A simple mechanism of bubble and slug formation in Taylor flow in microchannels

A simple mechanism is proposed to explain and predict the bubble and slug lengths in Taylor (slug) flow in microchannels. The results obtained using the proposed approach are in good agreement with a correlation based on numerical experiments [Qian, D. and Lawal, A., 2006, Numerical study on gas and liquid slugs for Taylor flow in a T-junction microchannel. Chem Eng Sci, 80: 7609–7625] and available experimental data.     

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.     

Continuous production of solid lipid nanoparticles by liquid flow-focusing and gas displacing method in microchannels

The liquid flow-focusing and gas displacing method is developed to produce solid lipid nanoparticles (SLNs) continuously in a microchannel, which has a cross-junction for the focus of lipid and aqueous solutions and a T-junction for the injection of gas bubbles. The liquid flow-focusing was achieved by introducing a lipid solution with a water-miscible organic solvent and an aqueous surfactant solution simultaneously through the two branches of the cross-junction into the main channel, while the gas displacing was accomplished by injecting an inert gas (N₂) through the T-junction at the downstream of the cross-junction into the main flow streams upward to form gas-liquid slug flow. Solid lipid nanoparticles were formed due to the local supersaturation of lipid induced by the diffusion of the solvent from the lipid solution stream into the aqueous phase. The liquid suspension containing solid lipid nanoparticles then passed freely through the microchannel without any blockage by the contribution of gas slug flow. The flow behaviors were observed by a digital inversion microscope system and the hydrodynamics of the liquid flow-focusing streams and the gas slug flow were investigated. Particle size distributions of the solid lipid nanoparticles obtained under various conditions were measured by dynamic light scattering and the particle morphology was examined by transmission electron microscopy. The influences of liquid velocity and lipid concentration under the gas displacing condition on the properties of solid lipid nanoparticles were studied experimentally. The solid lipid nanoparticles with small size (the mean size in the range of 120-200 nm) and narrow particle size distribution (with values of polydispersity index in the range of 0.14-0.19) had been produced by this method. The crucial roles of Taylor bubbles and liquid slugs in the formation of solid lipid nanoparticles were considered and the transfer mechanism of slug flow on the formation and passage of solid lipid nanoparticles in the microchannel were also discussed. Compared with other production methods for SLNs (e.g., hot homogenization, warm microemulsions and supercritical fluid technique), the proposed method in this work is simple and no overcritical operations are needed during the preparing process. Therefore, it can be employed to prepare SLNs with small sizes and a narrow diameter distribution.     

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.     

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.     

DEVELOPMENTS ON WETTING EFFECTS IN MICROFLUIDIC SLUG FLOW

Wetting effects form a dimension of fluid dynamics that becomes predominant, precisely controllable, and possibly useful at the micro-scale. Microfluidic multiphase flow patterns, including size, shape, and velocity of fluidic particles, and mass and heat transfer rates are affected by wetting properties of microchannel walls and surface tension forces between fluid phases. The novelty of this field, coupled to difficulties in experimental design and measurements, means that literature results are scarce and scientific understanding is incomplete. Numerical methods developed recently have enabled a shortcut in obtaining results that can be perceived as realistic and that offer insight otherwise not possible. In this work the effect of the contact angle on gas-liquid two-phase flow slug formation in a microchannel T-junction was studied by numerical simulation. The contact angle, varied from 0 to 140 degrees, influenced the interaction of the gas and liquid phases with the channel wall, affecting the shape, size, and velocity of the slugs. The visualisation of the cross-sectional area of gas slugs allowed insight into the existence of liquid flow along rectangular microchannel corners, which was affected by the contact angle and determined the occurrence of velocity slip. The velocity profile within the gas slugs was also found to change as a function of contact angle, with hydrophilic channels inducing greater internal circulation, compared to greater channel wall contact in the case of hydrophobic channels. These effects play a role in heat and mass transfer from channel walls and highlight the value of numeral simulation in microfluidic design. Supplementary materials are available for this article. Go to the publisher's online edition of Journal of Chemical Engineering Communications to view the supplemental file.     

