-The gas-liquid two-phase mass transfer process in microchannels is complicated due to the special dynamical characteristics. In this work, a novel method was explored to measure the liquid side volumetric mass transfer coefficient k L a. Pressure transducers were utilized to measure the pressure variation of upward gas-liquid two-phase flow in three vertical rectangular microchannels and the liquid side volumetric mass transfer coefficient k L a was calculated through the Pressure-Volume-Temperature correlation of the gas phase. Carbon dioxide-water, carbon dioxide-ethanol and carbon dioxide-n-propanol were used as working fluids, respectively. The dimensions of the microchannels were 40 μm×240 μm (depth×width), 100 μm×800 μm and 100 μm×2000 μm, respectively. Results showed that the channel diameter and the capillary number influence k L a remarkably and that the maximum value of k L a occurs in the annular flow regime. A new correlation of k L a was proposed based on the Sherwood number, Schmidt number and the capillary number. The predicted values of k L a agreed well with the experimental data.
Abstract:Bubble formation in an opposite-flowing T-shaped microchannel with 40 μm in depth and 120 μm in width was real-time visualized and investigated experimentally by means of a high speed camera. N 2 bubbles were generated in glycerol-water mixtures with different concentrations of surfactant sodium dodecyl sulfate (SDS). And the images were captured by the high speed camera linked to a computer. Results indicated that the bubble formation process can be distinguished into three consecutive stages, i.e., expansion, collapse and pinching off. The bubble size decreases with the increase of liquid flow rate and viscosity of liquid phase as well as the decrease of gas flow rate. The surface tension of the liquid phase has no measurable influence on the bubble size. Moreover, a new approach to predicting the size of bubbles formed in the T-shaped microchannel is proposed. And the predicted values agree well with the experimental data. Keywords:microchannel; bubble formation; high speed camera; opposite-flowing Great efforts have been made to explore multiphase flow in microfluidic devices in recent years. Microscopic gas bubbles find numerous applications in fields such as high-throughput screening experiments, material synthesis [1] , drug discovery [2] , chemical engineering [3] and fluidic logic [4] . It is essential to predict and control the flow performances (e.g., flow patterns and bubble size) in these applications, which are closely related to flow rates, phase properties and geometries of microfluidic devices.The most popular geometries for the production of microbubbles are hydrodynamic flow-focusing devices [5][6][7] , geometry-dominated breakup microfluidic devices [8] and T-junction microchannels using crossflowing rupture technique [9][10][11] . However, the bubble formation mechanisms are quite different for different microfluidic devices. In flow-focusing devices, the bubble size is determined solely by the ratio of gas and liquid flow rate [6,7] . The bubble size in the geometry-dominated breakup technique increases with the viscosity of continuous phase, but is independent of surface tension and flow rate [8] . For cross-flowing rupture technique, the bubble size changes with flow rate and the viscosity of continuous phase [9,10] . In order to study the bubble formation mechanism in T-shaped microchannels using cross-flowing rupture technique, van Steijn et al [11] measured the transient flow field of fluid during the formation of bubbles using microscopic particle image velocimetry (µ-PIV) and obtained three-dimensional velocity distributions in continuous phase. But so far, the microbubble formation by opposite-flowing technique in T-shaped microchannels has not been reported yet.In this study, the bubble formation in an oppositeflowing T-shaped microchannel was investigated experimentally by a high speed camera. The influences of gas and liquid flow rates, and the properties of liquid phase on the bubble size were investigated.
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