Study On Gas-Liquid Two-Phase Flow And Mass Transfer In Microchannels

In recent years, micro-chemical technology has become a new developing direction and research focus for chemical engineering. Microchannels with diameter ranging from nanometer to micron are main sections of the micro-chemical equipments, therefore, the characteristics of fluid flow and mass and heat transfer in microchannels are of key importance for the design and application of micro-chemical processes.The flow pattern, void fraction and pressure drop for gas-liquid two-phase flow in rectangular microchannels with hydraulic diameter of 100μm magnitude were investigated experimentally by a high-speed camera and a pressure transducer system. The flow pattern maps and transition equations were obtained. The results showed that, the aspect ratio of rectangular cross section influenced the void fraction and two phase pressure drop remarkably, hence, two new models were proposed to predict the void fraction and two phase pressure drop in microchannels, respectively, and the predicted values were accordant with experimental results of both this thesis and literature.The liquid volumetric mass transfer coefficients kLa were obtained by measuring the pressure variation during physical absorption process. Results showed that the channel diameter and liquid phase capillary number influenced the kLa remarkably, and a new dimensionless correlation was proposed to predict the kLa. The diminishing process of Taylor bubble during physical absorption was recorded by the high-speed camera, and the temporal evolution process for the Taylor bubble was obtained, subsequently, the liquid mass transfer coefficient kL and specific surface area a for two-phase flow were calculated. Results showed that, the increase of kLa in microchannels was mostly due to the increase of specific surface area a, and the kL in microchannels was close to that in conventional tubes.Concentration distributions on the liquid side during the formation of CO₂ Taylor bubble at the inlet of microfluidic T-junction were studied using a micro laser holographic interferometry. The shift of the interference stripes were recorded by an image acquisition system, and then the images were treated by a digital image processing system to obtain the concentration distributions on the liquid side and the thickness of concentration boundary layer. Experimental results showed that there was obvious mass transfer during the generation processes of Taylor bubbles, and both the liquid side concentration near the interface and the thickness of concentration boundary layer decreased with the increases of liquid and gas velocities.On the basis of Navier-Stokes equation and surface tension model, the Taylor bubble flow and the influence of various parameters on the Taylor flow were simulated by the VOF method. The agreement between numerical simulation results and experimental results confirmed the validity of the present model. Furthermore, based on the simulated stable Taylor bubble flow, by solving the convection-diffusion equation, the mass transfer process in a unit Taylor cell was simulated. Results showed that, for physical absorption in the unit Taylor cell, the primary mass transfer process occurred in the liquid film arround the bubble, and the simulated results of kLa are accordant with the experimental results, as well as the predictions by the penetration model. The influences of various parameters on mass transfer process in a unit Taylor cell were obtained in the simulation, and the results could be served as theoretical basis for the design and application of microfluidic devices.