Enhanced reaction kinetics and reactive mixing scale dynamics in mixing fronts under shear flow for arbitrary Damköhler numbers. (February 2017)

• Record Type: Journal Article
• Authors: Bandopadhyay, Aditya; Le Borgne, Tanguy; Méheust, Yves; Dentz, Marco
• Is Part Of: Advances in water resources. Volume 100(2017)
• Page Start: 78
• Page End: 95
• Publication Date: 2017-02
• Subjects: Reactive front -- Mixing -- Arbitrary Damköhler number -- Shear flow -- Reaction width; Hydrology -- Periodicals; Hydrodynamics -- Periodicals; Hydraulic engineering -- Periodicals; 551.48
• DOI: 10.1016/j.advwatres.2016.12.008
• Languages: English
• ISSNs: 0309-1708

Abstract

Highlights: Combined effect of linear shear and reaction kinetics on system evolution studied. Dramatic enhancement in the reaction kinetics for a stronger imposed shear quantified. Predictions of the length of the reaction mixing scale provided. Graphical abstract: Abstract: Mixing fronts, where fluids of different chemical compositions mix with each other, are known to represent hotspots of chemical reaction in hydrological systems. These fronts are typically subjected to velocity gradients, ranging from the pore scale due to no slip boundary conditions at fluid solid interfaces, to the catchment scale due to permeability variations and complex geometry of the Darcy velocity streamlines. A common trait of these processes is that the mixing interface is strained by shear. Depending on the Péclet number Pe, which represents the ratio of the characteristic diffusion time to the characteristic shear time, and the Damköhler number Da, which represents the ratio of the characteristic diffusion time to the characteristic reaction time, the local reaction rates can be strongly impacted by the dynamics of the mixing interface. So far, this impact has been characterized mostly either in kinetics-limited or in mixing-limited conditions, that is, for either low or high Da . Here the coupling of shear flow and chemical reactivity is investigated for arbitrary Damköhler numbers, for a bimolecular reaction and an initial interface with separated reactants. Approximate analytical expressions for the global production rate and reactive mixing scale are derived based on a reactive lamella approach that allows for a general coupling between stretching enhanced mixing and chemical reactions. While for Pe < Da, reaction kinetics and stretching effects are decoupled, a scenario which we name "weak stretching", for Pe > Da, we uncover a "strong stretching" scenario where new scaling laws emerge from the interplay between reaction kinetics, diffusion, and stretching. The analytical results are validated against numerical simulations. These findings shed light on the effect of flow heterogeneity on the enhancement of chemical reaction and the creation of spatially localized hotspots of reactivity for a broad range of systems ranging from kinetic limited to mixing limited situations.