(142ck) Mass Transfer Characteristic of Gas-Liquid Segmented Flow in T-Junction Microchannels

In this paper, the mass transfer characteristic of gas-liquid segmented flow in T-junction microchannels was systematically investigated, in which the mass transfer in both the formation and flowing stage was distinguished and studied respectively. Gas-liquid segmented flow was realized in T-junction microchannles with different contacting angles and an in situ method was developed to measure the mass transfer characteristic in the formation stage. The result showed that the slug formation time was in the range of 0.2-0.4 s and the mass transfer during the formation stage could contribute around 30-40% of the total transferred solute. The overall mass transfer coefficient during the slug formation stage was in the range of 1.4-5.5 * 10^-4 m/s. The comparison between the experimental results and the calculated data by diffusion model indicated that the recirculation flow in the gas slug significantly intensified the mass transfer with the enhancement factor in the range of 5-15. A correlation equation was developed to predict the enhancement factor, considering both the influences of the surface renewal and the moving speed of the gas phase. The calculated data was in good coincidence with the experimental results. On the other hand, a systematical study on the mass transfer performance of segmented gas-liquid flows  during flowing stage was conducted and the perpendicular shearing method was applied to generate the gas-liquid microflow. An in situ measuring method was developed, with which the overall mass transfer coefficient in the flowing stage could be determined by measuring the change of gas slug volume versus flowing distance. Meanwhile, the overall mass transfer coefficient in both straight and curve shaped microchannels were determined and compared. The influences of both the gas and liquid flow rates, the gas concentration and the curve radii of main channel on the overall mass transfer coefficient were investigated. The overall mass transfer coefficient in both straight and curve shaped microchannels respectively ranged in 1-3* 10^-4 m/s and 2-7 * 10^-4 m/s, indicating the mass transfer in segmented gas-liquid flows can be enhanced by changing the main channel shape. Semi-empirical equations have been developed to correlate overall mass transfer coefficient in straight and curve shaped microchannels and calculated results show that the equations can predict the experimental values well.