Efficient mixing in micro-channels is one of the most debated topics in the design of microfluidic devices for the outmost importance it has in many industrial processes. Different mixing mechanisms could be easily found in literature but a usual accepted classification divides the different devices in three major categories: diffusion, distribution-based and convective devices. For all of them, the goal is to break the intrinsic laminar character of very small Reynolds number flows typical of microfluidic to enhance the mixing of the different interacting streams. In this study, we focused on the numerical modeling of the so-called injection mixer. In these devices, the mixing is driven by the dynamic of the injection streams. Attention is given to the inlet region where the different streams first mix together. Computations tackled with the case of a three inlet micro-channel - thickness: 400μm, width: 3mm, length: 30mm - where the injection rates are modulated according to prescribed wave form and normalized to have a constant flow rate at the outlet. Linear and sinusoidal waveforms have been tested. For each configuration, an assessment of the mixing efficiency based on standard deviation of the stream concentration is evaluated together with a typical mixing length and time. Results are then compared with the case where the injection rate is constant. Preliminary results shows that injection mixer with modulated injections can achieve mixing efficiency that are at least comparable with the diffusion based systems. The waveform that is more suitable depends on the characteristic injection period and on the phase shift between different injections. Pulse waveform determines high efficiencies, but requires higher injection rates and creates pressure fluctuations that could be disturbing in applications. Experiments have also been carried and results are compared with the numerical ones. The results obtained are useful to better understand the behavior of injection mixing devices and to provide some hint to reduce the mixing length by coupling a preliminary mixing due to modulation of the inlet streams with other mixing configurations.
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