Optical Depth Effects on Aluminum Oxide Spectral Emissivity

Measurements of high-temperature spectral emissivities of aluminum oxide were made within a heterogeneous shock tube over the spectral range of 650-900 nm. The spectral emissivity of optically thin micrometer-scale alumina particles scaled approximately as lambda(-1.4) from 2800 to 3500 K. Results from optically thin clouds of nanoscale alumina showed that a lambda(-1.2) dependence, closer to the lambda(-1) predicted by the Rayleigh limit, is appropriate over the same spectral and temperature ranges. For temperatures below the melting point of alumina, the emissivity of nanoalumina shows a significant temperature dependence. The effect multiple scattering has on the apparent emissivity is studied, and it is determined to contribute to the discrepancy between the current and previous works. A Monte Carlo simulation showed qualitative agreement with the experimental work. It was found that, at small to moderate optical depths, scattering is responsible for a small change in the spectral distribution of particle emissivity. At large optical depths, absorption has a much stronger effect on the apparent spectral emissivity. It was determined that choice of optical depth can strongly affect the results of pyrometry measurements.