Effect of geometry modifications on the engulfment in micromixers: Numerical simulations and stability analysis

The effect of geometry variations on the engulfment regime in micromixers is investigated. The engulfment regime is a steady flow regime resulting from a symmetry-breaking pitchfork bifurcation as the Reynolds number (based on the hydraulic diameter and bulk velocity of the outlet conduit) is increased above a critical value. This flow regime is particularly interesting because it leads to an increase of mixing in micromixers. Here, starting from a T-mixer, the inclination α of the inlet channels is systematically varied, considering both arrow-like (α<0) and Y-mixers (α>0), α=0° denoting a T-mixer. It is shown by direct numerical simulations (DNS) that the engulfment regime is present in all the considered geometries and it starts at progressively lower values of the Reynolds number as α is decreased. The main differences in the flow topology are limited and mainly confined at the confluence region between the two incoming flows. The instability leading to the engulfment regime is further investigated by linear stability analysis. This allows a more accurate prediction of the critical Reynolds number for the onset of engulfment as α is varied and, through an adjoint-based sensitivity analysis, the localization of the corresponding instability core.