Void fraction measurement and calculation model of vertical upward co-current air–water slug flow

The focus of this paper is on the measurement and calculation model of void fraction for the vertical upward co-current air–water slug flow in a circular tube of 15 mm inner diameter. High-speed photography and optical probes were utilized, with water superficial velocity ranging from 0.089 to 0.65 m·s^-1 and gas superficial velocity ranging from 0.049 to 0.65 m·s^-1. A new void fraction model based on the local parameters was proposed, disposing the slug flow as a combination of Taylor bubbles and liquid slugs. In the Taylor bubble region, correction factors of liquid film thickness C_δ and nose shape C_Z^* were proposed to calculate α_TB. In the liquid slug region, the radial void fraction distribution profiles were obtained to calculate α_LS, by employing the image processing technique based on supervised machine learning. Results showed that the void fraction proportion in Taylor bubbles occupied crucial contribution to the overall void fraction. Multiple types of void fraction predictive correlations were assessed using the present data. The performance of the Schmidt model was optimal, while some models for slug flow performed not outstanding. Additionally, a predictive correlation was correlated between the central local void fraction and the cross-sectional averaged void fraction, as a straightforward form of the void fraction calculation model. The predictive correlation showed a good agreement with the present experimental data, as well as the data of Olerni et al., indicating that the new model was effective and applicable under the slug flow conditions.     

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     

Investigation of the hydrodynamics of slug flow in airlift pumps

A slug flow model considering the dispersed bubbles entrained from the tail of Taylor bubble (TB) and re-coalesced with the successive TB was proposed. Experiment was conducted to test the validity of this model by using a high-speed camcorder and particle image velocimetry (PIV). It was found that the model was valid for predicting the characteristics of slug flow in airlift pump within average error of 14%. Moreover, large pipe diameter was found to accelerate the rise velocity of TB and decreases void fraction in liquid slug by a small margin.     

用统计方法研究未充分发展弹状流动

在一内径19 mm、长2 m的垂直有机玻璃管内,采用自制的电导探针对未充分发展的气-液二相弹状流中的弹状气泡上升速度、液塞上升速度、弹状气泡长度和液塞长度进行了测量。得到了各自随表观气速或表观液速的变化规律。结果表明:在未充分发展的弹状流状态下,弹状气泡的上升速度略高于液塞的上升速度:弹状气泡长度随表观气速的增大而增大,随表观液速的增大而减小。文章对弹状气泡长度进行了统计分析。未充分发展弹状流中弹状气泡长度符合正态分布律。     

Bubble lengths in the gas–liquid Taylor flow in microchannels

Experimental results of measurements of the bubble and slug lengths in Taylor (slug) flow are presented. The experiments were carried out using 3 different straight microchannels (microreactor with square cross-section made of polydimethyloxosilane (PDMS); microreactor with circular cross-section made of glass; microreactor with rectangular cross-section made of polyethylene terephthalate modified by glycol (PETg)) and 4 different liquids (water, ethanol propanol and heptane). The results have been compared with the available literature correlations. It is concluded, that the values obtained from the correlation proposed by Laborie et al. [Laborie, S., Cabassud, C., Durant-Bourlier, L., Laine, J.M., 1999. Characterization of gas–liquid two-phase flow inside capillaries. Chem Eng Sci 54, 5723–5735] do not agree with the results of measurements, while the agreement of these results with the predictions obtained using the correlation proposed by Qian and Lawal [Qian, D., Lawal, A., 2006. Numerical study on gas and liquid slugs for Taylor flow in a T-junction microchannel. Chem Eng Sci 61, 7609–7625] is good. New, corrected values of the pre-exponential constant and the exponents in the Qian and Lawal [Qian, D., Lawal, A., 2006. Numerical study on gas and liquid slugs for Taylor flow in a T-junction microchannel. Chem Eng Sci 61, 7609–7625] correlation are proposed.     

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。     

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.     

Void fraction in vertical gas-liquid slug flow: Influence of liquid slug content

In this study we develop a model for computing the mean void fraction and the liquid slug void fraction in vertical upward gas-liquid intermittent flow. A new model for the rate of gas entrained from the Taylor bubble to the liquid slug is formulated. It uses the work done by the pressure force at the rear of the Taylor bubble. Then an iterative approach is employed for equating the gas entrainment flux and the gas flux obtained via conservation equations. Model predictions are compared with experimental data. The developed iterative method is found to provide reasonable quantitative predictions of the entrainment flux and of the void fraction at low and moderate liquid slug void fraction conditions. However, with an increased liquid slug void fraction experimental data indicate that the flow in the liquid slug transits to churn-heterogeneous bubbly flow thus gas entrainment flux tends to zero. Considering this effect in the iterative model significantly improved the predictions for large liquid slug void fraction conditions.     

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.     

