A planar micromixer with rhombic microchannels and a converging-diverging element has been proposed for its effective mixing. Both CFD-ACE numerical simulations and experiments were used to design and investigate the effect of three parameters (number of rhombi, turning angle and absence or presence of the converging-diverging element) on mixing. Mixing efficiency is dependent upon Reynolds number and geometrical parameters. Through the results of numerical simulation, it is evident that smaller turning angle (a), higher Reynolds number and increasing number of rhombi will result in better fluid mixing due to the occurrence of larger recirculation. The large recirculation is beneficial for both the increased interfacial contact area between two species and the convective mixing. In the numerical simulations, mixing efficiency of 99% was achieved with a most efficient system consisting of threerhombus mixer with a converging-diverging element at a = 30°and Re = 200. An experimental mixing efficiency of about 94% has been obtained with the same design parameters. As expected, it is lower than the theoretical efficiency but is still very effective. A micromixer with such design can be potentially useful in the future applications of rapid and high throughput mixing.

A planar three-rhombus micromixer with two constriction elements for good mixing more than 84% at Re ⩾ 20 has been demonstrated by simulations and experiments. Higher constriction elements with low blockage ratios may enhance significant fluid mixing by combining principles of focusing/diverging, recirculation and Dean vortices. The local high flow velocity induced by the high constriction element provides both high inertial forces and centrifugal forces for enhancing mixing efficiency under asymmetrical flow. Recirculations and Dean vortices are strongly influenced by blockage ratios and Reynolds numbers. The smaller blockage ratio and higher Reynolds number resulted in higher mixing efficiency. In simulation, the 84% mixing efficiency was achieved at the blockage ratios of 1/8 and Re = 20 together with a low pressure drop about 3630 Pa. The trend of the verified experimental result is in good agreement with the simulation result. A good mixing efficiency can be achieved using this simple micromixer with less mixing units at lower Reynolds number and pressure drop compared to the conventional chaotic micromixers.

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Over a wide Reynolds number range (0.1 B Re B 40), the new planar obstacle micromixer has been demonstrated over 85% mixing efficiency covering the mixing improvement in both convection-enhanced (higher Re flow) and diffusion-enhanced (lower Re flow) mechanisms. Mixing behavior between two operation windows was investigated by numerical simulations and experiments. For the adaptive design, numerical simulations and Taguchi method were used to study the effect of four geometrical factors on sensitivity of mixing. The factors are gap ratio (H/W), number of mixing units, baffle width (W b ) and chamber ratio (W m /W). The degree of sensitivity using the Taguchi method can be ranked as: Gap ratio [ Number of mixing units [ Baffle width [ Chamber ratio. Micromixer performance is greatly influenced by the gap ratio and Reynolds number. Beside the wide Reynolds number range, good mixing efficiency can be obtained at short distance of a mixing channel and relatively low-pressure drop. This micromixer had improved both complex fabrication process of multi-layer or 3D micromixers and low mixing efficiency of planar micromixer at Re \ 100. The trend of the verified experimental results is in agreement with the simulate results.

A planar micromixer with rhombic microchannels and a converging-diverging element has been systematically investigated by the Taguchi method, CFD-ACE simulations and experiments. To reduce the footprint and extend the operation range of Reynolds number, Taguchi method was used to numerically study the performance of the micromixer in a L(9) orthogonal array. Mixing efficiency is prominently influenced by geometrical parameters and Reynolds number (Re). The four factors in a L(9) orthogonal array are number of rhombi, turning angle, width of the rhombic channel and width of the throat. The degree of sensitivity by Taguchi method can be ranked as: Number of rhombi > Width of the rhombic channel > Width of the throat > Turning angle of the rhombic channel. Increasing the number of rhombi, reducing the width of the rhombic channel and throat and lowering the turning angle resulted in better fluid mixing efficiency. The optimal design of the micromixer in simulations indicates over 90% mixing efficiency at both Re > or = 80 and Re < or = 0.1. Experimental results in the optimal simulations are consistent with the simulated one. This planar rhombic micromixer has simplified the complex fabrication process of the multi-layer or three-dimensional micromixers and improved the performance of a previous rhombic micromixer at a reduced footprint and lower Re.