Dispersion versus diffusion in mixing fronts

TL;DR

This paper investigates how local dispersion and diffusion influence the properties of steady mixing fronts in fluids, revealing that dispersion dominates in porous media and affects mixing rates and interface behavior.

Contribution

It provides analytical solutions and experimental validation for the effects of dispersion versus diffusion on mixing fronts in various flow regimes.

Findings

Dispersion dominates local diffusion in porous media at high Péclet numbers.

Mixing scale grows with dispersion in non-uniform flows, unlike pure diffusion.

Flow acceleration coupled with dispersion leads to a Péclet independent mixing interface.

Abstract

Mixing fronts form when fluids with different chemical compositions are brought into contact. They influence a large range of biogeochemical processes in hydrological systems. An important mechanism governing mixing rates in such fronts is stretching by non-uniform flows that accelerates diffusive mass transfer by enhancing concentration gradients. In a range of systems, including porous media at Darcy scale, hydrodynamic dispersion dominates over diffusion to control local mixing rates. As it differs from diffusion through its velocity-dependent dispersion tensor, it is not known how local dispersion interacts with macroscopic mixing front stretching. Here, we investigate the impact of local dispersion versus diffusion on the properties of steady mixing fronts created by both uniform and non-uniform flows. We derive analytical solutions for the concentration profile, mixing scale and mixing rate across the fronts. We validate these predictions by comparison with numerical simulations and experiments performed in quasi two-dimensional tanks over a broad range of P\'eclet numbers. Without porous media, the mixing scale is governed by local diffusion coupled with flow: it increases diffusively along streamlines in uniform flows while it is constant in converging flows due to the balance between fluid compression and local diffusion. With porous media, the Batchelor scale is no longer sustained and the mixing scale grows with dispersion in non-uniform flows. In addition, the coupling between flow acceleration and dispersion results in a P\'eclet independent mixing interface, in contrast with the local diffusion scenario. We discuss the consequences of these findings on mixing rates in mixing fronts.