The dominant effect of alumina on nearfield plume radiation

Abstract

Solid propellant rocket motors can achieve high specific impulse with metal fuel additives such as aluminum. Combustion of aluminum produces condensed alumina particles. Besides causing performance losses in the nozzle, the condensed Al 2O 3 particles are the major source of primary smoke in the exhaust plume. The particulate matter can also have major effects upon the plume i.r. signature. High number densities of particles can block gas-phase radiation from the plume. They can also be the source of radiation, especially the larger particles which exit the nozzle not in thermal equilibrium with the gas. In the past, the expected effects of particle size on the plume i.r. signature have been determined almost exclusively from predictions made with flow and radiation codes. The aim of the present work was to investigate the role of the Al/Al 2O 3 particles from a highly loaded solid propellant (up to 16% in weight) on the plume radiation of a small rocket motor (5 cm in diameter). The spatial variation of particle size distribution was simultaneously measured with the overall radiation of a portion of the plume in the i.r. band (3.5–5.0 μ). In micro-motors, operating with highly aluminized solid propellant, the condensed particles in the near exhaust plume were the major source of radiation in the 3.5–5 μ wavelength band. Motors with longer residence time and operating at medium chamber pressures produced more particles in the micron sized range. The role of afterburning was predominately confined to reheating of the alumina particles to a higher temperature, at which the condensed Al 2O 3 radiated more than gaseous species. Even with 30% Al 2O 3 in the plume, the plume of small motors can be considered as approximately conical in shape, with volume distributed radiating sources. Motor conditions producing larger particles in the plume core were thus found to increase plume radiation from that region. The overall apparent emissivity of the plume was between 0.15 and 0.25 (dominantly 0.16–0.19). Nearfield particle size distributions were tri- or quadra-modal and the farfield contained significantly fewer large particles. Thus, assumed monomodal distributions should not be expected to result in the correct prediction of the effects of particles onplume signature, nor can particle size data obtained in the farfield be expected to be applicable in the nearfield.