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Surprisingly effective dispersion in porous medium for non-Newtonian fluids : highlighted by NMR techniques

Abstract : Dispersion is the mixing of a solute in a fluid flow which combines the diffusive and advective transport phenomena. Dispersion in porous media remains, today, a highly topical subject. Mixing in flow through porous media is ubiquitous in engineering research (processes, environment, oil recovery). The nuclear magnetic resonance (NMR) provides a nearly ideal tracer method by marking and tracking the molecules in their movement. More precisely, we used the PFG-NMR (Pulsed Field Gradient NMR) technique [1] to derive the dispersion coefficients. The PFG-NMR method is now a standard method to measure the molecular diffusion coefficients. It is based on the measurement of molecular displacements between a matched pair of magnetic gradient pulses. It provides likewise a convenient means to measure dispersion coefficients which characterize the diffusive-convective transport. The average displacement of molecules regardless of the regime (asymptotic or not) is studied by the formalism of propagators (the probability distribution of displacements) (Fig.1) [2]. The cumulant method is applied to measure the average velocity of the fluid in Stokes flow regime and to determine the longitudinal dispersion coefficient. We worked with a random pack of spherical PMMA beads (diameter =75-90µm) compacted in a plastic tube (porosity = 0.37) and we have studied a shear thinning fluid, a Xanthan gum solution at 0.2wt%. Particular emphasis is placed here on a comparison of the dispersion mechanisms between Newtonian and non-Newtonian fluids. We show that regardless the value of Péclet number (Pe = 10, 20 and 40) in a packing of beads, the dispersion is more efficient for Xanthan than for water (Fig.2). This observation was not expected given the rheology of xanthan. As a matter of fact, the opposite phenomenon is observed in a capillary tube (Poiseuille flow). The appearance of stagnant zones in the case of Xanthan (increase of the dynamic viscosity when the shear stress decreases) can explain this findings.
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Contributor : Maude Ferrari <>
Submitted on : Wednesday, January 10, 2018 - 12:14:10 PM
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Maude Ferrari, Christian Moyne. Surprisingly effective dispersion in porous medium for non-Newtonian fluids : highlighted by NMR techniques. Interpore 2017, May 2017, Rotterdam, Netherlands. ⟨hal-01678350⟩

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