Burning aluminum particles inside a laboratory-scale solid rocket motor

Single and agglomerated aluminum droplets were studied in a solid rocket motor 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 ammonium perchlorate/hydroxyl-terminated poly-butadiene 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 burning rate laws were developed. A high-speed video charge-coupled device with high-magnification optics imaged the flame/smoke cloud surrounding the burning liquid droplets. The intensity profiles of the droplet images were deconvoluted using an Abel inversion to give true intensity profiles. Both single and agglomerated droplets were studied, where agglomerates comprise 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 mean diameter law is D^0.75 = D₀ ^0.75-8 · t, and, for agglomerated droplets, D^1.0 = D₀ ^1.0-20 · t. For both single and agglomerated droplets, the burning rate slope k did not change significantly for the chamber pressure range studied.