Macro- and micromixing in a plane turbulent channel flow with a second-order chemical reaction

We analyse a database obtained from a direct numerical simulation of the reacting flow in a plane channel at low Reynolds number (Reτ = 180). One reactant is released along a line source at the center of the channel and the other near the walls of the channel. This distribution produces two reacting mass transfer mixing layers. The fluctuations of the concentration of the chemical species that react following a second-order chemical reaction (Da = 1, Sc = 0.7) are processed to determine the contribution of the large-scale and small-scale fluctuations in the reacting mass transfer mixing layers. The analysis is extended to reduce the flow structures responsible for the large and the small fluctuations of concentration in the mixing layers. The conditional sampling technique used reveals that hairpin-shape vortices that convect flow from the walls towards the center of the channel produce the extreme large-scale fluctuations. In the case of the small-scale fluctuations, the conditional sampled flow structure consist in a periodic distribution along the streamwise direction of counter rotating eddies that are located in the central part of the channel and convect flow from one mass transfer mixing layer to the other one.