Study On Gas-liquid Flow And Mass Transfer In Microchannels

Nowdays microreactors have attracted more and more attentions for their advantages brought from size reduction and produced process intensification, intrinsic safety and'number-up', which lead development of reactor design toward a new horizon. The aim of this paper is to investigate the gas-liquid two-phase flow and mass transfer in microchannels by both experiment and simulation methods and establish foundations for the design and optimization of microreactors.In the experimental part, two glass capillaries microchannels and one metal rectangular microchannel were self fabricated, whose hydrodynamic diameters were 300μm, 500μm and 90μm respectively, where two-phase flow, pressure drop and mass transfer performances of H2O-CO2 system was investigated. Bubbly flow, slug flow and slug-annular flow were recorded in both 300μm and 500μm microchannels under the experimental conditions by a high-speed CCD camera. After images analysis we concluded that diameters of microchannels affected the flow pattern maps because of the dominance of surface tension in micro channel. In slug flow, lengths of bubbles and liquid slugs changed with diameters and superficial velocities of two phases; the conventional fomulae for calculating pressure drop were unsuitable to microchannel and a modified equation was proposed for fitting experimental data well. The mass transfer rates in microchannels were higher 2-3 order of magnitude than large-scale gas-liquid contactors; however, part of the surface-to-volume for mass transfer would be invalid because the liquid in microchannels was so small to be satureatd easily in some conditions.In the simulation part, gas-liquid distributions and bubble formations were investigated numerically in microchannels with different geometries by computional fluid dynamics method. The results showed that cross-section shapes, hydrodynamic diameters of microchannels and the angles between gas and liquid inlets could affect the lengths of bubbles and liquid slugs, two correlations for predicting LB/d and LB/d in different geometries were proposed. During the bubble formation surface tension and pressure in gas and liquid inlets were dominanted over viscous force. The influence factors acted on the collapse of gas thread were different in different cross-sections of microchannels. Comparing with bubble formation in unbounded condition, the collapse rates of gas thread in microchannels were slower 1-2 order of magnitude, which caused the uniform distribution of gas and liquid in microchannels.