Heterogeneous propellant combustion

In earlier work it has been shown that spheres of various sizes can be randomly packed to simulate the morphology of ammonium perchlorate (AP)/binder heterogeneous propellants. A model is now formulated in which propellants defined in this way can be burnt, allowing for complete coupling between the gas-phase physics, the condensed-phase physics, and the unsteady nonuniform regression of the propellant surface. The gas-phase kinetics is represented by a two-step model with parameters fitted to experimental data for the one-dimensional combustion of pure AP and the one-dimensional combustion of a fine-AP/binder blend. In the discussion of the pure AP fits, the issue of intrinsic instabilities arises, and these are explored using nonlinear direct numerical simulation and a numerical linear-stability strategy. Results for two-dimensional heterogeneous burning show that surface regions where the AP and binder can mix regress more rapidly than those dominated by AP: local extinction can occur where the binder dominates. Time variations in the integrated flux-based equivalence ratio are shown to be large. Variations in the average burning rate with pressure are consistent with the (three-dimensional) experimental record.