Spectral emissivity and radiance temperature plateau of self-supporting alumina melt at rapid solidification

Abstract

Free solidification of a pool of molten alumina formed by local heating of solid alumina by concentrated CO 2 laser radiation was investigated. During free rapid cooling from 2700 - 2900 K in ambient air and in the course of solidification a high-speed pyrometer measured the thermograms of two radiance temperatures at the wavelengths of 0.55 μm and 0.72 μm. A high-speed scanning spectrometer measured the intensity of emitted radiation in the wavelength range from 0.5 to 1.3 μm. An infrared spectrometer measured the intensity of radiation of one discrete wavelength, which could be chosen in the range from 2 to 12 μm. In addition, the pool surface images during solidification were recorded with a CCD camera. Owing to semitransparency of the melt, its emitted radiation depends in the general case on temperature distribution and the optical properties of the emitting surface layer. During solidification the spectral emissivity e λ in the whole of the scanned spectral region is high and lies in the range from 0.8 to 1. A horizontal plateau of e λ and, accordingly, the radiance temperature is observed only in the infrared range of high absorption from 6 to 10 μm. The plateau of e λ is absent in the range 1 to 4 μm. First an increase and then a decrease of e λ occurs in the visible range and at the beginning of the infrared region. The experimental data are analysed by comparison with the results of a numerical simulation.