# Numerical and Experimental Investigations of a Micromixer with Chicane Mixing Geometry
## 3 Results and Discussion  
### Flow Behavior in the Micromixer  
The flow behavior in the micromixer was investigated using streamlines. The flows from different inlets were marked with different colors: red and blue. As a sample, a rectangular grid on inlets with spacing 0.005 (0.08 mm) was chosen. By stratified flow, due to domination of viscous forces in relation to the inertia forces, no vortices form and all streamlines are strictly parallel. The diffusion is the main mass-transfer process and convective mixing is absent. Consequently, a distinct interface between two flows can be clearly seen in the middle of the channel; for example at a Reynolds number of Re = 2.6 (Figure 5a). The streamlines are uniformly distributed for all cross-sections, therefore the absence of only small stagnation zones in the cells can be stated. It was found that the volume flow rate does not affect the streamlines within the stratified flow regime if Re < 2.6. Figure 4: Dependency of mixing index at the micromixer outlet compared with the number of grid cells (Re = 42.4). Starting from Re = 13, the flow regime can be defined as vortex flow (according to Figure 6). This causes an increase of the interface area between species and reduces the diffusion length. In Figure 5b, there is an example of weak vortex flow with relatively small vortex formations. Size and form of the vortices depend on the Dean number, i.e., Reynolds number in case of the same geometry. The larger the Reynolds number, the higher the amplitude of vortices. For the purpose of detection of stagnation zones, normalized velocity vectors (black arrowheads) and velocity contours for the symmetry plane for the second meander element are represented in Figure 7. At Re = 2.6 (Figure 7a) there are no recirculations. In Figure 7b, the velocity field and vector field at Re = 156 are shown. A stagnation zone is located directly after the sharp bend. The velocity magnitude in this area is almost zero and velocity vectors also indicate recirculation areas. By increasing the Reynolds number to Re = 259 (Figure 7c) the stagnation zone with recirculations has an increased area. The second smaller recirculation zone is located after the meander element in the place of transition to the rounded channel. The position of stagnation zones corresponds with the result, obtained in the same micromixer by [PERSON] et al. using Diazo Coupling reaction [24]. The recirculation effect is also reported in some articles related to the zig-zag (or similar) form of the channel [19; 32]. In addition, it was shown by [PERSON] and [PERSON] [33] that the consecutive recirculation zones are responsible for the mass-transfer rating. The efficiency of recirculation was discussed by [PERSON] et al. [34] for some different geometries of microchannels and the presence of such zones was reported as intensification of the mixing process in comparison with a straight channel under the same conditions. Figure 5: Simulated streamlines for different flow regimes: (**a**) stratified flow (Re = 2.6); (**b**) weak vortex flow (Re = 26); (**c**) vortex flow (Re = 156); and (**d**) vortex flow (Re = 259). The distribution of concentration field is shown in Figure 6. The red and blue colors indicate two different species in the micromixer. The green color shows the ideal mixed flow: by \(\mathrm{Re}=2.6\) (Figure 6a), the interface between species at the outlet is vertical. In the case of \(\mathrm{Re}=26\) (Figure 6b), the Dean vortices appear, resulting in an increased interfacial area. Absolute qualitative different forms of the interface can be noticed by \(\mathrm{Re}=156\) and \(\mathrm{Re}=259\) (Figure 6d,c). Due to large recirculation and an extremely large magnitude of secondary flows, an enhanced mixing acceleration effect can be observed. Moreover, it is shown that an asymmetrically formed channel provides greater effect than a symmetrical one. This can be explained by the presence of high quantity of curvatures, through which the flow has to pass and therefore increase the vorticity of the flow.