Prediction of directional spectral emittance of packed ceramic spheres at high temperature
Rogério Lopes, Luís M Moura, Agnès Delmas, Jean-François Sacadura
The directional spectral emittance of absorbing and scattering isothermal ceramic (alumina, Al2O3) is predicted by the use of a radiative model based on the discrete ordinates method associated with the control volume technique to solve the radiative transfer equation. The extinction (Qe) and scattering (Qs) efficiencies for radiation incident upon spherical particles of alumina are calculated from the Mie theory for a temperature range from 1200 °C to 1700 °C, the particle radius lying from 25 mm to 80 mm, and a range of wavelengths from 2 mm to 6 mm. These calculations are based on published values of the complex index of refraction (ñ = n – ik) at high temperature for the monocrystal of sapphire (Al2O3). The independent scattering assumption is used, whereby each particle in a densely packed system acts independently in the absorption and scattering of radiation, unaffected by the presence of other particles. The directional spectral emittance of packed spheres at high temperature is calculated by changing the composition and the grain size for a specimen of 5 mm thickness and with a porosity of 30%. Calculations are performed and analysed for three different microstructural compositions: (100% Al2O3), (99% Al2O3, 1% MgO), and (99% Al2O3, 0.5% MgO, 0.5% Fe). The aim was to predict the emittance of an isothermal medium made of a dispersion of spherical particles of alumina according to its porosity, grain size, purity, and temperature. The normal spectral emittance, determined theoretically, is in agreement with the experimental values from the literature.