微通道内气液(液液)二相流的实验研究进展

综述了微通道内气液(液液)二相流的流型特征.微通道内气液二相流常见的流型为泡状流、弹状流、环状流和翻腾流;液液二相流常见的流型为液滴流、塞状流、平行流及环状流.分析了不同操作条件对气液(液液)二相流行为的影响.介绍了微通道内气液(液液)二相流流型判别谱图,对常用的弹状流、液滴流和塞状流进行了重点介绍.指出了微通道内气液...     

微通道内醇胺水溶液吸收CO2的压力降研究

采用高速摄像仪对400μm×400μm T型微通道内N-甲基二乙醇胺(MDEA)与单乙醇胺(MEA)混合醇胺水溶液(含0.2%SDS)吸收CO_2过程的气液两相流压力降进行了实验研究。观测到了泡状流、泡状-弹状流、弹状流、弹状-环状流流型。考察了弹状流型下气液两相流流量、醇胺溶液浓度、气液雷诺数与增强因子对压力降的影响。结果表明:压力降随着气液两相流量、气液雷诺数及增强因子的增大而增大。当Q_G﹥80 mL/h时,压力降随醇胺浓度的增大而减小。以分相流模型为基础,提出了微通道内伴有化学吸收的气液两相流压力降预测模型,平均偏差为8.46%,模型计算值与实验值吻合良好。     

大深宽比双T形微通道内液-液两相流可视化研究

通过高速摄像对水力直径0.176mm、深宽比2.4的双T形矩形微通道内的液-液两相流动进行了可视化实验研究。改变连续相（硅油）和分散相（水）的流量比,记录分析了微通道不同部位油-水两相流的流型和流型发展演化情况。实验结果表明,在微通道上游T形部位的油-水两相流型主要包括滴状流、弹状流、波平行流和平行流;在微通道的中间部位,绘制了基于水和硅油量纲为1韦伯数的流型图,并将其与相关文献进行了比较。同时,发现微通道内液塞及液滴的长度（量纲为1）与油/水流量比之间存在线性关系,液塞/液滴速度比两相混合物表观速度大,建立了能够准确描述液塞/液滴运动速度的实验拟合公式。最后,研究了液滴在微通道下游T形部位的行为,观察到断裂和不断裂两种模式并进行了分析,给出了划分断裂与否的流型图。     

绝热状态下板壳式换热器壳侧的流型与压降

实验研究了板壳式换热器波纹通道内垂直向上气-液两相流动的流型和压降特性,讨论了圆形波纹通道内流型特征及转变机理,根据相界面形态特征将流型划分为泡状流、弹状流、膜状流和搅混流;同时分析了流型与压降之间的关系,发现泡状流中的压降波动幅值最小,弹状流与膜状流次之,搅混流中压降波动幅值最大;获得了波纹通道内单相以及气-液两相压降的分布规律,拟合了单相压降关联式,并基于Lockhart-Martinelli理论,通过分析两相摩阻系数与Martinelli参数的关系拟合了波纹通道内两相流动压降关联式,发现Chisholm参数C的值与Chisholm最初建议的光滑管内层流-层流的值接近。     

微小通道内低Reynolds数液-液两相流动与换热特性实验研究

实验研究了圆形微小通道内液-液两相流的流动和换热特性。选用去离子水为分散相,高黏度二甲基硅油为连续相。通过处理高速摄像所拍摄的可视化图像,总结了液-液两相流流型和液滴的长度/形状特征。并在此基础上考察了低Reynolds数下液-液弹状流对微小通道的换热作用。结果表明,平均Nusselt数随着Reynolds数的增加而增加,且油水比越大传热系数增加幅度越明显。Nu随着含水率的增加而降低。虽然含水率增加会使两相平均热容量提高,但在低Reynolds数下,这种提高被其长液滴内较弱的循环强度所抵消。选用三种不同形式接头在相同混合速度和含水率的情况下生成不同长度的液滴,发现短液滴更有利于换热。相同工况下,液滴长度的优化可以使整体传热系数提高近26%。     

微通道内气液两相流行为研究进展

综述了微通道内的气液两相流行为及传质特性。在微通道内流型一般分为泡状流、弹状流、环状流和弹状-环状流,没有分层流。气液传质效率比常规尺度中的提高了2～3个数量级。讨论了气泡对气液两相流的影响及其生成、生长和聚并规律。介绍了微通道内气液两相流的计算机模拟结果。从实验、理论和数值模拟3个方面对微通道内气液两相流的研究和应用前景进行了展望。     

