The role of surface tension in microgravity slug flow

In the analysis of slug flow under gravity conditions surface tension is usually neglected. The liquid slug is treated as a homogeneous mixture and the liquid film adjacent to the wall, in the Taylor bubble zone behind the slug, is treated using the one-dimensional approach (channel flow theory). Although the use of the one-dimensional approach is not accurate, especially close to the bubble cap, it is considered as a valid approximation and it yields reasonable results for the modeling of pressure drop, bubble length and void fraction in slug flow. Since for the case of microgravity flow, surface tension is expected to be a dominant force that should not be overlooked, one may be tempted to use the same procedure for the analysis of slug flow under microgravity conditions with the surface tension included (this can be done also for non-microgravity conditions). In this work, it is shown that the inclusion of the surface tension in the one-dimensional approach for the film analysis leads to erroneous and unacceptable results near the bubble cap that cannot be used even as an approximation. It is also shown that far away from the cap the solution with and without the surface tension is practically the same. Thus, a simplified model for slug flow in microgravity is suggested that assumes a spherical shape of the bubbles at the nose that is matched with the conventional one-dimensional viscous solution far downstream. In this procedure the effect of surface tension at the nose is in fact taken into account indirectly by the imposition of a spherical cap. That is, the assumption that the bubble nose behaves similar to the behavior of small size bubbles that are controlled by surface tension.     

Hydrodynamic characteristics of gas-liquid slug flow in a downward inclined pipe

Gas-liquid slug flow in a downward inclined pipe was studied experimentally by employing a wire-mesh sensor that enables quantitative measurements of the cross-sectional void fraction distribution. Processing of the wire-mesh sensor data was applied to carry out a statistical analysis of characteristic parameters of downward slug flow, such as bubble and liquid slug length distributions, as well as to determine the ensemble-averaged shapes of the bubble nose, liquid film and bubble tail. It was found that the pipe inclination affects mainly the bubble length, while variation in the gas flow rate affects both bubble and slug length. The bubble nose shape is more sensitive to the flow conditions than the bubble tail. The 3D structure of an elongated bubble in downward slug flow was reconstructed from the wire-mesh sensor data.     

水平与微倾斜管内间歇流中长气泡的形态特征

利用双平行探针技术和摄像方法对水平和近水平微倾斜管内长气泡的形态特征进行了实验研究.实验结果表明气泡头部以及气泡体的形态特征取决于气液混合Froude数和管道倾角,而尾部特征还与气泡长度有关;小气泡通过单纯水跃面的长气泡尾部向液塞区弥散,而具有阶梯状尾部结构的长气泡并不向液塞区弥散小气泡,所以气泡尾部结构特征的变化决定了弹状流向段塞流的转变;管道倾角对长气泡形态特征有显著的影响,下倾管内的长气泡在低Froude数时出现头尾倒置现象,同时下倾管内的长气泡比上倾管更易保持阶梯状的尾部结构,所以下倾管的弹状流区比上倾管宽.     

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.     

水平管内气液段塞流液塞长度发展规律的实验和模拟研究

In petroleum industry, the slug flow is a fre-quently encountered flow regime in multiphase flowpipeline. For pipeline designers, the liquid slug lengthdistribution is important for the proper design ofdownstream facilities, such as slug catcher and sepa-ration system. However, for its transient and unsteadynature, it is a great challenge for engineers to correctlypredict the flow parameters of slug flow, especiallythe maximum liquid slug length. The unit cell model for slug flow in horizontal…     

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.     

Low-velocity pneumatic conveying of coarse particles in a horizontal pipe

Slug flow was studied for air velocities less than 8 m/s with polyethylene pellets in a straight pipe 32 mm in diameter and 5.5 m in length, and was classified into 2 regimes: successive slug flow and solitary slug flow. A settled layer of particles did not form in the latter and high air pressure was required to push the slug as compared with the former. Successive slug flow was found in the low-velocity region where the power consumption reaches a minimum, a minimum that is lower than the minimum of the suspension flow region.     

The effect of gas injection on the hydraulic transport of slurries in horizontal pipes

The hydraulic transport of solid particles in a horizontal pipe is a well known practice in chemical plants and mining industry. The injection of gas into the flowing slurry results in a variety of flow patterns that affect the pressure gradient of the three-phase mixture in comparison with solid-liquid slurry flow. Furthermore, it may reduce or increase the pressure gradient relative to the conventional hydraulic transport of solids. This study constitutes the first attempt to formulate one-dimensional hydrodynamic models for evaluating the pressure gradients for stationary and moving solid beds overlaid by three-phase slug flow and for fully suspended three-phase slug flow of non-settling suspensions. The models for slug flow over stationary and moving beds are formulated by coupling the solid-liquid two-layer models of Doron et al. [1987. Slurry flow in horizontal pipes—experimental and modeling. International Journal of Multiphase Flow 13, 535-547] with a three-phase slug flow model. The proposed model for fully suspended three-phase slug flow constitutes an extension of the simple model for gas-liquid slug flow in horizontal pipes of Orell [2005. Experimental validation of a simple model for gas-liquid slug flow in horizontal pipes. Chemical Engineering Science 60, 1371-1381]. The proposed models, that are applicable to Newtonian slurries, were tested against the experimental data available in the literature over a wide range of operating conditions. In general, a good agreement was obtained between the predicted and experimental results.     

