Analysis of thermal radiation from burning aluminium in solid propellants

Thermal emission from burning aluminium droplets in solid propellants has been analysed using radiative transfer theory and infrared intensity measurements from emission images of burning droplets at 1-5 atm. Below 5 atm, infrared emission is dominated by the burning droplet, not the detached flame zone around the droplet. Each droplet consists of a molten metal portion and a molten oxide (cap) portion. Emission from the metal portion of the droplet surface is spatially uniform and corresponds with opaque surface theory. Emission from the oxide cap portion of the droplet surface is spatially non-uniform and corresponds with volumetric emission from a non-opaque surface. The oxide cap was approximated as a truncated section of a sphere. The burning droplet temperature was found to be approximately 300 K below the metal boiling point. Account was also taken for non-isothermal particulate emission by the sub-micron oxide smoke cloud formed in the detached flame zone around the droplet. The analysis was able to represent infrared emitted intensity reasonably well for various oxide cap thicknesses and orientation angles. Extending the analysis to include all wavelengths suggested that smoke envelope emission becomes more important at shorter (e.g., visible) wavelengths. A simple, linear relation for the incremental increase in total (spectrally and directionally integrated) droplet emissivity owing to smoke envelope emission was obtained at 1 atm: Δ ϵ = 0.001D(μ m).