Second Law Analysis of Passive Micro-Mixing in Rectangular Microchannels With Flow Obstacles

Abstract

Passive micromixers have application in the biosciences area. In particular, passive micromixers that may be used as part of point-of-care (POC) diagnostic testing devices are becoming common-place and have application in developed, developing, and relatively undeveloped locales. Characterizing and improving mixing efficiency in these devices is an ongoing research effort. Different channel geometries and flow obstacles lead to varying degrees of mixing effectiveness and serve to increase chaotic advective mixing in contrast to the molecular diffusive mixing that occurs even in the absence of these obstacles. Entropy is generated due to these, and other, irreversible processes. Efficient micromixer design is of interest to biomedical and mechanical engineers working in the biosciences area. The entropy generation rate, we contend, can provide an aid in determining how thoroughly mixed fluids in the channel have become, as well as provide insight into improving channel design to maximize desired outcomes, such as mixing, and minimizing losses due to heat transfer and power consumption. In this paper, we focus our analysis on numerical simulations conducted using computational fluid dynamics (CFD) on a supercomputer-cluster to do simulations with extended mesh refinement and very small residuals. This enabled us to test a wide range of flows with varying Reynolds numbers. The configuration of flow and species parameters within the simulations were compared to experimental results to confirm their validity. We show that varying the geometry of the channel can lead to a measurable increase in entropy generation via the Second Law of Thermodynamics. Further, we show that this increase in entropy is linked to mixing from obstacle-induced chaotic advection and diffusion. We provide evidence of a positive correlation between the efficiency of the mixing process and entropy generation. These findings will aid in the design of more efficient portable health care-related devices, particular in remote or underdeveloped regions where power utilization is a critical concern.