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ABSTRACT The quality of cometary surface activity simulations and erosion models of icy moons depends on a good knowledge of the surface layer permeability to gas flow. Therefore, we study various models of the Knudsen diffusion coefficient and the viscous permeability, which are used to describe the flow of rarefied gases through porous materials. Usually, these models are expressed for monodisperse packed beds. In this work, we describe a generalization to polydisperse packed beds and compare them with experimental results. In addition, we analyse non-spherical packings to test how well the recently developed models are applicable. For this purpose, the gas flow parameters of these samples are measured in a dedicated measurement set-up. Special attention had to be paid to biases in measuring the porosity and the pressure drop in the sample, which are discussed in detail. Our measurements confirm that the Knudsen diffusion coefficient is inversely proportional to the specific surface area of the grains and that the viscous permeability is inversely proportional to the specific surface area squared. Further, we were able to identify a relation between the gas flow parameters, represented by a parameter β, which seems to be an indicator of the mean orientation of the grains. The findings give further evidence of the importance of the grain size distribution and the grain shape for rarefied gas flow. In particular, the results show that the standard polydisperse model is not sufficient when a considerable part of the pore space consists of traps or other rarely percolated parts.
Zivithal et al. (Fri,) studied this question.