Investigation on the microscale combustion characteristics of AP/HTPB propellant under wide pressure range

This study developed a combustion model for ammonium perchlorate (AP)/hydroxyl–terminated polybutadiene (HTPB) composite propellant to capture the flame structure and elucidate the mechanism governing burning rate at different pressures, AP sizes and surface morphologies. A numerical model was proposed to characterize gas domain by solving Navier-Stokes equations of reacting flow field with 3-step and 12-species global kinetics; and the condensed domain was coupled by considering interfacial energy balance. The present model was validated with experimental results of both pure AP and AP/HTPB combustion, in which the mean absolute percentage error of burning rate for all cases was less than 5%. The standoff distances of AP monopropellant, premixed binder and primary diffusion flames were determined as 14.2, 12.1 and 25.8 μm at 2 MPa, respectively, reducing with pressure. The front and height of final diffusion flame were determined by axially varied radial gradient of temperature and its height was obtained as 152 μm at 2 MPa, which rises as the pressure or AP particle size increases. The increase of burning rate with pressure is attributed to the raise of surface heat flux dominated by standoff distances of AP monopropellant, premixed binder and primary diffusion flames, in which the effect of primary diffusion flame enhances as the AP size decreases resulting from enlarged lateral diffusion of species. Finally, the effects of protrusive, recessive AP surfaces and their combination with planar AP surface on the flow and flame properties were examined.