Mixing And Flow Behavior In Microfluidic Devices With Arborescent Structure

Due to its distinguished mass and heat transfer performance, and inherent safety, microfluidic technology is not only becoming one of the most important developments in chemical engineering and technology, but also the most effective approach for process intensification. In addition, the application of the microfluidic technology attracts more and more interesting in the last decade, because of its highly continuous operation and precise control over the fluid. The multi-phase system, especially, the gas-liquid two-phase system is one of the most frequently involved systems in chemical processes. With the benefit of the advantages, microfluidic device is suitable for some hazardous reactions, such as, highly exothermic, corroding, and toxic reactions, which are difficult to be accomplished and may cause serious problems if conventional reactors were used. However, most microfluidic devices developed so far have the problems of small throughput, high energy dissipation, and very costly fabrication. Numbering-up is though a simple concept to achieve process scale-out. It may lead to some fluid distributing and operating issues. Therefore, microfluidic technology has a great gap to meet the needs of industrial applications. The studies on gas-liquid two-phase flow and mass transfer in micromixers and microchannels were carried out in this dissertation. Moreover, two-phase flow in some parts of the micromixer was also studied. The results of these researches would provide scientific basis for the design and optimization of the industrial level micromixers on the one hand. And on the other hand, these results would deepen the understandings of gas-liquid two-phase flow in microfluidics, and facilitate the development of the models for flow pattern, pressure prediction, and mass transfer.First of all, a novel multi-scale micromixer with arborescent structure for high throughput gas-liquid mixing is evaluated by absorbing pure CO2into alkaline solutions, and the volumetric mass transfer coefficient, interface area, liquid side mass transfer coefficient and pressure drop were determined for different configurations and operations of the micromixer. When the two fluids are first partitioned into sub-streams and impinging in opposite directions, and the mixing chamber has also an arborescent structure, the mixer has a superior performance when the gas feeding rate is sufficiently high. For a small gas feeding rate, the mixer will have a higher mass transfer coefficient if the gas is introduced from the center plate, flows through a perforated plate after contacts with liquid, and convergences in the side arborescent plate. However, the pressure drop of the latter is much higher than the former. The former has the best performance under all operating conditions if both mass transfer coefficient and pressure drop are taken into account.In order to further improve the mass transfer performance of the arborescent micromixer, a better understanding on the fundamentals of fluid distribution and mass transfer of its core component, the arborescent structure, is necessary. For fluid distribution, the computational fluid dynamics (CFD) method was employed to investigate the gas and liquid distribution performance of the two arborescent distributors, the original arborescent structure and a novel arborescent structure with smoothed junctions. It is found that the uniformity of gas distribution is better than the liquid distribution in both structures. As compared with the original arborescent structure, the smoothed one distributes both gas and liquid with much higher uniformity. But surprisingly, the pressure drop of the smoothed structure is higher than the original one. By the analysis, the pressure drop of the distributor is mainly caused by the inlet and the outlet. Because of the reducing in diameter near the outlets, a large pressure drop can be arisen in the vicinity of the outlets of the smoothed distributor.For mass transfer, the mixing performance of the contacting structure of the arborescent micromixer for miscible liquid-liquid mixing and gas-liquid mixing are studied numerically. Firstly, the performance of miscible liquids mixing was investigated in three different contacting structures, including the original structure, the smoothed structure, and the smoothed structure with reduced side channel. For all these contacting structures, the mixing performances of impinging mixing and jet-mixing are studied. The mixing quality, a, at the outlet plan of each structure can be calculated to quantitatively evaluate the mixing performance. The results imply that the original contacting structure outperforms other structures when the fluids are impinging in opposite directions. For jet-mixing, the original structure has a better mixing quality when mixing channel Reynold number, ReM, is small, the smoothed contacting structure with reduced channel width has larger a when ReM is large. The pressure drop of the cross-flow mixing for all the structure is higher than the impinging mixing. Besides, for both mixing principles, the smoothed structure reduces the pressure drop. Moreover, the Volume of fluid (VOF) method was employed to simulate gas-liquid two-phase mixing in two contacting structures which has better mixing quality for the miscible liquids system. The interfacial area was calculated to determine the gas-liquid mass transfer performance of the contacting structures. Gas superficial velocity has insignificant effect on the interfacial area, while interfacial area increases remarkably with the increment of liquid superficial velocity. For smaller liquid superficial velocityies, the original structure has larger interfacial area, but for larger liquid superficial velocities, the interfacial area in smoothed structure with reduced channel is relatively larger. The pressure drop of the former is lower than the latter for smaller liquid superficial velocities. On the contrary, for larger liquid velocities, the latter has relatively lower pressure drop. The original structure has better mixing efficiency for smaller liquid velocities, while the smoothed structure with reduced side channel performs better for larger liquid velocities if both mass transfer and pressure drop are taken into account.Before carrying out the study on gas-liquid two-phase distribution, the factors that affect gas-liquid two-phase flow in single microchannel should be studied. By employing the VOF method, gas-liquid two-phase flow was simulated over a wide range of gas and liquid superficial velocities and fluid physical properties. The impacts of geometric parameters, such as gas inlet angle, cross-section aspect ratio, cross-section shape, and channel diameter, on bubble length were investigated comprehensively. It is found that the operating conditions, such as gas to liquid velocity ratio, physical properties, such as interfacial tension and liquid viscosity, as well as geometric parameters, such as gas inlet angle, aspect ratio, and shape of cross-section, have great influences on the bubble length, while the effects of liquid density and hydraulic diameter can be neglected. For most circumstances, the shortest bubble can be obtained in mcirochannels with a60°gas inlet. A correlation is proposed to predict the influence of geometric parameters, operating conditions and physical properties on bubble length in cross-flow microchannel, and a good agreement with the simulation data can be achieved.Finally, a novel microcontactor with arborescent distribution structure for gas-liquid two-phase distribution was evaluated using visualized experiment. For most gas and liquid feed rate, a good distribution of the two-phase flow can be observed. Relatively higher gas to liquid feed rate ratios can lead to maldistribution of the two-phase flow in each channel. The bubble length as well as the uniformity of the two-phase distribution decrease with the increase in the gas feed rate. On the contrary, the bubble length and the distribution uniformity increase with the increment of liquid feef rate. Moreover, the relative deviation of the bubble length for each channel from the average bubble length of the contactor increases with the increase in gas feed rate, while decreases with the increase in liquid feed rate.