弯曲正方形截面微通道内气液两相流流型研究

以空气-水为工质,在不同曲率比下利用高速摄影仪对正方形截面为0.8mm×0.8mm的弯曲微通道进行了气液两相流流动特性实验研究,获得了典型流型毛细泡状流、弹状流、间歇流和环状流,发现了非典型毛细泡状流和少见的波状分层流,并将实验结果与相关文献进行对比,为合理设计微型换热器和微化工混合器的气液流动分布结构、保证微通道内优异的传热传质特性提供理论指导和技术支撑。     

正弦型微通道内液-液两相流型及流动特性实验研究

采用实验的方法对不混溶的液液两相流体在不同入口结构下的正弦微通道(直通道正弦、波峰正弦和波中正弦)内液滴的流动特性进行了分析。硅油作为离散相,含有0.5% SDS的蒸馏水作为连续相,观测到弹状流、滴状流和射状流。分析了两相流动参数及不同的微通道入口结构对流型和液滴长度的影响。流型受微通道入口结构影响较大,波峰正弦微通道能够生成最大范围的稳定的流型。液滴长度随离散相体积流量和离散相与连续相体积流量之比的增大而增大,随连续相的体积流量和毛细数的增大而降低。微通道入口结构对液滴长度有影响,直通道的正弦微通道内液滴长度最短,更有利于液滴的形成。三种通道生成的液滴中,最大的液滴尺寸是最小的液滴尺寸的1.15~1.39倍,但正弦流动段对液滴速度几乎没有影响。     

微通道内伴有化学吸收的气液二相流型

采用高速摄像仪对T型进口的矩形微通道内气液二相流型进行了实验研究,实验物系采用单乙醇胺(MEA)水溶液-N2和单乙醇胺水溶液-CO2。对于无相间传质的单乙醇胺水溶液-N2二相流动过程,观测到了泡状流、弹状流、弹状-环状流和液环流;对于伴有化学吸收的单乙醇胺水溶液-CO2二相流动过程,未观测到泡状流,而观测到弹状-泡状流。在实验范围内,随着深宽比减小,无论是否伴有化学吸收,弹状流区域均减小;对伴有化学吸收的气液二相流,随化学反应速率的增大,流型转换线向右移动。以化学反应速率为控制参数,分别给出流型转换判别式,预测结果与实验数据吻合良好。在弹状-泡状流型中,随着气相表观流速的下降和液相表观流速、深宽比以及化学反应速率的上升,微通道内临界泡状距离减小。     

入口角度及壁面性质对蛇形微通道内弹状流流动特性影响的格子Boltzmann模拟

在Y型汇流的矩形截面蛇形微通道内,采用格子Boltzmann方法对不同壁面性质的蛇形微通道内弹状流流动进行了数值计算。首先以空气和水为工作流体对气液两相流动进行模拟研究并通过实验进行验证。通过验证实验后,模拟计算了气相速度,Y型夹角和壁面性质对气泡长度的影响,以及Y型夹角对微通道内弹状流压降和流动阻力的影响;探讨了粗糙度与壁面润湿性对流动阻力的影响;同时,针对蛇形微通道弯管部分,分析了角度和壁面性质对弹状流流动的影响。通过计算,发现当壁面接触角及Y型夹角为90?时,气泡长度最大;当直微通道为亲水性光滑壁面,回转弯道为粗糙度较大的疏水壁面时,Po数较小。     

Slug flow in curved microreactors: Hydrodynamic study

The need for eco-friendly and energy saving processes which are substantially compact and give higher efficiency has led to the concept of process intensification (PI). Curved microchannel is such innovative device, which has potential for the intensification of processes currently carried out in conventional straight or T-type microchannels. Curved microchannels utilize the benefits of centrifugal force to its advantage. The present study deals with the numerical simulation of the Taylor flow in curved microchannels, particularly on gas and liquid slugs with varying curvature ratios (i.e., coil to tube diameter=5, 10, 20 and 30). The three-dimensional, unsteady slug flow development in the curved microchannel was carried out using control volume finite difference method (CVFDM). The gas and liquid slug lengths at various operating and fluid conditions were obtained. The slug flow development for different inlet conditions and geometries (premixed feed, T-type and Y-type inlets) was also studied in the curved microchannels. It was found that for low curvature ratio (D/d=3), the phenomenon of flow reversal and slug freezing takes place due to centrifugal and buoyancy forces. For the similar process conditions, with an increase in curvature ratio to 5 and 10, the phenomenon of flow reversal and slug freezing observed was very minor. The non-uniformity in the slug formation was observed for low curvature ratio as compared to the higher curvature ratios. Further the influence of surface tension, viscosity and wall adhesion was studied on slug flow development in the curved microchannels. From the results it was observed that the surface tension, viscosity and wall adhesion have significant influence on slug flow development in curved microchannels.     

