EFFECTIVE DIFFUSIVITIES AND CONDUCTIVITIES OF RANDOM DISPERSIONS OF NONOVERLAPPING AND PARTIALLY OVERLAPPING UNIDIRECTIONAL FIBERS

Computer simulation results are presented for the effective bulk trans ition, and Knudsen diffusion coefficients in structures consisting of parallel nonoverlapping (impenetrable) or partially overlapping (penet rable) fibers. The numerical procedure is based on a discrete step-by- step random walk mechanism used to determine the mean square displacem ent of molecules traveling in the interior of the porous medium. The c omputed transition regime diffusivities are compared with the predicti ons of the reciprocal additivity effective diffusivity expression. The bulk diffusivity results are also expressed as formation factors desc ribing other transport properties (e.g., thermal and electrical conduc tivity, magnetic permeability, and dielectric constant) of the fibrous structures or in general of the corresponding arrays of cylinders (ei ther nonconducting or infinitely conducting with respect to the matrix phase). The formation factors of nonoverlapping cylinders are compare d with variational bounds and analytical and numerical results of the literature.