New kinetics for a model of heterogeneous propellant combustion

In earlier work we describe an unsteady, three-dimensional, phase-coupled combustion code which, with the use of a random packing algorithm to construct model propellants, and the use of a homogenization strategy to account for unresolvably small propellant particles, can be used for the simulation of heterogeneous propellant combustion. This work uses a simple two-step kinetic model for ammonium perchlorate (AP)/hydroxyl-terminated polybutadiene (HTPB) combustion which fails to accurately predict variations in the burning rate with AP concentration for a homogenized AP[HTPB blend supporting a one-dimensional flame. Here we describe a three-step model, one which captures the three flames of the Beckstead-Derr-Price (BDP) combustion model, and show that kinetic parameters can be adopted so that one-dimensional AP burning rates and one-dimensional AP/HTBP blend burning rates can be correctly predicted. We discuss the stability of the underlying flame structures and highlight a difficulty that arises in these instability-prone systems when simple kinetic models are used to describe them. The combustion model, with the new kinetics, is used to reexamine the burning of random packs, and improved agreement with the experimental burning rates of Miller packs is demonstrated. We also reexamine the problem of sandwich-propellant combustion and investigate the trend in surface shape and burning-rate variations with pressure and binder width. These trends are compared with experimental results of Price. The sandwich configuration is used to measure the importance of the primary diffusion flame of the BDP model.