Influence of hydrodynamics on liquid mixing during Taylor flow in a microchannel

The miscible liquid‐liquid two phases based on Taylor flow in microchannels was investigated by high‐speed imaging techniques and Villermaux/Dushman reaction. The mixing based on Taylor flow was much better compared with that without introducing gas in microchannels, even the ideal micromixing performance could be obtained under optimized superficial gas and liquid velocities. In the mixing process based on Taylor flow, the superficial gas and liquid velocities affected the lengths and the velocities of Taylor bubble and liquid slug, and finally the micromixing performance. The formation process of Taylor flow in the inlets, the initial uniform distribution of reactants and the internal circulations in the liquid slug, and the thin liquid films all improved the mixing performance. Furthermore, a modified Peclet number that represented the relative importance of diffusion and convection in the mixing process was proposed for explaining and anticipating micromixing efficiency. © 2011 American Institute of Chemical Engineers AIChE J, 58: 1660–1670, 2012     

微通道内气-液弹状流动及传质特性研究进展

气-液弹状流,又称Taylor流,是一种以长气泡和液弹交替形式流动的流动形态。微通道内气-液弹状流因其气泡与液弹尺寸分布均一、停留时间分布窄、径向混合强等优点,是一种适于强化气-液反应的理想流型。本文首先介绍了微通道内气泡的生成机理、气泡和液弹长度,以及气泡生成阶段的传质特征。其次系统综述了主通道中弹状流动及传质过程的研究进展,包括气泡形状与液膜厚度、液弹内循环和泄漏流特征、气-液传质系数的测量与预测,以及物理与化学吸收过程中的传质特性等方面内容。最后阐述了当前研究的不足并展望了气-液弹状流的研究方向。     

An experimental investigation of gas-liquid two-phase flow in single microchannel contactors

This paper describes two-phase flow pattern and pressure drop characteristics during the absorption of CO₂ into water in three horizontal microchannel contactors which consist of Y-type rectangular microchannels having hydraulic diameters of 667, 400 and , respectively. With the help of a high-speed photography system, flow patterns such as bubbly flow, slug flow (including two sub-regimes, Taylor flow and unstable slug flow), slug-annular flow, churn flow and annular flow were observed in these microchannels. The applicability of the currently available correlations for describing flow pattern transitions in microchannels has been examined. Generally, the predicting performance of these correlations deteriorates as the channel diameter further reduces. Toward solving this discrepancy, an empirical correlation based on the superficial Weber numbers was developed to interpret the transition from Taylor flow to unstable slug flow in three microchannels. Taylor bubble formation process in microchannels was found to be in the squeezing regime at lower superficial liquid velocities (Ca ranging from 0.0019 to 0.029) while the transition to the dripping regime was observed at the highest superficial liquid velocity of 1.0 m/s. Lengths of Taylor bubbles formed in the squeezing regime can be well represented by the scaling relation proposed by Garstecki et al. [Formation of droplets and bubbles in a microfluidic T-junction—scaling and mechanism of break-up. Lab on a Chip, 6, 437-446]. For flow patterns including slug-annular flow, annular flow and churn flow, a simple analysis based on the separated flow model has been performed in order to reveal the observed effect of the superficial liquid velocity on two-phase frictional multiplier in the present microchannels. Then, reasonable correlations for the prediction of two-phase frictional pressure drop under these flow patterns were suggested.     

3‐D numerical simulations on flow and mixing behaviors in gas–liquid–solid microchannels

Three‐dimensional (3‐D) gas‐liquid–solid flow and mixing behaviors in microchannels were simulated by coupled volume of fluid and discrete phase method and simulations were validated against observations. The detachment time and length of gas slug are shortened in liquid–solid flow, compared with those in liquid flow due to higher superficial viscosity of liquid–solid mixture, which will move the bubble formation toward the dripping regime. Solid particles mainly distribute in liquid slug and particle flow shows obvious periodicity. With the increase of contact angle of the inner wall, gas slug (0–50°), stratified (77–120°), and liquid drop (160°) flows are observed. The residence time distributions of solid and liquid phases are similar because particles behave as tracers. The backmixing of solid and liquid phases in liquid drop flow is the weakest among the three flow patterns, and the backmixing of gas phase in slug flow is weaker than that in both stratified and liquid drop flows. The results can provide a theoretical basis for the design of microreactors. © 2013 American Institute of Chemical Engineers AIChE J, 59: 1934–1951, 2013     