微通道中气-液-液三相流流型及传质研究

使用双T型微通道,以体积分数为30%的磷酸三丁酯（TBP）的环己烷溶液-乙酸水溶液为萃取体系,研究了不同油水两相流量比及油水两相总流量条件下,气相的引入及气相流量分率α对流型及传质的影响,并获得总体积传质系数k_La与油水两相流量比q、气相雷诺数Re_g、液相平均雷诺数Re_M的关系式。研究结果表明,第1个T型接口处气相的引入所带来的气相剪切作用能促进第2个T型接口处分散相液滴的形成,可使液-液并行流转化为气-液-液三相弹状流;同时,由于气-液-液弹状流具有较高的相接触面积及内循环作用,传质系数k_La得到显著提高。     

壁面润湿性对微通道内二氧化碳-水两相流流动及传质性能的影响

在1 mm×1 mm矩形截面下微通道内,以二氧化碳-水为工作流体,研究壁面润湿性和气液表观流速对气-液两相流型和气液传质的影响,并研究了气、液表观流速对弹状流流体力学性质的影响。在亲水微通道中观测到了泡状流、泡状-弹状流、弹状流;在疏水微通道中观测到了非对称弹状流、拉长的非对称弹状流、分层流。实验表明亲水微通道中弹状流区域下气泡长度大体上随气相表观流速的增大而增大,随液相表观流速的增大而减小;液弹长度大体上随气相表观流速的增大而减小,随液相表观流速的增大先增大后减小;液侧体积传质系数k_La均随气、液相表观流速的增大而增大,随通道壁面润湿性的增强而增大。     

CFD Simulation of hydrodynamics of air sparging in a vertical tubular membrane

Computational fluid dynamics (CFD) simulation of the hydrodynamics of slug flow which is generated by air sparging in a vertical tubular membrane has been investigated. The results of simulation have been reported in the form of parameters such as shape, velocity profile, surface shear stresses and gas slug (Taylor bubble) rising velocities, and evaluated with experimental data which were presented in previous articles. This study showed that CFD modeling is able to accurately simulate the shape and velocity field around the gas slugs. Also the shear stress induced by slug flow passage and rising velocity of gas slugs for high-velocity liquid and low-velocity gas fit appropriately to values in reference data. Simulation results for gas slug rising velocity showed about 0.35–9% error in the different conditions investigated in respect to experimental data.     

T型微通道内弹状流流动特性

以空气为气相介质和不同表面张力的液体(纯水、0.01%SDS溶液、0.50%SDS溶液、乙醇)为液相介质,对矩形截面为100μm×800μm的T型微通道内的气液弹状流进行了相分离可视化实验。实验结果表明:气相表观速度一定时,液相采出分率随着液体表观速度的增加而减小;液体表观速度一定时,气体表观速度变化对液相采出分率影响不大;两相的表观速度一定时,液相采出分率随着液体的表面张力降低而减小。另外,将所得的相分离实验数据与其他管径尺寸的数据进行比较,发现不同尺寸T型通道内弹状流相分离特性比较相似。     

小曲率蛇形微通道弯头处弹状流流动及传质特性的数值研究

采用CLSVOF（coupled level set and volume of fluid）方法,以空气和水为工作流体对小曲率矩形截面蛇形微通道内气液两相流动进行模拟研究。验证模型的合理性后,研究了曲率对弯通道内压降的影响,曲率及气相速度对弹状流气泡及液塞长度的综合影响;同时深入分析了弯管内气液两相流动的传质特性,包括不同曲率下气泡长度的变化,弯管内液侧体积传质系数与液膜体积传质系数的比较,曲率及气相速度对液相体积传质系数的影响。同时,对比了回转弯道与直微通道传质系数的差异,发现弯微通道可以强化传质。     

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。     

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。     

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.     

Numerical simulation of the mass transfer process of CO2 absorption by different solutions in a microchannel

The flow and mass transfer characteristics of CO₂ absorption in different liquid phases in a microchannel were studied by numerical simulation. The mixture gas phase contained 5 vol% CO₂ and 95 vol% N₂ , and the different liquid phases were water, ethanol solution, 0.2 M monoethanolamine solution, and 0.2 M NaOH solution, respectively. Based on the permeation theory, the distribution of velocity and concentration in the slug flow was obtained by local simulation of flow and mass transfer coupling and was described in depth. The influence of contact time and bubble velocity on the mass transfer of the whole bubble was highlighted. The volumetric mass transfer coefficient on the bubble cap and liquid film, CO₂ absorption rate, and enhancement factor were calculated and analyzed. The results showed that the volumetric mass transfer coefficients of chemical absorption were ~3 to 10 times that of physical absorption and the CO₂ was absorbed more completely in chemical absorption. The new empirical correlations for predicting the mass transfer coefficient of the liquid phase were proposed respectively in physical absorption and chemical absorption, which were compared with the empirical formulas in the literature. The volumetric mass transfer coefficients obtained by predictive correlations are in good agreement with those obtained by simulation in this paper. This work made a basic prediction for CO₂ absorption in microchannel and provides a foundation for later experimental research.     

