Residence Time Control in Micromixers with Vortex Shedding

Residence time control is an important indicator of micromixer design. When using vortex shedding to enhance mixing efficiency in a micromixer, the relationship between residence time and vortex shedding becomes important; if residence time is shorter than shedding time, the fluid elements flow through the channel too quickly with no contribution of vortex shedding to mixing. Both residence time and vortex shedding depend on geometrical and flow parameters and hence in order to optimize micromixer design the effect of these parameters on mixing need to be well understood. Furthermore, the onset of vortex shedding in confined flows such as those encountered in micromixers need be elucidated. In this work, the flow field past a single cylindrical pin in a microchannel is studied experimentally using a high-speed PIV system. The effects of confinement on vortex formation are examined. Vortex shedding was observed for a channel height of two pin diameters and the shedding frequency increased with increasing lateral confinement (i.e. upon decrease in channel width at the same pin diameter). Therefore, controlling residence time via wake oscillations in pin microchannels is highly dependent on confinement.