Combustion of aluminum in solid rocket motor flows

The combustion of aluminum droplets was studied in a solid-rocket chamber flow-field with variable chamber pressure. Rocket motor chamber conditions were generated directly from an aluminized solid propellant combustion products flow field. The experimental test conditions were 1-20 atm pressure and ~2300 K temperature. Images of the burning aluminum droplets were made with a high-speed CCD video camera, and were de-convoluted with an Abel transformation to give radial intensity profiles of the flame and Al₂O₃ smoke cloud surrounding each droplet. High-magnification experiments have shown major differences in the combustion of aluminum between well-studied 1 atm cold air environments and solid rocket motor conditions. The aluminum droplet flame burns with a much greater intensity at chamber pressures, indicating an increase in flame temperature, and the flame/Al₂O₃ cloud region thickens and extends close to the droplet surface, possibly indicating a reaction-limited mechanism in the H₂O/CO₂ environment. The mean value for the location of the peak intensity in the flame/Al₂O₃ cloud region is (r_peak/r_surf) ~ 2.5, and there was no apparent correlation between flame/Al₂O₃ smoke size and chamber pressure. However, as droplet diameter decreases, the overall Al₂O₃ smoke cloud size increases, indicating a reaction-limited system becoming more fuel-lean as the droplet surface area decreases. Lastly, studies were conducted to observe combustion events at the surface of the aluminized propellant. Aluminum particles were first almost fully 'cleaned' before ignition began, but there was still enough binder to hold the aluminum particles onto the surface during ignition, which can lead to agglomeration.