Experimental characterization of shear-enhanced dispersion in porous media

Abstract

The shear flow generated by natural convection in porous media considerably influences the dispersion of contaminants in various geological formations. Previous studies have revealed that the dispersion coefficient depends on the Péclet number (Pe), shear rate, and complex pore structures. In this study, the dispersion behavior of a NaI tracer cloud was investigated under different shear rates ranging from 0.001 to 0.04 s−1 and Pe ranging from 10 to 200 using an innovative experimental apparatus via an X-ray technique. In contrast to uniform flow, a considerably enhanced dispersion was observed in shear flow along the longitudinal direction, and spreading in the longitudinal and transverse directions grew with time as ∼t3 and ∼t, respectively. In the power-law regime, the dispersion behaviors in the transverse and longitudinal directions were strengthened by shear action, with a larger dependence of the power law on the shear rates and Pe compared with that in the uniform flow. The strengthened dispersion promoted the formation of a hyper-mixing state, which was evaluated using the temporal evolution of the maximum concentration of the tracer plume, dilution index, and scalar dissipation rate. The results indicate that the dispersion behavior in a two-dimensional shear flow is quantitatively equivalent to that in a four-dimensional uniform-flow field. The findings of this study can provide a deeper understanding of the influence of shear flow on the hydrodynamic dispersion in porous media.