Effects of Fiber Orientation and Overlapping on Knudsen, Transition, and Ordinary Regime Diffusion in Fibrous Substrates

Abstract

AbstractWe prescnt effective diffusion coefficients of gases in porous media whose structure can be represented as an assemblage of cylindrical fibers, such as the media used as substrates in chemical vapor infiltration. Structures consisting of non-, partially, or freely overlapping fibers of various orientation distributions are considered, and effective diffusion coefficients are computed by means of a Monte Carlo simulation scheme. In order to be able to examine the interrelation of ordinary, transition, and Knudsen diffusivities and tortuosities, computations are carried out over the whole diffusion regime, i.e., from bulk to Knudsen. Our simulation results are compared with variational bounds and experimcntal values of tortuosity of fibrous beds reported by other investigators.