Vortex-shedding from micropins has the potential to significantly enhance and intensify scalar transport in microchannels, for example by improving species mixing. However, the onset of vortex-shedding and the mixing efficiency are highly sensitive to the confinement imposed by the microchannel walls. In this work, the time dependent flow past a cylindrical pin in microchannels with different levels of confinement was studied experimentally. The onset of vortex-shedding in such flows is associated with high, kilohertz range frequencies that are difficult to resolve using conventional laser-based microscale particle image velocimetry (μPIV) techniques. Hence, in this study, a high-speed μPIV technique was implemented in order to obtain time-resolved measurements of the velocity fields downstream of the micropin to estimate the corresponding vortex-shedding frequencies and quantify the mixing in the pin wake. The vertical confinement (pin length to diameter ratio) was found to delay the onset of vortex-shedding. When vortex-shedding was present, the shedding frequency and the corresponding Strouhal numbers were found to be greater in channels with higher lateral confinement for the same Reynolds number. Finite-time Lyapunov exponent analysis was performed on the acquired velocity fields to estimate the mixing performance. The results clearly illustrated the significant enhancement in both the mixing in the wake and the mass flux across the centerline of the wake induced by vortex-shedding.
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