Low pressure gas transfer in fibrous media with progressive infiltration: correlation between different transfer modes

Abstract

Permeation of gases in fibrous preforms of C/C composites has been studied with various degrees of infiltration, between 60% and 12% pore volume fraction, both experimentally with steady-state flow vs. pressure drop experiments and numerically with X-ray computerized micro-tomography (μ-CT), image processing and image-based computations. An excellent agreement has been found for Darcian permeabilities and for Knudsen diffusivities, when taking the mean solid chord length as a characteristic length scale. The reported tendencies of the mass transfer coefficients vs. porosity are in agreement with fibrous filters literature values, that are available for pore volume fractions above 55%. Accordingly, this work proposes useful extensions of these known correlations towards lower values of porosity. Finally, the dimensionless friction factors related to Knudsen diffusion, binary diffusion and viscous flow have been found to be correlated between each other, regardless the flow direction or the type of fibrous media. They are easy-to-use tools for large-scale numerical simulations of mixed-mode gas flow, eg. in Chemical Vapor Infiltration (CVI) or filter clogging simulations.