GAS-DIFFUSION IN RANDOM BINARY MEDIA

The binary representation of porous media can provide a direct means f or the characterization of their internal structure and is becoming pr ogressively fashionable as a result of the rapid development of novel, powerful image analysis techniques. In the present work wr provide ac curate predictions of the Knudsen and intermediate diffusivities based on molecular trajectory computations in three-dimensional pixelized p orous media. The main advantage of this approach is that it avoids res orting to the commonly used concepts of parr, grain, or fiber models. which introduce inevitably a significant degree of approximation to th e actual structure. a second advantage is that this method is valid ev en far from the continuum limit where rarefied transport prevails. An analytical expression for the Knudsen diffusivity in random binary med ia is suggested that combines simplicity and accuracy over the entire porosity range. In the intermediate diffusion regime, it is found that the serial combination of the overall bulk and Knudsen diffusional re sistances provides excellent estimates of the effective diffusional re sistance and, hence, a Bosanquet type of approximation is quite valid. Finally, numerical results for the accessible porosity and specific s urface area in random binary media are presented using large random re alizations that, practically, eliminate statistical errors. (C) 1998 A merican Institute of Physics. [S0021-9606(98)70440-4].