Steady and unsteady regimes in a T-shaped micro-mixer: Synergic experimental and numerical investigation

Despite the very simple geometry, T-shaped micro-mixers are characterized by different flow regimes when operated for liquid mixing. In this work, experiments and direct numerical simulations are employed jointly to investigate such flow regimes. First, a novel methodology is proposed to provide a quantitative comparison with numerical predictions starting from simple flow visualization experiments that use non-collimated light. This method is applied and validated in the steady flow regimes. Moreover, flow visualizations provide for the first time an experimental support to the flow dynamics scenarios for the periodic unsteady flow regimes previously proposed in the literature on the basis of numerical simulations. In particular, at a Reynolds number, based on the bulk velocity and on the hydraulic diameter of the mixing channel, of approximately 225, the flow passes from a steady engulfment regime to an unsteady asymmetric regime, which is characterized by a periodic dynamics of the three-dimensional structures present in the mixer. In this regime, the flow in the mixing channel always remains asymmetric promoting, thus, the mixing between the two streams. By further increasing the Reynolds number, another change in flow dynamics is observed. In the new regime, called periodic symmetric regime, the flow always maintains a double mirror symmetry in the mixing channel and this leads to a dramatic decrease of mixing. The flow unsteadiness is mainly due to a periodic shift of the vortical structures in the top part of the mixer along the direction parallel to the incoming flow.