2019 Volume 75 Issue 2 Pages I_113-I_124
This study develops a numerical technique for the analysis of mass diffusion in unsaturated porous media. In the present approach, a periodic porous solid model is generated numerically by packing arbitrary shaped particles into the unit cell. The pore water of predetermined amount is then distributed over the pore space so that the total interfacial free energy is minimized. Finally, a random walk particle tracking is implemented on the rectangular grid generated uniformly over the fluid phase domain. To characterize the macroscopic diffusion, an overall diffusion constant and the abnormality of the macro diffusion are evaluated quantitatively based on the temporal evolution of random walkers’ mean-square displacement. With the numerical methods, the effects of such parameters as the degree of saturation, solid phase packing structure, and the solid particle size distribution on the macro diffusion characteristics are investigated. It was found, as the results, that the origin of the anomalous diffusion is the tortuosity of the diffusion paths, while the grain size distribution has more to do with the magnitude of the macro diffusion constant.
