Emissivity of Aluminum-Oxide Particle Clouds: Application to Pyrometry of Explosive Fireballs

Pyrometry measurements of clouds of high-temperature particles require an estimate of the spectral dependence of the particle emissivity. Common assumptions for this dependence range from epsilon(lambda) similar to lambda(-2) to epsilon(lambda) similar to constant. Depending upon the assumption used, there is uncertainty in the temperature of 100 s to a 1000 K in high-temperature clouds. Such errors are not apparent in goodness of fit of spectral data. A heterogeneous shock tube was used to measure the emissivity of aluminum oxide in an inert environment as a function of temperature (2000-3500 K), wavelength (0.55-0.95 mu m), and particle diameter (50 nm-10 mu m). In micro-sized alumina particles, the spectral dependence upon temperature transitions from decreasing with wavelength to increasing with wavelength with the dependence being roughly gray at about 3000 K. Because of local minima in the epsilon(lambda) vs lambda curve, a power-law (lambda(n)) dependence is insufficient to describe the emissivity. However, if such a dependence is assumed, n transitions from -1.4 to 0.5 as temperature increases from 2500-3500 K. Nano-sized alumina particles exhibit an even stronger spectral dependence. At 2678 K, n is approximately -1.2 but reaches as high as 2.1 at 3052 K. Considering optical depth issues, there is merit in gray emissivity approximations for high-temperature (similar to 3000-3300 K) particles typical of aluminum particle combustion.