Slug flow characteristics of gas–miscible liquids in a rectangular microchannel with cross and T-shaped junctions

The slug flow of an inert gas and two miscible liquids in microchannels has found its applications in the preparation of solid lipid nanoparticles (SLNs) by the liquid flow-focusing together with Taylor bubbles in microchannel systems, synthesis of metal nanoparticles or colloid silica in microreactors and enhancement of micro-mixing by interaction using gas bubbles in microfluidic devices. In this work, the flow characteristics of the slug flow generated by nitrogen gas and two miscible liquids (the aqueous surfactant solution and acetone or ethanol) flowing in a rectangular microchannel were investigated experimentally by using the high-speed optical imaging method. The microchannel system has a straight main channel for introducing one of the miscible liquids, a cross-junction for injecting of the other miscible liquid, and a T-junction for feeding the gas phase. The pressure drops were measured and images of Taylor bubbles and slug units at various velocities were obtained, from which other flow parameters were determined. Correlations for the velocity and length of Taylor bubbles, the bubble nose length, the bubble tail length, the liquid slug length, the maximum and minimum thicknesses of the liquid films around bubbles, as well as the pressure drop, were proposed. The calculated values of these parameters by using the correlations were compared with the experimental data. The results showed that the proposed correlations are in a good or reasonable agreement with experimental data and then expected to be available in the estimation of the slug flow parameters of the inert gas and two miscible liquids in rectangular microchannels.     

对称分支并行微通道中气液两相流的均匀性规律

采用高速摄像系统研究了对称分支形并行微通道内气液两相流及弹状气泡均匀性规律。实验中分别采用含0.3% SDS的甘油-水溶液与氮气作为液相和气相。观察到弹状流和泡状流两种流型,作出了由两相操作条件构成的流型图及流型转变线。结果表明,气泡非均匀性主要由两微通道内流体之间的相互作用、下游通道中流体动力学的反馈作用以及通道制造误差造成。随液相黏度增大,气泡均匀性变好;在高液相流量以及低气相压力下操作,气泡尺寸分布更易达到均匀。基于压力降守恒原理和微通道内气液两相流阻力模型,构建了两通道中气泡尺寸的预测模型。     

Experiment and lattice Boltzmann simulation of two-phase gas-liquid flows in microchannels

The two-phase flows in microchannels have many advantages in heat and mass transfer compared to single-phase flows. In particular, segmented flows such as bubbly and slug flows are often used in microfluidic devices. In the present study, experiments and Lattice Boltzmann simulations were carried out to study the gas-liquid flow in microchannels under various conditions. Two types of mixer geometries were used, including the cross-shape and the converging shape channels. The bubble shape, bubble size, and formation mechanism were investigated for different flow rates and different mixer geometries. The simulation results and the experimental results were compared based on dimensionless numbers, and good agreement was found in general. Different flow regimes with different bubble shapes were found depending on the Capillary number of the flow. The simulation data confirmed that the breakup was induced by the pressure difference in the two phases for small Capillary numbers. The geometry of the mixing section was also observed to have an impact on the size of the gas and liquid slugs.     

Effect of Viscosity on the Hydrodynamics of Liquid Processes in Microchannels

The hydrodynamics of single‐phase liquid flow with relatively high fluid viscosities in a microchannel was investigated experimentally. The results showed that the conventional theory could predict the single‐phase flow with high fluid viscosities in microchannels. Furthermore, the effect of viscosity on the slug flow of two immiscible liquid phases in a microchannel was studied with high‐speed imaging techniques. It was found that a higher dispersed‐phase viscosity quickened the flow pattern transition from slug flow to parallel flow and resulted in smaller slugs. A modified capillary number representing the mutual effects of the viscosities of the continuous phase and the dispersed phase was proposed for predicting the slug sizes in microchannels.     

垂直上升气液两相弹状流模型

基于等效弹单元思想,改进了预测垂直上升管中充分发展气液弹状流流动特性的模型。 模型中考虑了界面切应力对液膜运动的影响;并在液弹空隙度预测中引入临界气体夹带速度的概念,以此来描述弹状流中大气泡尾部的混合特性。本文提出的模型还考虑了管径对液弹空隙度的影响。弹状流模型的计算结果得到本文及其他作者实验数据的验证。