Characteristics Of Gas-liquid Flow And Mass Transfer In Annular Microchannel

Annulus microchannel,composed of two concentric cylinders with different size,is a new microchannel structure,with advantages of easy catalyst coating,simple catalyst regeneration and higher heat transfer rate.At present,few studies were conducted in annulus microchannels.In this paper,the characteristics of gas-liquid flow and mass transfer in the annulus microchannel are studied systematically.Two scale horizontal annular microchannel devices are designed for experiment.The large one has an outer tube of transparent quartz glass with13mm inner diameter,and an inner stainless steel tube.The small one is consisted of an acrylic inner tube,and a plexiglass outer tube with 6.5～7mm inner diameter.The annulus gap sizes for both devices are 0.5 mm and1 mm.The CO₂ and H₂O were adopted as the fluid media.The influences of gas flow rate,liquid flow rate,annular gap size,outer diameter of the annular gap and the internals on the gas-liquid flow and dispersion characteristics were investigated with high-speed camera.The pressure drop was tested via pressure transducer.The gas-liquid mass transfer coefficients in the annulus microchannel were measured through titration.Ansys Fluent software was used to simulate the gas-liquid flow in the annular gap.The optimal internals structure design is obtained by numerical simulation.The simulation results were verified by experiments.The gas-liquid flow experiments show that the bubble size in the microchannel increases with the increasing gas rate.However,the bubble size decreases with the increase of liquid rate.Reducing the annulus gap size results in a better dispersion and breakage for gas bubble.The internals enhance the gas dispersion and gas-liquid mass transfer.The inertia force and surface tension play main role for on the gas-liquid flow in the large device,while the surface tension for the small device.The pressure drop results in the annulus microchannel show that the increase of gas rate and/or liquid rate leads to an increasing pressure drop.Smaller annulus gap size brings a higher pressure drop.The internals result in significantly rising pressure drop.Based on the experimental data,a Mathematical model for pressure drop of splitter flow was proposed.The experimental results of gas-liquid mass transfer reveal that the gas-liquid mass transfer coefficient increases with the increasing gas rate or liquid rate.The decrease of annulus gap size leads to the increase of gas-liquid mass transfer coefficient.The internals enhance the gas-liquid mass transfer effect.Smaller outer diameter of the annulus effectively improves the mass transfer coefficient.A dimensionless prediction model of gas-liquid mass transfer for annulus microchannel is presented,which can predict the experimental results well.The numerical simulation of the gas-liquid flow and dispersion characteristics in the annulus microchannel was conducted by Ansys Fluent software.The optimal design of the internals was found via numerical simulation.The verification experiments were performed.The experimental results agreed well with the simulation results,indicating the internals can effectively improve the gas-liquid flow,dispersion and mass transfer.