New mixture rules affect kinetic predictions of energetic material combustion: A case study for RDX, ammonium perchlorate, and nitroglycerin

Abstract

Recent theoretical and computational advancements have unlocked new possibilities for the consideration of mixture dependence of complex-forming reactions. Previous work has shown that implementation of the new linear-mixture-rule-in-reduced-pressure (LMR-R) along with ab initio temperature-dependent third-body efficiencies has a substantial impact on combustion predictions of fuel/air mixtures. Compared to combustion in air, whose primary constituents (N${}_2$, O${}_2$) have third-body efficiencies close to unity, combustion of monopropellants often produces mixtures almost exclusively composed of species that are polyatomic, polar, or both and therefore would tend to have high third-body efficiencies—such that composition dependence may be equally (if not more) significant. This paper presents the first implementation of LMR-R and accompanying third-body efficiencies for energetic material combustion. The impact of composition dependence is illustrated through deflagration simulations for cyclotrimethylene trinitramine (RDX), ammonium perchlorate (AP), and nitroglycerin (NG), three common energetic materials whose kinetic mechanisms are the subject of extensive previous and ongoing investigation. Our present analysis reveals that consideration of mixture rules has a major effect on predictions of burn rates and species profiles, also impacting features that describe flame thicknesses and dark zone structures. Importantly, the results demonstrate the importance of proper treatment of composition dependence in future energetic material models and motivate future characterization of different complex/collider pairs than those important to fuel/air combustion.

Novelty and significance statement: This study presents the first systematic investigation of the impact of rate-constant composition dependence on deflagration predictions for energetic materials. Consistent with the abundance of species with high third-body efficiencies encountered in such scenarios, composition dependence is found to significantly influence predictions of burn rates and species profiles, which in turn have implications for flame thicknesses and dark zone structures. Based on these results, we recommend future energetic material models include systematic treatment of composition dependence and we identify specific complex/collider pairs worth further fundamental characterization.