捕捉微通道内Taylor流特性的一种渐变网格划分方法

提出了一种适用于捕捉通道内Taylor流液膜厚度的渐变网格划分方法。为了平衡模拟精度和计算效率,提出了针对此方法的具体规则来获得一种高效的网格划分。利用CFD软件Fluent进行了一系列的二维数值模拟,采用VOF模型来捕捉微通道内气液两相界面,所用微通道横截面宽度分别为0.1,0.2,0.3,0.4,0.5,0.8和1.0mm。采用提出的网格划分方法获得的模拟结果与关联式计算值和实验数据很接近。基于模拟结果,提出了新的气液弹长度的关联式,并且利用实验数据对此关联式进行了验证。     

Numerical study on gas and liquid slugs for Taylor flow in a T-junction microchannel

The rapid development of microfabrication techniques creates new opportunities for applications of microchannel reactor technology in chemical reaction engineering. The extremely large surface-to-volume ratio and the short transport path in microchannels enhance heat and mass transfer dramatically, and hence provide many potential opportunities in chemical process development and intensification. Multiphase reactions involving gas/liquid reactants with a solid as a catalyst are ubiquitous in chemical and pharmaceutical industries. The hydrodynamics of the flow affects the reactor performance significantly; therefore it plays a prominent role in reactor design. For gas/liquid two-phase flow in a microchannel, the Taylor slug flow regime is the most commonly encountered flow pattern. The present study deals with the numerical simulation of the Taylor flow in a microchannel, particularly on gas and liquid slugs. A T-junction empty microchannel with varying cross-sectional width (0.25, 0.5, 0.75, 1, 2 and 3 mm) served as the model micro-reactor, and a finite volume based commercial computational fluid dynamics (CFD) package, FLUENT, was adopted for the numerical simulation. The gas and liquid slug lengths at various operating and fluid conditions were obtained and found to be in good agreement with the literature data. Several correlations in the T-junction microchannel were developed based on the simulation results. The slug flows for other geometries and inlet conditions were also studied.     

The pressure drop experiment to determine slug lengths in multiphase monoliths

The length of the liquid slugs, that separate the elongated bubbles in Taylor flow, is an important parameter for mass transfer, flow stability and pressure drop in capillary microchannels. In this work, pressure drop measurements are used to determine the length of slug in Taylor flow in downflow monoliths. The method is sensitive if the slugs are relatively short, less than 10 times the channel diameter. The pressure drop measurements are a cheap and fast alternative to tomographic or electric methods. Experiments using different distributors indicate that the slug length varies significantly with changes in the hydrodynamics in the feed section of the monoliths. Slug length correlations that are based on parameters inside the channels can therefore not safely be used for a different setup. As a result, the slug length should be measured in each experimental setup, which makes a inexpensive and robust method to do so very welcome.     

倾斜上升弹状流中Taylor气泡运动速度研究

用高速动态分析仪对倾斜上升管中气液两相弹状流中Ｔａｙｌｏｒ气泡的运动速度进行了研究。获得了无干扰流场下Ｔａｙｌｏｒ气泡运动参数的测量结果，并分析了混合物流速及管倾角对气泡头部位置的影响，以及由此而引起的气泡漂移速度及液体速度影响系数的变化情况，在理论分析的基础上，推荐了计算气泡运动速度的实验关联式。并且与可利用的结果进行了比较，两者符合较好。     

倾斜管内低温液弹和Taylor汽泡的长度分布

An experimental study was carried out to understand the phenomena of the boiling flow of liquid nitrogen in an inclined tube with closed bottom by using a high speed motion analyzer.The experimental tube is 0.018 m ID and 1.0 m in length.The range of the inclination angle is 45°-90° from the horizontal.The experiment focused on the effect of the inclination angle show that the mean liquid slug length and Taylor bubble length increase with the increasing x/D at various inclination angles.At the same x/D,the mean liquid slug length and Taylor bubble length increase first,and then decrease with decreasing inclination angles,with the maximum at 60°.In the vertical tube,standard deviation of the nitrogen Taylor bubble length increase with the increasing x/D.For the inclined tube,standard deviation of the nitrogen Taylor bubble length increases first,and then decreases with the increasing x/D.Standard deviation of the liquid slug length increases with increasing x/D for all inclination angles.     

