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This paper was presented at the 3rd Micro and Nano Flows Conference (MNF2011), which was held at the Makedonia Palace Hotel, Thessaloniki in Greece. The conference was organised by Brunel University and supported by the Italian Union of Thermofluiddynamics, Aristotle University of Thessaloniki, University of Thessaly, IPEM, the Process Intensification Network, the Institution of Mechanical Engineers, the Heat Transfer Society, HEXAG - the Heat Exchange Action Group, and the Energy Institute. Flow regimes and mixing performance in a T-type micromixer at high Reynolds numbers were studied by numerical solution of the Navier–Stokes equations. The Reynolds number was varied from one to one thousand. The cross section of the mixing channel was 100 μm×200 μm, and its length was 1400 μm. The transverse inlet channels were symmetric about the mixing channel, and their cross-section was 100 μm×100 μm, and the total length was 800 μm. Five different flow regimes were identified: (i) stationary vortex-free flow (Re < 5); (ii) stationary symmetric vortex flow with two horseshoe vortices (5 < Re < 150); (iii) stationary asymmetric vortex flow (150 < Re < 240); (iv) nonstationary periodic flow (240 < Re < 400); (v) stochastic flow (Re>400). Maximum mixing efficiency was obtained for stationary asymmetric vortex flow. In this case, an S-shaped vortex structure formed in the flow field. The effect of the slip conditions on the flow pattern and mixing efficiency are studied. The Russian Foundation for Basic Research (Grant No. 10-01-00074) and the Federal Special Program “Scientific and scientific-pedagogical personnel of innovative Russia in 2009-2013” (projects No. P230, No. 14.740.11.0579 and No. 14.740.11.0103).

This paper was presented at the 2nd Micro and Nano Flows Conference (MNF2009), which was held at Brunel University, West London, UK. The conference was organised by Brunel University and supported by the Institution of Mechanical Engineers, IPEM, the Italian Union of Thermofluid dynamics, the Process Intensification Network, HEXAG - the Heat Exchange Action Group and the Institute of Mathematics and its Applications. A method for simulating fluid flows in microchannels is proposed. The method is tested using available experimental data obtained in micro-PIV studies of microchannel flows. Flow regimes in Y- and Ttype micromixers are studied. Passive and active mixers are considered. The dependence of the mixing efficiency on the Peclet number is examined, and the possibility of using hydrophobic and ultrahydrophobic coatings is analyzed. An active mixing method using a T-mixer with a harmonically varying flow rate at one of the inlet channels is studied. The dependence of the mixing efficiency on the frequency and amplitude of flow rate variation is determined. This work was supported in part by the Russian Foundation for Basic Researches (grant No. 07-08-00164) and by the grant of the President of the Russian Federation for Support of Leading Scientific Schools (project no. NSh-454.2008.1).