VORTEX INDUCED TRANSPORT PHENOMENA IN FLOWS PAST MICROPINS

Abstract

The potential of vortex shedding behind cylindrical micropins to enhance mixing at the microscale is investigated. The confinement present in microchannels may prevent or delay the formation of vortices and weaken the performance of such a passive micromixer. Thus, the onset of vortex shedding in confined flows in micromixers need be elucidated to optimize mixing performance. In this work, the flow past a single cylindrical pin in microchannels with different levels of confinement was studied experimentally using a custom high-speed micro particle-image velocimetry (µPIV) system. This system can resolve the instantaneous velocity fields and the corresponding vortex shedding frequencies. Results show that the onset of vortex shedding is delayed by enhancing the vertical confinement for a channel width of three pin diameters. However, when vortex shedding is present, the vortex shedding frequency and corresponding Strouhal numbers (St) are greater in more confined channels for the same Reynolds number (Re). Finite-Time Lyapunov Exponent (FTLE) analysis was performed on the acquired velocity fields to estimate the mixing performance. Preliminary results with a channel three pin diameters wide and three pin diameters high indicated that vortex shedding induces an enhancement in both the mixing in the wake and the mass flux across the centreline of the wake. We also discuss our results in the context of prior works focussing on exploiting vortex shedding for enhancing thermal transport.