Mixing by Helical Flows in Curved and Meander Micro Channels

Mixing by Helical Flows in Curved and Meander Micro Channels Most Relevant Citations 

The generation of secondary helical flows in suitably curved channels, known as Dean vortices, is not an entirely new fluidic phenomenon discovered at the micro scale actually it was found already also for wound tubings of conventional diameter (see a summary in [152]). In-depth studies concerning Dean vortices in curved channels were made in the framework of various applications such as filtration, heat exchange, friction and mixing. 

When guiding fluids through curved channels, the maximum in the velocity profile is displaced towards the outer channel wall and Dean vortices form (as reported, e.g., in [152]). The latter are typically characterized by two counter-rotating vortices above and below the symmetry plane of the channel coinciding with its plane of curvature. Fluid is transported outwards in this plane by means of centrifugal forces. By recirculation, back transport along the channel walls is induced. 

A Dean number of -140 is a kind of threshold value [47,152], For lower values, two counter-rotating vortices are found, whereas for higher values, two additional counter rotating vortices appear which are close to the center of the outer channel wall. Means to achieve this are changes in the flow velocity, the hydraulic diameter and the radius of curvature. 

Helical flows are not only found for 2-D mixer designs. In analogy with findings for macro-scale alternating helical coils, microstructured 3-D designs were proposed [49, 153], For reasons of limitations of today s micro fabrication, not real helices were made, but easier to fabricate structures such as complex micro channels based of L-shaped elements arranged in a 3-D fashion. 

---