BUBBLE AND SLUG FLOW AT MICROGRAVITY CONDITIONS: STATE OF KNOWLEDGE AND OPEN QUESTIONS

Based on the experiments carried out over the past decade at microgravity conditions, an overview of our current knowledge of bubbly and slug flows is presented. The transition from bubble to slug flow, the void fraction and the pressure drop are discussed from the data collected in the literature. The transition from bubble to slug flow may be predicted by introducing a critical void fraction that depends on the fluid properties and the pipe diameter: however, the role of coalescence which controls this transition is not clearly understood. The void fraction may be accurately calculated using a drift-flux model: it is shown from local measurements that the drift of the gas with respect to the mixture is due to the non uniform radial distribution of void fraction. The pressuredrop happens to be controlled by the liquid flow for bubbly flow whereas for slug flow the experimental results show that pressure drops is larger than expected. From this study, the guidelines for future research in microgravity are given.     

倾斜窄矩形通道中气弹运动速度

利用高速摄像系统,对4种倾角下窄矩形通道(3.25 mm×40 mm)内弹状流进行了可视化研究。实验中发现,低流速时,倾斜条件下,由于浮力的影响,气弹头部偏离管道中心,头部变尖,因而气弹运动速度加快,系数C₀随倾角增加而减小,漂移速度V₀呈相反变化趋势;高流速时,流动趋于稳定,倾角对气弹速度影响不显著,对C₀和V₀影响较小。通过实验数据评价了竖直条件下4种气弹速度计算关联式。以Froude数3.5为界,分别提出了倾斜条件下C₀和V₀计算关联式。     

Gas–liquid mass transfer of aqueous Taylor flow in monoliths

The gas–liquid mass transfer of a monolith operating in the Taylor flow regime is presented. Mass transfer measurements are compared with a literature model derived for single capillaries. The comparison resulted in a prediction of the unit cell length (gasbubble+liquidslug). Independent measurements of the liquid slug length showed that the predicted unit cell length is close to the measured ones. This leads to the conclusion that mass transfer models for single capillaries may indeed be used for monoliths. Additionally, it is shown that the liquid slug length may also be estimated from pressure drop measurements.     

Assessment of well trajectory effect on slug flow parameters using CFD tools

Abstract

Oil and gas industry uses different well trajectories to enhance reservoir production rates. However, two-phase flow characterization can significantly change due to the well configuration. This study assesses this effect on slug flow development using computational fluid dynamics. An orthogonal mesh was used to assure well modeling of the slug flow dynamics. Slug flow parameters such as frequency and length were predicted using the VOF model. Four well trajectories were analyzed: toe-down, toe-up, and one-undulation with a hump or sump. Results revealed a vertical–lateral sections interaction, which increases slug length and decreases slug frequency on the horizontal pipeline. Moreover, using a toe-down well over an only-horizontal trajectory causes an increment in the slug length up to two times. Finally, a severe slugging condition was predicted in the toe-up well configuration.     

Design and operation of gas–liquid slug flow in miniaturized channels for rapid mass transfer

The influences of operating parameters such as channel size, flow rate, and void fraction on the mass transfer rate in the gas–liquid slug flow are investigated to establish a design method to determine the parameters for rapid mass transfer. From the experimental results, the turnover index, including the slug linear velocity, its length, and the channel size that represents the turnover frequency of the internal circulation flow, is proposed. For PTFE tube in which no liquid film exists in slug flow, a master curve is derived from the relationship between the mass transfer coefficient and the turnover index. For each channel material, the Sherwood number is also roughly correlated with the Peclet number. These correlations make it possible to arbitrarily determine a set of operating parameters to achieve the desired mass transfer rate. However, the turnover index and the Peclet number include the slug length, which cannot be controlled directly. The relationship between the slug length and the operating parameters is also investigated. The slug volume mainly depends on the inner diameter (i.d.) of a union tee. At a fixed union tee i.d., the slug length is controlled through the exit i.d. of the channel connected to the union tee and the void fraction. Thus, the final slug length depends on the union tee and exit channel inner diameters. At low flow rates, the gas and liquid collision angle is significant in determining the slug length.     

