Unsteady Laminar Flow Regimes and Mixing in T-Shaped Micromixers

Abstract

Convective micromixers create vortices in curved channel elements and allow characteristic mixing times below 1 millisecond for gaseous and liquid media. The flow regimes in the T-shaped junction of rectangular microchannels determine the mixing characteristics of the device. This contribution gives an overview about the flow regimes of symmetrical 1:1 mixing in T-shaped micromixers for Reynolds numbers from 0.01 to 1000 in the mixing channel. CFD simulations with the CFD-ACE+ code of ESI group give a detailed picture of the flow and mixing regimes in the investigated range of Reynolds numbers Re. First symmetrical vortices are formed at the entrance of the mixing channel for Re > 10. Due the symmetrical flow and the undisturbed interface between the components, the mixing quality at a distinct mixing channel length decreases with increasing Re number. At a certain Re number of about 140, the flow symmetry is disturbed, fluid from one side swaps to the opposite side and creates a double vortex within the mixing channel. For Re > 240 the flow becomes unsteady. The vortex formation at the mixing channel entrance is disturbed and a kind of wake flow establishes within the mixing channel. From 240 < Re < 500 the wake flow is periodic with a dimensionless frequency, the Strouhal number Sr of about 0.2. The Sr number is found to be the same in scaled mixer geometries. The mixing quality shows also a periodic behavior and reaches its maximum at this point. With further increasing Re number, the flow starts to become chaotic and the two components are often flowing parallel in the mixing channel leading to a decreasing mixing quality. Besides detailed CFD simulations, the periodic flow is observed in experimental studies with colored flow and stroboscopic imaging and has the same frequency. The decreasing mixing quality is also reflected in a lower selectivity of parallel, chemical test reactions for Re numbers larger than 500. With the knowledge of the flow regimes in microchannels, design criteria can be formulated for efficient mixing devices.