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。     

Squeezing-to-dripping transition for bubble formation in a microfluidic T-junction

The aim of this paper is to investigate the squeezing-to-dripping transition for bubble formation in a microfluidic T-junction by cross-flowing rupture technique using a high-speed digital camera. Experiments were conducted in a glass microfluidic T-junction with the cross-section of the microchannel of 120 μm wide and 40 μm deep. N₂ bubbles were generated in glycerol–water mixtures with several concentrations of surfactant sodium dodecyl sulfate (SDS). Three different regimes were identified for generating different kinds of bubbles: squeezing, dripping and transition regimes. Various forces exerted on the gaseous thread in different regimes were analyzed. Long slug bubbles were formed in the squeezing regime, while dispersed bubbles in the dripping regime. The transition regime formed short slug bubbles. The bubble sizes in various regimes could be correlated with several dimensionless numbers such as the ratio of gas/liquid flow rates and capillary number. The two-step model for droplets (Steegmans et al., 2009) was extended to describe the bubble formation.     

High-frequency formation of bubble with short length in a capillary embedded step T-junction microdevice

The performances of slug flow gas–liquid reactors are mostly determined by slug length, especially for the high gas–liquid flow rate ratio condition. This work is the first time to report the short bubble generated with high frequency in a capillary embedded step T-junction microdevice. The aspect ratio of bubble could be around 0.5 with a frequency higher than 750 s⁻¹ when the gas–liquid flow rate ratio is even higher than 5. The specific surface area of the generated gas–liquid microdispersion system is larger than 10,400 m²/m³. The short bubble formation process includes two periods, and its formation mechanism is mainly because of the relatively higher pressure drop in the step T-junction, which provides a much higher breakup force for the squeezing flow. Finally, two models are developed to predict the bubble frequency and volume. This work provides a highly promising dispersion technology for the gas–liquid process intensification in microreactors.     

Simulation of fluid dynamics in a microchannel Fischer-Tropsch reactor

Fluid dynamics in a microchannel of a new-generation Fischer-Tropsch reactor for artificial petroleum synthesis is reported. Liquid and gaseous product downflows are modeled in the annular flow approximation. The conjugate flow of gaseous and liquid products is investigated in terms of integral momentum balance equations with capillary effects and microchannel wall roughness taken into consideration. A simulation procedure appropriate for the method of deposition of catalyst particles onto the inner wall is suggested for modeling a random inner surface of the microchannel. The simulation procedure takes into account the variation of the synthesis product composition and the variation of the thermal properties of the liquid and gas phases along the microchannel length. A stable computational procedure has been devised for calculating fluid dynamic parameters of the two-phase flow. The effects of the synthesis gas flow rate and conversion, microchannel diameter, chain propagation constant, temperature, and pressure on the two-phase flow parameters and on the aerodynamic resistance of the gas have been investigated.     

Immiscible liquid–liquid slug flow characteristics in the generation of aqueous drops within a rectangular microchannel for preparation of poly(2-hydroxyethylmethacrylate) cryogel beads

The microchannel liquid-flow focusing and cryo-polymerization is an efficient method for the preparation of cryogel beads with a narrow diameter distribution. In order to prepare cryogel beads with expected diameters, it is necessary to get insights in the liquid–liquid immiscible flow characteristics of the flow-focusing fluid and the monomer solution in microchannels. In this work, the slug flow behaviors of two immiscible liquids regarding the preparation of poly(2-hydroxyethylmethacrylate) (pHEMA) cryogel beads in a rectangular cross-junction microchannel were investigated experimentally by the high-speed imaging method. Correlations of the immiscible liquid–liquid slug flow parameters like the aqueous slug velocity and length, the aqueous slug nose and tail lengths, the water-immiscible slug length as well as the aqueous droplet size were obtained. The pHEMA cryogel beads were prepared under certain flow conditions and the bead sizes were measured by laser particle size analyzer. The obtained correlations were then employed to estimate the bead sizes and compared with those obtained experimentally. The results showed that the present correlations gave reasonable estimations of the mean bead diameters at various conditions and thus, could be useful and helpful in the preparation of cryogel beads with expected size distributions in rectangular microchannels.