The effective diffusion in porous media, De, depends on the structure of the porous material and the phases involved. Dependent on whether the transport of diluted species occurs in free flow or in saturated or unsaturated porous media, the effective diffusivity is defined as:
Here DF,
DL, and
DG are the single-phase diffusion coefficients for the species diluted in a fluid, a pure liquid, and a gas phase, respectively (SI unit: m
2/s), and
τF,
τL, and
τG are the corresponding tortuosity factors (dimensionless).
For saturated porous media θl =
εp. The fluid tortuosity for the Millington and Quirk model is