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Network Modeling of Diffusion Coefficients for Porous Media: II. Simulations

Dep. of Agricultural Engineering, North Dakota State Univ., Fargo, ND 58105-5626

J. L. Nieber

Dep. of Agricultural Engineering, Univ. of Minnesota, 1390 Eckles Ave., St. Paul, MN 55108

*Corresponding author (steele{at}plains.nodak.edu).

ABSTRACT

Gas diffusion often dominates constituent transport in porous media (PM) and is dependent on pore geometry, water content, and water distribution in PM. Network models of PM offer the ability to investigate the influence and interaction of pore-scale PM properties and fluid properties on macroscopic properties of the system. This study was conducted to investigate the macroscopic relative gas diffusion coefficient vs. air-filled porosity relationship or diffusion characteristic (DC) of PM using a network model. The network model was used to simulate DCs in wetting and drying PM containing air and water. A network size of nine by nine by nine spheres was used; increasing the network size to 19 by 19 by 19 produced essentially no change in the DC. The DC was independent of Henry's law gas-liquid partition coefficient (H) for H values of 0.1, 1.0, and 5.0. The product HR_w, where R_w is the ratio between the bulk gas- and liquid-phase diffusion coefficients, strongly influences the DC when H values of 1000 to 10 000 are considered; this indicates that certain organic compounds have DCs independent of air-filled porosity. Hysteresis in DCs was found in selected network cases, with the wetting DCs being greater than the drying DCs for most air-filled porosities, in accord with some experimental results reported in the literature. Spatial correlation of network pore space was shown to bring the simulated DCs into better agreement with some experimental DCs.

Contribution from the Minnesota Agric. Exp. Stn., Scientific Journal Series, Paper no. 20190.

Received for publication January 4, 1993.