【作者】 侯璟鑫；

【导师】 周兴贵；

【作者基本信息】 华东理工大学 ， 过程系统工程， 2013， 博士

【摘要】 微流体技术因为具有卓越的传质、传热及安全性能,连续化操作和可实现流体精确调控等特性,成为过程强化的一个重要手段,也是近年来化学工程学科的主要研究内容之一。微流体设备因其固有的一些优良特性,非常适合于进行一些强放热、涉及强腐蚀和剧毒介质的反应。但是,目前微流体设备的处理量通常较小,而且制造成本较高。以数增的方式实现过程放大的概念虽然简单,但实际过程中会出现诸如流体分布不均、阻力降大、反应过程难以控制等问题。因此,微流体技术离实际工业应用还有一定距离。互溶液液体系和液液或气液两相体系在化工生产过程中比较常见。本文探究一种具有枝形结构的微流体设备中的互溶液液体系和气液两相体系混合与流动,重点研究其中的气液两相流及其传质规律,并对微反应器局部的流动特性进行研究。这些研究一方面有助于认识微流体设备中的气液两相流规律,另一方面可为微混合器与分布器的优化设计提供依据。本文首先研究了一种带有枝形结构的新型高通量枝形微混合器的气液传质性能。通过CO2在碱溶液中的吸收实验,测定了微混合器的体积传质系数、比界面面积、液侧传质系数以及压降,并研究了不同混合方式(撞击流和射流混合)以及微反应器配置(结构及气、液入口位置)的影响。结果表明,在气体流量足够高时,若气、液两相以相反的方向撞击混合、且使用具有枝形结构的混合腔,传质性能最好。在气体流量较低时,若气体从具有枝形结构的中部板进入混合器、与液体错流接触后再通过小孔进入带有枝形结构的侧板,传质性能最好,但压降相对较高。如果同时考虑传质效果和压降,则前者最优。要进一步改进枝形微混合器的传质性能,就要从对其核心结构——枝形结构的流体均布及混合性能进行研究着手。流体均布特性方面,采用计算流体力学(CFD)模拟的方法对两种枝形结构的气、液分布效果分别进行模拟。一种是枝形微混合器使用的原始结构,另一种是在原始结构基础上将流动转向处以及出口处做平滑处理获得新型枝形结构。结果显示,两种结构的气体分布性能均好于液体均布性能。平滑枝形结构其流体均布性能更好但压降也相对较高。通过对枝形结构各部位压降的构成进行分析发现,两种枝形结构的主要压降来自于进、出口处,后者出口处的渐缩结构形成了相对较高的阻力损失,因此,平滑枝形结构的总压降反而较高。混合特性方面,对枝形微混合器接触区的均相及多相混合进行了CFD模拟研究。首先,分别模拟了原始接触区、平滑接触区及缩小侧向通道的平滑接触区三种结构内的液液均相混合。对各接触区结构的撞击与错流混合性能分别进行了考察。通过计算了各接触区结构的出口截面混合效果指数a对混合性能进行量化。研究结果表明,撞击混合的原始接触区的混合效果最好。错流混合时,当混合通道雷诺准数ReM时,原型结构在低混合效果较好,而当ReM较大时,缩小侧向通道的平滑接触区混合效果较好。此外,撞击混合的压降普遍低于错流混合,而平滑处理有效的降低了阻力损失。然后,采用流体体积法(VOF法)模拟了均相混合效果较好的两种接触区结构内的气液两相撞击混合,并通过定量计算各接触区内的界面面积对其气液传质性能进行了比较。结果显示,气体表观流速度界面面积几乎没有影响,而液体表观流速的增大能够显著提高界面面积。当液体表观流速较低时,原始接触区的界面面积较大,反之缩小侧向通道的平滑接触区具有更大的界面面积。在低液体表观流速较低时,前者的压降低于后者,而当液体表观流速较高时,后者的压降则相对较低。若同时考虑界面面积和压降,在低液体表观流速时,原始接触区的传质效率较高：而在液体表观流速较高时,缩小侧向通道的平滑接触区的传质效率较高。对于气液两相流的分散研究从研究单个微通道内气液两相流的影响因素入手。采用VOF法在较广的气、液表观流速比和流体物性的范围内,对错流微通道的气体入口角度、截面宽高比、截面形状、通道直径等几何参数对两相流气泡长度的影响进行了系统研究。研究发现,气液表观流速比、界面张力、液相粘度、气体入口角度、截面宽高比以及截面形状对微通道内的气泡长度有显著影响,而液体密度和通道直径的影响则可以忽略。在大多数条件下,气体入口角度为60°的微通道内气泡长度最短。通过分析各因素与气泡长度的关系,建立关联式预测具有不同几何参数的错流剪切微通道内不同操作条件下的气泡长度,预测结果与CFD模拟结果彼此吻合较好。在对单根微通道内气液两相流具有较深入认识的基础上,采用可视化方法对基于构造理论设计的带有枝形分布结构的新型平行多通道微接触器内气液两相流的分散进行研究。在研究考察的大多数气、液流量下,该接触器内能够获得较为理想的两相流分散,气液流动分布不均主要发生在气、液入口流量比较大时。随着气体流量的增加,气泡长度变长,且两相分散的均匀性降低：而随着液体流量的增加,气泡长度变短,且两相分散均匀性提高。研究还发现,各通道气泡长度与接触器平均气泡长度的相对偏差随气体流量的升高而增大,随液体流量的升高而减小。更多还原

【Abstract】 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.更多还原