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Prediction of Diffusion Coefficients in Porous Media Using Tortuosity Factors Based on Interfacial Areas
Author(s) -
Saripalli K. Prasad,
Serne R. Jeffery,
Meyer Philip D.,
McGrail B. Peter
Publication year - 2002
Publication title -
groundwater
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.84
H-Index - 94
eISSN - 1745-6584
pISSN - 0017-467X
DOI - 10.1111/j.1745-6584.2002.tb02512.x
Subject(s) - tortuosity , porous medium , diffusion , saturation (graph theory) , porosity , capillary action , thermodynamics , materials science , permeability (electromagnetism) , shape factor , chemistry , composite material , mathematics , geometry , physics , biochemistry , combinatorics , membrane
Determination of aqueous phase diffusion coefficients of solutes through porous media is essential for understanding and modeling contaminant transport. Prediction of diffusion coefficients in both saturated and unsaturated zones requires knowledge of tortuosity and constrictivity factors. No methods are available for the direct measurement of these factors, which are empirical in their definition. In this paper, a new definition for the tortuosity factor is proposed, as the real to ideal interfacial area ratio. We define the tortuosity factor for saturated porous media (τ s ) as the ratio S/S o (specific surface of real porous medium to that of an idealized capillary bundle). For unsaturated media, tortuosity factor (τ a ) is defined as 2 aw l a (ratio of the specific air‐water interfacial area of real and the corresponding idealized porous medium). This tortuosity factor is suitably measured using sorptive tracers (e.g., nitrogen adsorption method) for saturated media and interfacial tracers for unsaturated media. A model based on this new definition of tortuosity factors, termed the interfacial area ratio (IAR) model, is presented for the prediction of diffusion coefficients as a function of the degree of water saturation. Diffusion coefficients and diffusive resistances measured in a number of saturated and unsaturated granular porous media, for solutes in dilute aqueous solutions, agree well with the predictions of the IAR model. A comparison of permeability of saturated sands estimated based on τ s and the same based on the Kozeny‐Carman equation confirm the usefulness of the τ s parameter as a measure of tortuosity.