Computational Modelling of AP/HTPB Heterogeneous Sandwich Propellant Combustion Considering Non-planar Surface Regression

Abstract This study presents a computational model aimed at simulating the transient reactive flow characteristics of ammonium perchlorate (AP)/hydroxyl-terminated polybutadiene (HTPB) sandwich propellant. The proposed numerical framework incorporates a simplified condensed phase kinetic mechanism to represent the pyrolysis processes of both AP and HTPB. Subsequently, a two-step global kinetic submodel is employed to capture the gas flame structure that arises from the combustion of AP/HTPB propellant. Specifically, the first step of the kinetic model characterizes the self-sustained monopropellant reaction of AP, while the second step delineates the interaction between the products of AP and the pyrolysis products of HTPB. In the gas phase, we solve the two-dimensional unsteady Navier-Stokes equations alongside the energy and species conservation equations. Concurrently, two-dimensional unsteady heat conduction is addressed within the solid phase. The model ensures the continuity of heat flux, mass flux, velocity, temperature, and species concentrations at the pyrolyzing regressing heterogeneous surface, thereby effectively coupling the solid and gas phases. The simulated average burning rates demonstrate strong agreement with the experimental data published by Miller et al., thereby validating the reliability of the proposed model. Furthermore, we conduct simulations of the transient burn behavior of the AP/HTPB sandwich propellant to investigate the variations of multiple parameters, exploring the combustion instability inherent to this propellant system. It is revealed that the combustion process of AP/HTPB propellant at a steady pressure regime is characterized as a progressively stable phenomenon. This stability is attributed to the gradual stabilization of the flame structure, surface morphology, and temperature as the surface regresses.