Aluminum agglomeration of AP/HTPB composite propellant

Aluminum (Al) powder agglomeration is one of the main reasons for the degradation in the performance of aluminized solid propellant rockets and so, understanding the combustion behavior of aluminum in solid propellants is of great importance. In this work, a laser ignition test bench was used to study the behavior of Al on the burning surface of an aluminum/ammonium perchlorate/hydroxyl-terminated polybutadiene composite propellant, under atmospheric pressure. Based on the images captured by a high-speed camera, the agglomeration process and behavior of the agglomerates were analyzed in detail. The size distribution and speed of motion of the agglomerates away from the burning surface were also considered. Results show that the formation of an agglomerate on the burning surface from multiple aluminum particles include three stages: accumulation, aggregation, and agglomeration. Local ignition promotes the collapse of the aggregate into a spherical agglomerate. Before detachment, the agglomerates often roll around on the burning surface and pick up more aluminum, promoting self-growth. The interesting phenomenon of an agglomerate droplet rupturing and ejecting liquid alumina on the burning surface was clearly captured for the first time. This is believed to indicate the heterogeneous composition characteristics of the agglomerate. The transformation of the polar oxide cap on one part of the agglomerate surface into an alumina shell that fully covers the droplet surface was also captured for the first time. The agglomerates have different shapes, diameters, and velocities when they leave the burning surface. The formation of non-spherical agglomerates consisting of more than one aluminum droplet is ascribed to the propellant microstructure. The 400 agglomerates that were counted had diameters that ranged from 51 μm to 815 μm and the majority of them (nearly 98%) were below 400 μm. The velocities of motion of 176 agglomerates exhibited great dispersion, with the maximum and minimum velocity being 196 cm/s and 13 cm/s, respectively. In general, the moving velocities of the agglomerates decreased with increasing diameter. •Interesting phenomenon of an agglomerate rupturing and oxide cap growth are unveiled.•Relationships between propellant microstructure and agglomerate shapes are analyzed.•Size distribution and speeds of motion of agglomerates are given.