On the evaluation of dispersion coefficients from visualization experiments in artificial porous media

Abstract

High-resolution single source–solute transport experiments in glass-etched pore networks presented in a previous publication are used for quantifying the hydrodynamic dispersion as a function of Peclet number ( Pe). This type of artificial porous media is ideal for understanding the effects of pore-scale phenomena on solute dispersion thanks to its well characterized topology and pore geometry. The hydrodynamic dispersion coefficients are estimated by matching the spatial and temporal distributions of the solute concentration over various regions of the network with the numerical solution of the advection–dispersion equation and using a parameter space analysis to ensure well conditioning of the parameters. The source release function and assumptions on boundary conditions are found to affect significantly the reliability of the dispersion coefficient estimates. The estimated longitudinal dispersion coefficients are in close quantitative agreement with literature data for dispersion in porous media. The extracted transverse dispersivity ( α T) values indicate an apparent decrease of α T with increasing flow velocity, which is at variance of constant dispersivity approximation adopted commonly in soil hydrology models and is attributed to incomplete diffusive mixing at the pore intersections. Calculated values of an effective mixing ratio at pore junctions are in agreement with published theoretical estimates and reveal a transition from partial mixing to streamline routing as Pe increases within the range investigated here.