This work deals with a numerical study to characterize the onset of vortex shedding from a rectangular cylinder with different length-to-width aspect ratios (AR) and confined in a channel under the consideration of constant blockage ratio (BR = 1/5). For AR ranging from 1/8 to 4, it has found that the onset of vortex shedding can be promoted and can take place at lower Reynolds numbers by reducing the aspect ratio of the cylinder, decreasing the critical Reynolds number almost linearly with AR for AR 5 2, being the lowest critical Reynolds number around 30 for an aspect ratio of 1/8. The drag coefficient, Strouhal number and pumping power needed to run the channel have been also characterized and correlations proposed to fit both the drag coefficient and pumping power as function of AR and critical Reynolds number. The gained knowledge has been later applied to mix two fluids in the channel at constant Reynolds number. The mixing has been characterized in terms of the mixing efficiency and mixing energy cost. It has been observed that configurations with vortex shedding can intensify mixing with efficiencies between 50% and 100% higher and two order of magnitude cheaper than those without vortex shedding.