Research On The Mechanism Of Diffusion And Mixing In The Micro-channel Flow And Micro-mixer

This thesis is mainly related to the numerical study on the diffusion and mixing in the micro-channel flow. Based on it, the micro-mixers with high efficiency are designed. The thesis can be divided into three parts.In the first part, the numerical simulation of the diffusion-based mixing of the species in the micro-channel with the width of 50-500 microns and some experiments were carried out on the basis of the introduction of the research background about this theme, using the basic equations of the fluid dynamics. The numerical results show that the Reynolds number crucially influences the diffusion and mixing in the micro-channel, the lower the Reynolds is, the wider the species diffuse, furthermore, the dimension and shape of the micro-channel of T-sensor also play an important role in the diffusion of the flow. The smaller the channel width is, the more obviously the species diffuse, and the inlet angle has a reversed impact on the diffusion process. The ratio of the diffusion dimension (d) to the channel width (w) also plays a role in diffusion, the velocity distribution along diffusion dimension is almost the same, and the velocity contribution along channel width is obviously parabola when the ratio d/w is large, so the influence of upper and lower wall is primary. Conversely, when d/w is small, the velocity contribution along both directions are parabola, thereby the influence of all walls (upper and lower, right and left) could not be ignored. The experimental results validate partially the numerical results.In the second part, after introducing the various mixing principles and chaotic mixing theory, a new helical micro-mixer with horseshoe map was designed. In order to illuminate the mixing efficiency of the helical micro-mixer, the diffusion and mixing of the multi-species in the helical micro-channel were simulated numerically, the numerical results show that the mixing efficiency is much better than that in the straight micro-channel and obviously better than that in the serpentine micro-channel because of the layered structure and more contact areas when the Reynolds number islow. The mixing efficiency is also much better than that in the straight channel, but no obvious difference from that in serpentine micro-channel when the Reynolds number is high because the convection dominates the flow. A new micro-channel type with good passive mixing efficiency is provided. An active micro-mixer with magnetic bead and external magnetic field was designed, and then the factors impacting on the mixing efficiency for the micro-mixer were analysed.In the third part, the numerical simulation of Electroosmosis flow in a micro-channel was carried out by using the full Navier-Stokes equation (the electric force is considered as the volume force in this equation) in order that the separating efficiency and sampling quality can be improved when the zeta potential along the micro-channel wall becomes non-uniform because of the joint of different capillaries with different material and dimension. The numerical results indicate that a step change in ^-potential causes a significant variation in the velocity profile and in the pressure distribution. A step change both in ^-potential and dimension will result in more violent variation of the velocity profile near the joint. This variation will reduce the separation efficiency and sampling quality. However, the small difference between the maximum velocity and minimum velocity is benefit to the separation efficiency of the capillary electrophoresis when narrowing channel with bigger step-changed ^-potential.