Real-time visualization of aluminum particle combustion in solid composite propellant at motor conditions: agglomeration, bubbling, explosion and pressure effects

Aluminum (Al) particles are widely used as energetic additives in propellants, pyrotechnics, and explosives due to their high energy density and ability to enhance performance. However, the agglomeration of Al during combustion can reduce efficiency, often causing the performance of metalized propellants to fall significantly short of theoretical expectations. Consequently, elucidating the detailed combustion behaviors and underlying mechanisms of Al particles is critical for the design and enhancement of high-performance propellants. The highly transient nature of propellant combustion, accompanied by violent reactions and intense luminescence, makes capturing its internal dynamics particularly challenging. This study integrates in-situ high-speed synchrotron X-ray phase-contrast imaging with combustion experiments at motor conditions to systematically investigate the combustion of Al particles within a hydroxyl‑terminated polybutadiene (HTPB)–based solid composite propellant. Experiments were conducted at environmental pressures ranging from 1.2 to 5.1 MPa. We examine the effects of pressure on combustion properties, Al agglomeration, particle size distributions, and the nucleation and explosion of bubbles within molten Al particles. Key phenomena such as droplet oscillation, bubble coalescence, and explosion are analyzed. Our results reveal that internal bubbles nucleate at the liquid metal-oxide interface and remain confined to this interface during subsequent growth, coalescence, and explosion. A comprehensive model of Al particle combustion is proposed, outlining both general and pressure-dependent behaviors. These findings not only advance the fundamental understanding of Al combustion in propellants but also provide critical insights for controlling particle behavior and designing high-performance metalized solid composite propellants.