Relative Gas Diffusion Coefficient Research Articles

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

AbstractGas 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 HRw, where Rw 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.

Network Modeling of Diffusion Coefficients for Porous Media: I. Theory and Model Development

AbstractGas diffusion often dominates constituent transport in porous media and is dependent on pore geometry, water content, and water distribution in a porous medium. Network models of porous media offer the ability to investigate the influence and interaction of pore‐scale porous media properties and fluid properties on macroscopic properties of the medium. This study was conducted to investigate the macroscopic relative gas diffusion coefficient vs. air‐filled porosity relationship (diffusion characteristic) of porous media using a network modeling approach. A cubic sphere‐and‐tube network model of porous media was adapted from petroleum engineering using Fick's law and the principle of conservation of mass to simulate one‐dimensional, steady‐state, isothermal, isobaric, molecular diffusion of a dilute binary gas in a nonadsorbing porous medium containing a single nonwetting fluid (air) and a single wetting fluid (water). The network model simulates hysteresis in air and water distributions in porous media for boundary drying and wetting curves of the soil water characteristic and demonstrates the effect of air‐filled porosity, Henry's law liquid‐gas partitioning coefficient, the ratio between gas‐ and liquid‐phase diffusion rates, and pore geometry on the diffusion characteristic.

Calibration of Jointed Tube Model for the Gas Diffusion Coefficient in Soils

AbstractModeling of gas and vapor transport in soils requires knowledge of the relative gas diffusion coefficient. The relative diffusion coefficient was determined as a function of air‐filled porosity, using a pore model based on two tortuous tubes of different radii joined in series. This model was fitted to measured relative diffusion coefficients on undisturbed samples of nine soil types at a wide range of water contents. It was found that model parameters vary with the soil types studied. Combined measurements of the water retention curve, air‐filled porosity, and relative diffusion coefficient were made to evaluate the effects of water distribution in the soil pores on the relative diffusion coefficient. After the samples were saturated with water, the residual air‐filled porosity ranged from 0.051 to 0.167 m3 m−3 and the relative diffusion coefficient was practically zero, which is due to pore blocking by soil water. At pressure heads corresponding to the air‐entry value in the water retention function, air‐filled porosity ranged from 0.095 to 0.256 m3 m−3 and the relative diffusion coefficient ranged from 0.002 to 0.013. From this point, with increasing air‐filled porosity, the relative diffusion coefficient increased rapidly to values ranging from 0.165 to 0.383 at zero water content. It was concluded that the air‐entry value is an important parameter, which indicates a priori the water content at which pore blocking becomes relevant.