Radiative properties of burning aluminum droplets

Abstract

The spectral intensity (790 nm) emitted during the combustion of pressed pellets composed of ammonium perchlorate (AP) and aluminum has been measured. Aluminum mass fraction content was varied from 0.5% to 6.0%. The combustion bomb pressure ranged from the low pressure deflagration limit (near 650 psia or 4.5 MPa) to 1100 psia or was well above the local gas temperature and in excess of the adiabatic flame temperature assuming complete Al 7.6 MPa. Based on the intensity measurements, the effective blackbody emission temperature of the burning droplets combustion. The higher intensities were attributed to continuous emission by molten Al 2O 3 in the flame envelope surrounding the molten Al droplets as well as emission by the Al droplets themselves. A two part model of the radiant transport in the burning droplet system and between droplets was developed for comparing the theoretical and measured spectral intensities to estimate the magnitude of the radiative parameters of the burning Al droplets. From this model, the optical depth of the Al 2O 3 flame envelope surrounding the droplet was estimated to be of the order of 10 −2. It was concluded that scattering and absorption by the envelope were negligible whereas emission was not. The effective radiative properties of the burning droplet system were predicted and the importance of radiative transfer in vapor phase combustion of liquid metal droplets was discussed.