AP/HTPB laminate propellant flame structure: Fuel-lean intrinsic instability

Two-dimensional laminate propellant flames of ammonium perchlorate (AP) and hydroxyl-terminated polybutadiene (HTPB) have been observed using infrared (IR) and ultraviolet (UV) emission and transmission imaging. Under fuel-lean conditions and at slightly elevated pressures (4 atm), intrinsic instability has been observed in the form of a leading-edge flame kernel whose location oscillates laterally about the central fuel binder layer. A mechanistic explanation for this behavior is described in terms of local gas-phase equivalence ratio, surface geometry, and gas–solid thermal coupling. The flame structure under these conditions is unique in having a leading-edge flame kernel that appears to be more spatially distinct from the trailing diffusion flame than under nonoscillatory conditions. Other results are reported, including gas-phase rotational and vibrational temperature estimates based on HCl emission imaging spectroscopy. These results add to a growing set of flame and burning surface observations being assembled for the purpose of comprehensive validation of multi-dimensional AP composite propellant computational combustion models.