NUMERICAL STUDY OF THE FLOW AND MASS TRANSFER IN MICROMIXERS

Computational fluid dynamic simulations are used to characterize the flow and the liquid mixing quality in a micromixer as a function of the Reynolds number. Two micromixers are studied in steady flow conditions; they are based on two geometries, respectively T-shaped and cross-type (T and + shapes). Simulations allow, in the case of T micromixers, to chart the topology of the flow and to describe the evolution of species concentration downstream the intersection. The streamline layout and the mixing quality curves reveal the three characteristic types of flow, depending on Reynolds number : stratified, vortex and engulfment flows. Vortices appear after impingement, in the exit channel. They become asymmetrical and gain in length with an increase in Re making the flow unsteady, which induces an enhancement of the mass transfer by advection between the two liquids. In the case of cross-type micromixers, the structure of the flow is strongly three-dimensional. It is characterized by symmetrical vortices in both output channels. In the zone close to the impingement, a back flow is observed which induces strong shear stresses. The results show that the + shaped system can improve the mixing process in comparison with the micromixers having T geometry. The numerical study also allows to select the locations of the most relevant zones of study, from an experimental point of view. It will allow to choose the location of PIV planes and local non intrusive sensors, such as electrochemical microprobes, in order to experimentally investigate the flow.