Mixing performance of an active mixer which mixes two fluids by three-dimensional flow surrounding a rotating shuttlecock rotor in continuous flow is numerically investigated. The mixer consists of a step contraction-expansion microchannel and a shuttlecock micro-rotor placed in the step. The obtained results show that mixing quality of solution does not depend on neither rotation speed nor mean velocity in the mixer, but rather on the ratio of tip paddle velocity of the rotor to the mean velocity (i.e. Strouhal number). Streaklines demonstrate that two fluids from the inlet can penetrate into the space between the paddles of the rotor, and then are mixed here before flowing to the exit channel. In small Strouhal number (∼10) cases, two fluids are twisted 90 degrees after passing the rotor region. In the other words, mixing in downstream flow behind the rotor takes place in the height instead of the width of the exit channel, which makes the mixer applicable for channels with high aspect ratio of the cross section. It is observed that mixing is dominantly enhanced in the rotor region and increasing Strouhal number results in faster mixing in the mixer.

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