AP/HTPB heterogeneous combustion with revised kinetics

This study first proposes a revised combustion model for composite propellants containing ammonium perchlorate (AP) and hydroxyl-terminated polybutadiene (HTPB). It comprises a detailed gas-phase kinetic mechanism, assembled leveraging recent work from the chemical kinetics community. A model for homogenized AP/HTPB decomposition at the burning surface is formulated to perform coupled flame/solid simulations with the proposed gas-phase mechanism. The obtained combustion model is validated against available experimental data on AP/HTPB combustion for propellants containing 60% to 80% mass fraction. Employing this combustion model, 2D heterogeneous combustion of an AP particle surrounded by a layer of homogenized AP/HTPB binder is then studied. These 2D simulations allow to investigate the flame structure and conditions on the propellant surface, such as burning rate. The effect of AP particle size and ambient pressure on the combustion regime is evidenced: conditions leading to a premixed regime or a diffusion-limited regime are identified. Production of chemical species within the flame is finally studied. Significant differences are observed, with respect to a widely used model from the literature, mainly in the formation of end products and NOx species. In particular, it is shown that C₄H₆ isomerization and NO formation are important chemical steps in the flame.