水平管道段塞流特征参数试验研究

通过对差压波动信号的详细分析 ,明确了液塞头和液塞尾到达和离开传感器测压点的时刻与差压信号特征点之间的物理关系 ,提出了采用差压波动信号分析技术获得液塞速度、液塞长度和液塞频率的计算公式。采用该方法 ,在室内长 2 4m、内径 0 .0 5 4m试验架上以空气 水、空气 柴油和空气 6 8# 机械油为介质进行了实际测试 ,经过和文献中数据以及经验公式对比 ,验证了该测试方法的正确性     

大输量多相混输管路气液两相流实验研究

为了研究大输量条件下多相混输管路的流动特性,以水和空气为实验介质,在长江大学多相流实验平台上进行了水平状态的高气液量两相流模拟实验研究。实验采用内径为60 mm、长9.4 m的透明有机玻璃管,并利用高速摄像仪记录实验过程中的流型。通过对实验流型进行整理,将水平管内的气液两相流流型划分为分层流、泡状流、段塞流和环状流,并与典型的Mandhane流型图进行对比分析。另外,对实验范围内的几种典型流型下的压降梯度变化规律进行了研究,泡状流区域压降梯度随气流速的增大而减小,段塞流区域压降梯度随气流速的增大而缓慢增大,环状流区域压降梯度随气流速的增加而继续增大。     

矩形截面螺旋通道内弹状流的流动特性

对水平放置矩形截面螺旋通道内弹状流的流动特性进行了实验研究.通过实验获得了不同周角下的气弹演变过程和局部流动特征,结果表明,其流动特性会随着螺旋周角位置的变化而变化.根据实验数据分析发现,同一工况下,不同转角气弹的运动速度、频率和长度分布不尽相同.重力和离心力的相对大小决定着内外壁面液膜的厚度,给出了同一条件下,不同时刻的液膜厚度的演变过程.最后对下降液膜的运动速度展开了分析研究,在螺旋上升过程中,液膜下降速度逐渐减小,在螺旋下降段,液膜速度明显增大.     

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。     

Influence of operating conditions on the observed reaction rate in the single channel monolith reactor

The catalytic hydrogenation of nitrobenzoic acid (NBA) to the aminobenzoic acid was used as a model reaction for a quantitative study of influences of the operating conditions on the observed reaction rate in a single channel monolith reactor operated in Taylor flow regime. A simple mathematical model was derived and used for the analysis of hydrogenation experiments carried out in batch mode. Results showed that in the investigated concentration range of NBA, i.e. 0.0005–0.02 mol/l and under the hydrogen pressure of 1 bar, the observed reaction rate is considerably limited by mass transport. At higher concentrations of NBA, the reaction is controlled by the hydrogen mass transport while at lower concentrations the mass transport of NBA is dominant. The analysis of experimental results, which were obtained when the length of gas bubbles and liquid slugs were varied, showed that the reaction took place in the thin liquid film surrounding the gas bubble. The liquid slug serves as exchanger of reactants and reaction products between bulk liquid slug and liquid film surrounding the catalyst surface.     

Flow in the nose region and annular film around a Taylor bubble rising through vertical columns of stagnant and flowing Newtonian liquids

The flow in the nose region and in the annular film around individual Taylor bubbles rising through stagnant and co-current vertical columns of liquid were studied, employing particle image velocimetry (PIV) and pulsed shadowgraphy techniques (PST) at the same time. The combined techniques enabled simultaneous determination of the bubble shape and the velocity profiles in the liquid film. Experiments were performed with water and aqueous glycerol solutions in a wide range of viscosities , in an acrylic column of 32 mm ID.Values for the distance ahead of the nose in which the flow is disturbed by the presence of the bubble are presented for the conditions studied. The bubble shapes in the nose region are compared with Dumitrescu's shape for potential flow. The velocity profiles show that after the nose region the liquid begins to accelerate downwards, and at a certain distance from the bubble nose the velocity profile and the liquid film thickness stabilise. The liquid film acquires characteristics of a free-falling film. Values of the developing length and film thickness are reported for the experimental conditions studied. Average velocity profiles in the fully developed film are also presented. A critical Reynolds number of around 80 (based on the mean absolute velocity in the liquid film and on the film thickness) is reported for the transition from laminar to turbulent regime. Shear stress profiles (in the fully developed film) are also provided.The data reported are relevant for the validation of numerical codes in slug flow.     

水平管道油气二相段塞流稳态流动特性

使用通过自行设计并搭建的实验装置,对水平管道油气二相段塞流稳态流动特性进行了实验研究。采用压力信号互相关方法测量了段塞流平均液塞速度,通过分析得到了液塞速度、液塞长度和液塞频率随气、液相折算速度的变化规律。结果表明,当液相折算速度恒定时,随着气相折算速度的增大,液塞速度基本上呈线性增大,而液塞长度呈双曲线减小;当气相折算速度恒定时,随着液相折算速度的增大,液塞频率基本呈线性增大,而液塞长度呈双曲线减小。