Simulating the Gas Diffusion Coefficient in Macropore Network Images: Influence of Soil Pore Morphology

Knowledge of the diffusion coefficient is necessary for modeling gas transport in soils and other porous media. This study was conducted to determine the relationship between the diffusion coefficient and pore structure parameters, such as the fractal dimension of pores ( D mp ), the shortest path length through the medium ( l min ), and the fractal dimension of the shortest path ( D min ). The finite element method (FEM) was used to simulate the gaseous diffusion process in an idealized soil system with a highly connected macropore network. The analysis was performed on binary images of soil thin sections. We show that the ratio ξ of the diffusion coefficient in soil ( D eff ) to that in free air ( D 0 ) is a function of not only the air‐filled porosity ε, but also of other parameters, and hence no universal relationship exists between ξ and D mp and D min Furthermore, ξ is shown to be strongly related to the pore‐space structure and the direction of the concentration gradient. The tortuosity ( T ) furthermore was found to be related to the weighted path length along the main diffusion direction.