A novel passive micromixer with fan-shaped cavity based on the principle of flow planar asymmetric split-and-recombine (P-ASAR) and focusing/diverging is designed. The micromixer consists of two split sub-channels with unequal widths and one fan-shaped cavity structure on the major sub-channel which are similar to a diamond ring structure. In order to yield optimum mixing effect, different parameters of geometry structure under a wide range of Reynolds numbers (1–80) have been investigated by numerical simulation with three-dimensional Navier-Stokes equations. The steady laminar flow was solved by using a finite-volume method and SIMPLE algorithm. Enhanced micromixing is achieved by utilizing a synergistic combination of unbalanced inertial collision, Dean vortices and expansion vortices. As a result of interplay between inertial, centrifugal and viscous effects, Dean vortices arise in the vertical plane of curved channel. Expansion vortices appear in the horizontal plane due to an abrupt increase of the cross-sectional area. The mixing index is used to evaluate the degree of mixing. Our studies show that vortices are observed in the channels at high Reynolds numbers. The geometry parameters of fan-shaped cavity structure affect the mixing index of micromixer. When the ratio of the widths of the major sub-channel and fan-shaped cavity channel is 1/3, the mixing index of this type micromixer could reach around 75% at Reynolds numbers larger than 60. The relation between mixing intensity and pump power consumption has been analyzed at a wide range of Reynolds numbers simultaneously.

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