Emission imaging of AP/HTPB propellant sandwich combustion

Ultraviolet emission imaging (305-315 nm) was used to study the combustion of sandwiches of ammonium perchlorate (AP) and hydroxyl-terminated polybutadiene (HTPB) in nitrogen at pressures up to 32 atm, with binder layers from 50 to 450 µm in thickness. The emission imaging was combined with a novel backlighting technique to allow determination of the corresponding surface shape during combustion. The results indicated that the interface regression rate of IPDI-cured samples undergoing laser-assisted deflagration (120 W/cm 2 average flux) is nearly independent of the binder thickness for binders thicker than 100 µm. The pressure exponent of the regression rate is 0.31 up to 15 atm, then increases with pressure from 15 to 32 atm. Two primary flame regimes were identified: a regime of high Peclet and Damköhler numbers which exhibits a split base in the ultraviolet flame emission, and a regime of low Peclet and Damköhler numbers which exhibits a merged flame base. A third, "lifted" flame region, in which the strongest flame emission starts several hundred microns above the solid surface, occurs with low Damköhler numbers and high Peclet numbers. The effects of Pe and binder oxygenation (by fine-AP) on the size of the diffusion flame and the location of its leading edges relative to the AP/binder interface were observed to be in agreement with Shvab-Zeldovich theory.