Agglomeration and ignition mechanism of aluminum particles in solid propellants

Aluminum powders added to conventional rocket propellants burn either as single particles or agglomerates which contain hundreds or even thousands of the original particles. Combustion efficiency and acoustic stability characteristics are very dependent on the final Al/Al 2 O 3 particle size injected into the chamber flowfield. High-speed photographs of burning homogeneous propellants provided data on agglomeration size and visualization of the flow processes as a function of pressure (1 to 10 MPa), initial particle size (5 to 100 μm), and aluminum mass fraction (0.1 to 13%). Photomicrographs of extinguished surfaces revealed the importance of particle accumulation in a thin mobile reaction layer adjacent to the burning surface. A model was developed that interpreted data and observations from several sources. The model accounts for accumulation of aluminum particles in the mobile reaction layer, retention of particles by surface tension forces, melting, and ignition at the surface. The following agglomeration and particle behavior items are categorized: decreasing agglomerate size with increasing pressure, minimum mass loading required for agglomeration, prominent agglomeration for particles with diameters less than the reaction layer thickness, and sharply reduced agglomeration for larger particles. The model provides an approach for controlling and interpreting agglomerate size behavior.