Pore scale mixing and macroscopic solute dispersion regimes in polymer flows inside 2D model networks
Résumé
A change of solute dispersion regime with the flow velocity has been studied both at the macroscopic and pore scales in a transparent array of capillary channels using an optical technique allowing for simultaneous local and global concentration mappings. Two solutions of different polymer concentrations ($500$ and $1000$~ppm) have been used at different Péclet numbers. At the macroscopic scale, the displacement front displays a diffusive spreading: for $Pe\ \leq \ 10$, the dispersivity $l_d$ is constant with $Pe$ and increases with the polymer concentration; for $Pe \ > \ 10$, $l_d$ increases as $Pe^{1.35}$ and is similar for the two concentrations. At the local scale, a time lag between the saturations of channels parallel and perpendicular to the mean flow has been observed and studied as a function of the flow rate. These local measurements suggest that the change of dispersion regime is related to variations of the degree of mixing at the junctions. For $Pe \ \leq \ 10$, complete mixing leads to pure geometrical dispersion enhanced for shear thinning fluids; for $Pe >10$ weaker mixing results in higher correlation lengths along flow paths parallel to the mean flow and in a combination of geometrical and Taylor dispersion.
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