Effects of Interacting Helical Flows on Solute Transport and Mixing in 3-D Porous Media

Heterogeneity and macroscopic anisotropy of porous media play an important role for dilution and reaction enhancement of conservative and reactive plumes. In this study, we performed numerical simulations to investigate steady-state complex flow and solute transport in three-dimensional heterogeneous porous media. We considered two macroscopic anisotropic inclusions resulting in helical flow with twisting streamlines in the 3-D flow-through domain. The inclusions were obtained by alternating two layers of angled slices of coarse and fine porous media with different hydraulic conductivity. We analysed different scenarios in which the geometry and the relative position of the two anisotropic inclusions were changed. Metrics of stretching and folding were applied to characterize the flow field, whereas, for the transport problem, dilution and reactive mixing were quantified by the entropy-based metric of flux-related dilution index. The results show that the two helixes result in different twisting streamlines patterns that cause distinct deformation of the plumes. However, mixing and reaction enhancement could not be directly related to the extent of plume deformation and the configurations with strong deformation can result in only moderate mixing enhancement. Finally, we analysed the impact of the reaction kinetics on reactive transport and its role on the entropy balance for a reactant undergoing transport and mixing-controlled reactions in the twisting flow fields.