The design and calculation of micro mixers is done by conventional analytical and numerical calculations. Due to the small dimensions, laminar flow is expected and the mass transfer is supposed to be dominated by diffusion. A detailed CFD-study by CFDRC-ACE+ of simple static mixers shows a deviation from strictly laminar flow in a wide range of Re numbers and channel dimensions. The static mixers under test have a T-Profile with rectangular cross sections and characteristic dimensions of 50 to 400 μm. The mean flow velocity is varied from 0,01 m/s to 5 m/s, which are typical values for chemical and biological applications, chemical analysis, and microfluidics with acceptable pressure losses.
With increasing flow velocity and increasing Re numbers the flow starts to develop a vortex at the entrance of the mixing channel. With further increasing velocity the flow tends to instabilities, which causes the break up of the flow symmetry. With the onset of the vortex formation and with the occurrence of the flow instabilities the mass transfer is enhanced by the exchange of fluid elements. The laminar diffusion model cannot describe this mixing effect. A simple analytical model for first calculations is proposed.