Numerical Simulation On Hydrodynamics And Mass Transfer Of Gas-Liquid Two-Phase Taylor Flow In Microchannel

With the rapid development of micro chemical technology,gas-liquid two-phase Taylor flow in microchannels has attracted extensive attention in recent years.Taylor flow consists of alternating gas-liquid slugs with double-circulating vortices inside.Due to the advantages of precise regulation and high transfer rate,Taylor flow is considered to be an ideal flow pattern for gas-liquid reactions.Through the exploration of hydrodynamics and mass transfer characteristics of Taylor flow,the flow behavior and reaction phenomenon in micro scale can be further supplemented,which provides a theoretical basis for optimizing the operating conditions of the microreactor.At present,the research on the formation of Taylor flow and gas-liquid two-phase mass transfer process still needs to be improved.Based on the computational fluid dynamics software ANSYS FLUENT 19.2,this paper numerically simulates the flow and mass transfer behavior of gas-liquid two-phase Taylor flow through the multiphase flow model.The research contents mainly include the following aspects:（1）The generation characteristics of Taylor flow in a T-junction microchannel with a diameter of 1mm are explored by using the fixed reference system method,taking CO₂/N₂mixture and water as the system.It shows that Taylor bubbles are formed in four stages:intake,blocking,extrusion and fracture.It is found that the gas-liquid superficial velocity and inlet structure will affect the generation length of slug.The regulation of gas-liquid section length by changing the gas-liquid superficial velocity and gas-phase inlet angle is obtained.（2）The moving reference frame method is employed to explorethe hydrodynamic details in a single Taylor unit.It is found that there are symmetrical circulating vortices in the gas-liquid slug.The Taylor bubble and the wall of the microchannel are separated by a thin liquid film.By changing the viscosity and surface tension,the variation of Taylor bubble shape under different capillary numbers is analyzed.The results indicate that enlarging Ca will lengthen the liquid film region and increase the curvature on both sides of the bubble.（3）Based on the simulation of Taylor unit,the mass transfer behavior of CO₂ Taylor bubble to liquid slug is investigated by compiling user-defined function and coupling VOF model with species transport model.The results show that the solute distribution is mainly affected by the flow field in the liquid slug.Therefore,the concentration near the phase interface and the mass transfer capacity of Taylor bubble surface are different,and the mass transfer capacity of liquid film region is the most prominent.The gas phase dissolution process under different bubble velocities is studied.It is found that the increase of bubble velocity will accelerate the renewal rate of phase interface,and the mass transfer rate in each region of bubble surface will increase accordingly.（4）The mass transfer behavior in double bubble system is discussed.Compared with single bubble,the mass transfer capacity of front bubble tail and back bubble head in multi bubble system is weakened,and the mass transfer rate of the back bubble is lower than that of the front bubble due to the influence of the flow direction.It can be deduced from the conclusions that the mass transfer capacity of Taylor bubble at the head is the strongest,and the subsequent bubbles gradually weaken.