Burning aluminum particles inside a laboratory-scale solid rocket motor

Single and agglomerated aluminum droplets were studied in a solid rocket motor (SRM) test chamber with optical access to the internal flow at 6-22 atm and 2300 K. The chamber was pressurized by burning a main grain AP/HTPB propellant, and the burning aluminum droplets were generated by a smaller aluminized solid propellant sample, center-mounted in the flow. A 35 mm camera was used with a chopper wheel to give droplet flame diameter vs. time measurements of the burning droplets in flight, from which burn-rate laws were developed. A high-speed video CCD with high-magnification optics imaged the flame/smoke cloud surrounding the burning liquid droplets. The intensity profiles of the droplet images were de-convoluted using an Abel inversion to give true intensity profiles. Both single and agglomerated droplets were studied, where agglomerates are comprised of hundreds of parent particles or more. The Abel inversions show that the relative smoke cloud size is not constant with diameter, but instead grows as the droplet shrinks, by ~ D^-0.5, for both the single and agglomerated droplets. Measured diameter trajectories show that for single droplets, the diameter law is D^0.75 = D₀^0.75 - 8·t[μm, msec], and for agglomerated droplets, D^1.0 = D₀^1.0 - 20·t, such that the single droplets burn faster than the agglomerates. For both single and agglomerated droplets, the burning rate slope k did not change significantly for the chamber pressure range studied.