Second Law Analysis of Different Channel Geometries for Micro-Mixing

Abstract

Micro-mixing in different channel geometries may increase entropy generation and lead to improved efficiency of fluid mixing. The entropy generation rate corresponds to irreversibility due to the heat transfer and viscous effects in fluid flow through a channel. The objectives of this study are to validate the entropy generation rate of three expansion/contraction geometries [1] by using an analysis based on the Second Law of Thermodynamics (SLT) numerically and to study how entropy generation rate changes by placing flow obstacles in the channel. The geometries presented are not unique. In this paper the focus is on using CFD combined with the SLT as a tool to explore the effectiveness of micro-mixers. The entropy generation field in the expansion/contraction region between a 100 micrometer wide and a 200 micrometer wide rectangular micro-channel was analyzed using computational fluid dynamics (CFD) ANSYS-Fluent, and compared with the experimental results from Saffaripour et al. [1]. The numerical velocity profiles in the fully developed region of the channel in the flow direction and normal to flow direction were compared with experimental profile [1], and determined to be in agreement with the experimental profile. Using CFD, the entropy generation rates were determined for combinations of channel expansion/contraction geometry and the presence/lack of flow obstacles. The results presented here show that flow obstacles, which generally lead to better mixing, also lead to higher entropy generation rates.