Numerical Study On Hydrodynamics And Mixing Process In Microchannels For Liquid-liquid Two-phase Flow

Microfluidic system has great advantages in terms of mass transfer, heat transfer and chemical reaction etc, which attracts wide interest in biochemistry, pharmaceutics and many other fields. As an important part of microfluidic systems, micromixing technology based on liquid-liquid two-phase flow gradually become one of the hotspots in scientific research, but the hydrodynamics and mixing laws have not been well understood. In order to get a detailed knowledge of the laws, we divided our research into three parts:perturbance effect at the entrance of the channel, mixing mechanism in the microchannel and the chaotic mixing within the micro-droplet.The Computational Fluid Dynamics (CFD) methodology based on the Volume of Fluid (VOF) model was used to simulate the droplet formation process in the T-shaped and cross-shaped microchannels. A user define scalar (UDS) method was implemented to investigate the mixing enhancement performance of the perturbance effect at the entrance of the channel and the mixing mechanism within the mciro-droplet while travelling in the serpentine channels. Furthermore, a simplified2D theoretical micro-droplet model and the Discrete Phase Model (DPM) methodology were used to investigate the detailed chaotic mixing process within the micro-droplet. The results are as follows:1) UDS method is suitable to simulate the mixing process with diffusion effect, while DPM method is suitable to simulate the mixing process without diffusion effect.2) Generation of the dispersed droplet causes a perturbance effect of the mixing process, which leads to radial mixing, thus the mixing process can be enhanced. The perturbance mechanism can be mainly divided into two steps:when the dispersed droplet is on the growth stage, the front part of the continous phase is impeded by the dispersed phase, which produces radial velocity gradient and leads to radial mixing; when the dispersed droplet is on the formation stage, the channel is partly impeded by the dispersed phase, which causes a narrow part of the flow channel and the leads to further radial mixing. The perturbance effect under different dispersed phase fraction (εd) is significantly different, the lower, the better mixing.3) Mixing process within the micro-droplet can be well enhanced while traveling along serpentine microfluidic channels. The mixing of fluid was found to follow two routes:a) minor circulations in the front and back of the droplet and b) major circulations within the droplet. When the droplet travels in the straight part of the channel, both of them keep equal between the left and right parts of the droplet. But when the droplet travels in the winding part of the channel, they become unequal, which leads to the reorientation of the fluids in the droplet. For droplets of dimensionless size less than2.0, the recirculation and minor circulations make it easy for the fluids distribute in axial direction, which leads to a fast mixing process. When the droplet dimensionless size is larger than2.0, the major and minor circulations become insignificant for the reorientation effect that the fluids within droplet mainly distribute in radial direction. Thus the mixing distance increases observably.4) The simplified2D theoretical micro-droplet model shows that when travelling in the serpentine channel, the streamlines at two different times cross leads to the chaotic mixing. Poincare map and Lyapunov index shows chaotic mixing occurs within the micro-droplets, while travelling in the serpentine microchannels. When the Reynolds number is under low value (Re≤100), a lager Re and a small T lead to the better mixing.