Mixing enhancement due to time periodic flows in a T-shaped micro-mixer

A Direct Numerical Simulation of the mixing process between two streams having the same flow rate in a T-shaped micro-device indicates that for a certain range of Reynolds numbers, Re, the flow regime might be unsteady, with time-periodic structures. Using a rectangular cross section with a 3:2 aspect ratio at the outlet and assuming fully developed flow at the inlet, pulsating flows are observed when 230<410; for example, when Re= 360, the time periodic structures were characterized by a Strouhal number, St  0.25, in good agreement with recent experimental results. Predictably, these pulsating flows strongly enhance the degree of mixing, that can be as large as twice the one that is measured in the steady engulfment regime. Then, when we increase the Reynolds number beyond the pulsating range, the flow is no more unsteady, with a consequent reduction of the mixing efficiency. In addition, we also observe that for our investigated geometry the unsteady, periodic behavior does not occur when the inlet flow profiles are blunt and not fully developed.