Large Eddy Simulation Of Flow Characteristics In Microchannel

Microchannels have attracted much wider applications as the progress of microfabrication techniques. Due to the advantages such as the large volume-to-surface ratio, high mass and heat transfer rates, short reaction time, and easy scaleup, etc., microchannels have promising applications especially in mixing, heat exchange, high risk reactions and bio-detections. However, the validity of conventional theories and the accuracy of experimental measurements are ambiguous, because of the small dimension of microchannels. This provokes us to study flow characteristics in microchannels, which further accelerates the development of microchannel techniques.With the significant development of computational speed and capacity, numerical simulation technology had made great progress and now it has been becoming an important tool to study fluid flows. In this thesis, large eddy simulation was used to study the liquid flow, gas-liquid two-phase flow and liquid-liquid two-phase flow in microchannels.For the liquid (water) flow in a trapezoidal channel with 237μm hydraulic diameter, the flow field and its evolution were revealed by the simulation. The computational velocity agreed well with the PIV experimental data. At the entrance region of the channel, the pressure gradient and vorticity magnitude were large while the turbulent intensity was low; as the liquid flow along the channel, the pressure gradient tended to be a constant and the turbulent intensity gradually inclined to be a uniform distribution.For gas-liquid (air-water) two-phase flow in a circular microchannel with 400μm diameter, we obtained the pressure drop, velocity and turbulent intensity, vorticity magnitude and turbulent shear stress, bubble profile and its corresponding flow pattern through calculations via the VOF model at different conditions. The velocity had a symmetric radical distribution; the turbulent intensity showed a saddle-shape distribution and increased with superifical velocities. The vorticity magnitude and turbulent shear stress distributed as a tsper shape, and also increased with superficial velocities. Most of bubbles located at the center of the channel, and they reduced the turbulent intensity magnitude and influenced the vorticity magnitude and turbulent shear stress to some extent. For the immiscible liquid-liquid two-phase flow (square channel with 200μm wide), the microchannel was specified by bas-relief structure on the inside wall or elbows so as to enhance the liquid-liquid interface. The simulation revealed the influence of the bas-relief and elbows on the flow flied and phase distribution, and the impact of the bas-relief height, width, intervals and slope angle on the flow flied.