An experimental study of air-water Taylor flow and mass transfer inside square microchannels

Flow and mass transfer properties under air-water Taylor flow have been investigated in two square microchannels with hydraulic diameters of 400 and 200 μm. Experimental data on Taylor bubble velocity, pressure drop and liquid side volumetric mass transfer coefficient (k_La) have been presented. It was shown that the measured Taylor bubble velocity in square microchannels could be well interpreted based upon an approximate measurement of the liquid film profile therein. Then, the obtained two-phase frictional pressure drop values in both microchannels were found to be significantly higher than the predictions of the correlation proposed by Kreutzer et al. [2005b. Inertial and interfacial effects on pressure drop of Taylor flow in capillaries. A.I.Ch.E. Journal 51, 2428-2440] when the liquid slug was very short, which can be explained by the inadequacy of their correlation to describe the excess pressure drop caused by the strong inner circulation in such short liquid slugs. An appropriate modification has been made to this correlation in order to improve its applicability in microchannels. Finally, the experimental (k_La) values in the microchannel with hydraulic diameter of 400 μm were found to be in poor agreement with those predicted by the existing correlations proposed for capillaries with diameters of several millimeters. The observed deviation was mainly due to the fact that mass transfer experiments in this microchannel actually corresponded to the case of short film contact time and rather poor mixing between the liquid film and the liquid slug, which was not in accordance with mass transfer assumptions associated with these correlations. A new empirical correlation has been proposed to describe mass transfer data in this microchannel.     

垂直及倾斜上升管内气液两相弹状流壁面切应力的模拟

用VOF模型对垂直及倾斜上升管内弹状流壁面切应力进行数值模拟。计算结果表明,垂直上升流动时,液膜厚度始终小于对应位置倾斜上升弹状流的液膜厚度,壁面切应力从气弹头部至尾部逐渐增大至恒定不变,在尾流区呈杂乱无章状态。倾斜上升流动时,气泡头部顶点偏向管中心线上方,倾角越小,相同轴向位置处测得的液膜厚度越大。当FrTB较小时,倾斜管内弹状流上管壁面的切应力曲线在液膜区有明显波动,而下管壁面在对应区域的切应力分布则比较光滑。随着FrTB的增大,上下壁面切应力分布曲线越来越靠近。     

Scaling of the bubble/slug length of Taylor flow in a meandering microchannel

In order to reduce or avoid the fluctuations from interface breakup, a meandering microchannel with curved multi-bends (44 turns) is fabricated, and investigations of scaling bubble/slug length in Taylor flow in a rectangular meandering microchannel are systematically conducted. Based on considerable experimental data, quantitative analyses for the influences of two important characteristic times, liquid phase physical properties and aspect ratio are made on the prediction criteria for the bubble/slug length of Taylor flow in a meandering microchannel. A simple principle is suggested to predict the bubble formation period by using the information of Rayleigh time and capillary time for six gas-liquid systems with average deviation of 10.96%. Considering physical properties of the liquid phase and cross-section configuration of the rectangular mcirochannel, revised scaling laws for bubble length are established by introducing Ca, We, Re and W/h whether for the squeezing-driven or shearing-driven of bubble break. In addition, a simple principle in terms of Garstecki-type model and bubble formation period is set-up to predict slug lengths. A total of 107 sets of experimental data are correlated with the meandering microchannel and operating range:0.001 < Ca_TP < 0.05, 0.06 < We_TP < 9.0, 18 < Re_TP < 460 using the bubble/slug length prediction equation from current work. The average deviation between the correlated data and the experimental data for bubble length and slug length is about 9.42% and 9.95%, respectively.     

垂直管内弹状气泡上升时的壁面传质

Wall-liquid mass transfer for Taylor bubbles rising through liquid column in vertical tubes is an important and fundamental topic in industrial processes.In this work,the characteristics of wall-liquid mass transfer for this special case of slug flow were studied experimentally by limiting diffusion current technique (LDCT). Based on the experimental results and the analysis of hydrodynamic mechanisms,it was proposed that four different zones exist,i.e.the laminar falling film zone,the turbulent falling film zone,the wake region and the remaining liquid slug zone.The corresponding correlations for all these zones were developed.     

Visualisation research on characteristics of the cryogenic slug flow in vertical and inclined tubes

The statistical parameters of Taylor bubbles in cryogenic slug flow along glass tube were studied experimentally for various inclination angles θ (0°–60° from the vertical direction) and four tube inner diameters D (14, 18, 25 and 32 mm) by using high speed digital camera. The distributions and mean values of initial formation position, and the length and velocity of Taylor bubbles along the tube at various inclination angles were obtained. Initial position of Taylor bubbles increases as tube inner diameter increases and the effect of tube diameter on Taylor bubble initial position becomes more obvious at θ ≥ 30°. Taylor bubble length shows an allometric decreasing trend with the increasing of tube inner diameter. The influence of inclination angle and axial position on the length and velocity of Taylor bubble in cryogenic slug flow agrees qualitatively with conventional air–water system. The maximum mean velocity of Taylor bubble occurs at 30° ≤ θ ≤ 45° and the minimum mean length of Taylor bubble occurs at 0° ≤ θ ≤ 20°. The Mean length of Taylor bubble increases along the tube for x/D ≤ 60. © 2011 Canadian Society for Chemical Engineering     

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。