Experimental study of solute dispersion in macroscopic suspension flow

Abstract

We experimentally investigate the influence of suspended neutrally-buoyant particles on the dispersion of a passive solute in pressure-driven axial flow in a constant aperture fracture (parallel plates configuration). A dye is employed as solute in order to measure its local concentration by means of a light transmission technique. In the experiments a dyed particle suspension displaces a transparent one at constant flow rate, for volume fractions ϕ ranging from 0 to 0.25 and for solute Péclet numbers (Pes) between 35 and 476 (mean flow velocities U between 0.004 and 0.0544 cm/s). The local time variation of the solute concentration in the measurement zone was well-fitted by the solution of the advection–dispersion equation, and a longitudinal dispersion coefficient D for the solute was measured. For Pes < 300, the values of D for flow with particles (ϕ > 0) and without particles (ϕ = 0) are equal within the measurement error. For Pes > 300, D decreases for ϕ > 0 compared to ϕ = 0. The magnitude of the reduction increases as ϕ increases, and also as Pes increases. This decrease of D in the presence of suspended macroscopic particles is analyzed in the light of theoretical, numerical and experimental results from other authors that studied suspension flow in similar geometries.