Vortex mixing in passive micromixers

Abstract

The advent of chaotic advection in microchannels has enabled us to explore the untapped potential of disturbance in the flow. The theory of fluid stretching, twisting, or folding, has facilitated a chaotic Lagrangian particle trajectory even for laminar flow with fixed Eulerian velocity. Although diffusion is existent in all types of flows, modern-day processes such as interfacial heat transfer, and dispersion are largely dependent on chaotic advection. The chaotic advection is highly useful in microchannels that work on a laminar flow regime. For the advection of chemical species, micromixers are used which are often integrated with micro total analysis systems (μTAS). Passive micromixers benefit from rapid analysis, improved mobility, and low cost but suffer from low mixing efficiency. The challenge to increase the mixing efficiency without compromising the benefits needs further evaluation of the underlying mixing mechanism. Further, the integration of microchannels with microelectromechanical systems (MEMS) necessitates further investigation into thermo-fluid dynamics. The high surface area to volume ratio of microchannels makes it an ideal competitor for thermal management systems. The rising need for more effective cooling systems has garnered the commitment toward the exploration of liquid flow characteristics in the microchannels heat sink (MCHS). For the advection of species in micromixers, a passive 3d design is presented which induces the swirling flow pattern by using the misaligned inlets and vortex generation through rectangular winglets. The flow is studied using the concepts of fluid stretching and twisting (chaotic advection). The swirling nature is quantified using a modified swirl number. The diffusive mixing was enhanced using the lateral Peclet number. A detailed analysis outlying the impact of geometrical features on vortex formation, propagation, and intensification is done. The vortex core region is identified using the lambda 2 approach and is quantified using the helicity and vortex index. The aspect ratio of the mixing channel has been identified as a significant parameter for vortex intensification. For Reynolds numbers varying from 1 to 70, mixing efficiency above 85% was achieved. The mixing length & pressure loss were kept within 5.5 mm and 3 kPa. Another study presents a numerical analysis of mixing in a misaligned serpentine micromixer with flow splitting and recombination. The design amalgamates two square-wave cross-sections with lateral misalignments thereby producing a vortex flow at each mixing junction. Flow analysis was done for Reynolds numbers 5 to 50 and a mixing efficiency above 90% was achieved. The results suggest strong vortex mixing along with cross-flow phenomenon. For application chaotic advection in microchannel heat sinks, manipulation of extended structures (longitudinal vortex generators) is done to increase the conjugate heat transfer. The channel was etched on an externally heated silicon substrate and deionized water was used as working fluid. The thermo-fluid analysis conceptualized the results obtained from flow stretching and showed an increase in heat exchange inside the MCHS. The thermal enhancement factor was evaluated for different configurations of the winglet and the heat transfer improved 2.9 times the benchmark and the overall thermal enhancement improved 2.24 times.