An acoustical study of gas transport through porous media

Modeling gas transport through unsaturated porous media (such as soils) is enhanced by the accurate measurement of site-specific gas phase effective diffusion coefficients (D_e). A novel acoustic gas analysis technique was used to test the veracity of results gathered using the traditional methodology, to provide a superior analytical technique by which results can be gathered, and to probe toward a deeper understanding of gas transport through porous media, specifically with respect to non-Fickian transport phenomena.; The acoustic gas analyzer is composed of an electroacoustic system for measuring the change in concentration of a pseudo-binary mixture of gas using a planewave resonator and a phase-locked-loop automatic resonance frequency tracker. The resonator consists of a thermally insulated copper vessel with o-ring seals and Swageloc™ valves. The validity of applying this acoustic methodology for measuring diffusion coefficients through soil was tested by benchmarking gaseous free air binary diffusion coefficients at ambient (∼297K) temperature. Diffusion coefficients were determined for helium (He), methane (CH₄) and krypton (Kr) through a geometrically simple diffusive pathway (capillary leak). The resultant diffusion coefficients were found to be in agreement with published measured values where they are available for comparison.; Gas flux through porous media was observed with the acoustic gas analyzer using different porous media core lengths, multiple gas tracers, alternate device orientation, and flux observations recorded throughout a wide tracer concentration range. Measurements were made using He, CH₄, and Kr. Results show that, depending on binary gas mixtures employed, the device measured buoyant flow (natural convection), and non-equimolar diffusion, in addition to simple Fickian diffusion. Furthermore, the past use of He in the “Gas Diffusivity” standard method for measuring the tortuosity of unsaturated porous media provided a good estimate, although the flow regime in which the device has been operated in the past is outside the realm of pure Fickian diffusion.