The permeability of alumina over an extended temperature range
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A semiempirical mathematical model has been developed to predict the permeability of porous alumina over a wide temperature range at atmospherie pressure. Although the thermal expansion of the alumina is of the order of 1 % over the range of temperatures considered, the apparent permeability of the sample to gas flow varies by over 400%. This behavior is due to the dependence of the mean free path of a gas on temperature and to the corresponding slip conditions that occur in the pores of the solid. The model developed correlates the “apparent permeability” data with temperature, true permeability, gas viscosity, and gas molecular weight, variables suggested by kinetie theory on mean free path and slip. Apparent permeability was found to be a very strong function of temperature. It exhibited both a direct thermal dependence and an indirect dependence, manifest through thermally driven variations in the gas viscosity. A mathematical model from the literature was used for gas viscosity. The inverse correlation with gas molecular weight, suggested by kinetie theory, is demonstrated. The model covers the temperature range between 250 and 1600 K. Small molelcule gases including air, nitrogen, argon, and helium were used in the development.
Key wordshigh-temperature gas flow permeability porous ceramics slipflow
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