Enhanced and reduced solute transport and flow strength in salt finger convection in porous media

Abstract

We report a pore-scale numerical study of salt finger convection in porous media, with a focus on the influence of the porosity in the non-Darcy regime, which has received little attention in previous research. The numerical model is based on the lattice Boltzmann method with a multiple-relaxation-time scheme and employs an immersed boundary method to describe the fluid–solid interaction. The simulations are conducted in a two-dimensional, horizontally periodic domain with an aspect ratio of 4, and the porosity ϕ is varied from 0.7 to 1, while the solute Rayleigh number R a S ranges from 4 × 10 6 to 4 ×   10 9. Our results show that, for all explored R a S, solute transport first enhances unexpectedly with decreasing ϕ and then decreases when ϕ is smaller than a R a S-dependent value. On the other hand, while the flow strength decreases significantly as ϕ decreases at low R a S, it varies weakly with decreasing ϕ at high R a S and even increases counterintuitively for some porosities at moderate R a S. Detailed analysis of the salinity and velocity fields reveals that the fingered structures are blocked by the porous structure and can even be destroyed when their widths are larger than the pore scale, but become more ordered and coherent with the presence of porous media. This combination of opposing effects explains the complex porosity dependencies of solute transport and flow strength. The influence of porous structure arrangement is also examined, with stronger effects observed for smaller ϕ and higher R a S. These findings have important implications for passive control of mass/solute transport in engineering applications.