require their consideration in gas-phase phenomenological models. Although a select number of systems have already been investigated, such as flame speed and ignition delay predictions in common hydrocarbon combustion scenarios, the influence of chemically termolecular reactions on the kinetics of energetic materials has not yet been explored. As an initial investigation into energetic materials, we performed a case study for RDX, for which abundant computational and experimental data are available. To aid in assessing the impact of chemically termolecular reactions, for which almost no data are available, this study leverages an automated procedure to identify and estimate rate constants for potential chemically termolecular reactions based exclusively on data available for related reactions. Four detailed kinetics models for RDX have been independently screened for potential chemically termolecular reactions. Model predictions including these chemically termolecular reactions reveal they have significant potential impact on profiles of major species, radicals, and temperature, including the temperature gradient at the burner surface affecting burn rates. The analysis pinpoints  ∼ 20-40 chemically termolecular reactions, out of the thousands of possibilities, estimated to have the largest impact. These reactions, including many mediated by ephemeral NNH** and HNO** complexes, are therefore worthwhile candidates for more accurate quantification via master equation calculations. More generally, just as the importance of including chemically termolecular reactions in hydrocarbon combustion models is becoming recognized, the present results show compelling evidence for the need for their inclusion in energetic material models as well.