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.     

下倾管中活塞流液塞长度波动特性

通过采集下倾管中活塞流的差压信号,应用互相关法分析得到液塞长度时间序列.利用统计理论与分形理论分析液塞长度时间序列来揭示液塞长度波动特性.结果表明：气相折算速度对液塞长度的影响远大于液相折算速度.液相折算速度较大时,随着混合速度的增加,液塞长度波动的混沌程度增加;而液相折算速度较小时,随着混合速度的增加,液塞长度波动的混沌程度减小.在管线下倾角较大的情况下,液塞长度分布集中在较小的范围内.管线下倾角越大,液塞长度波动的长程相关性越好.     

Comparative study of gas–oil and gas–water two-phase flow in a vertical pipe

A wire-mesh sensor has been employed to study air/water and air/silicone oil two-phase flow in a vertical pipe of 67 mm diameter and 6 m length. The sensor was operated with a conductivity-measuring electronics for air/water flow and a permittivity-measuring one for air/silicone oil flow. The experimental setup enabled a direct comparison of both two-phase flow types for the given pipe geometry and volumetric flow rates of the flow constituents. The data have been interrogated at a number of levels. The time series of cross-sectionally averaged void fraction was used to determine characteristics in amplitude and frequency space. In a more three-dimensional examination, radial gas volume fraction profiles and bubble size distributions were processed from the wire-mesh sensor data and compared for both flow types. Information from time series and bubble size distribution data was used to identify flow patterns for each of the flow rates studied.     

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.     

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。     

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…     

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。     

下倾管中段塞流液塞长度波动的非线性分析

利用非线性分析技术中的分形理论在较宽的流速范围内对下倾管中空气-水段塞流的液塞长度波动特性进行了研究.结果表明,下倾管中段塞流的液塞长度符合对数正态分布,其波动是对初始条件敏感的混沌振荡,且具有持久性.通过对不同液相折算速度下的液塞长度波动特性的分析发现,液相折算速度较大时,随着混合速度的增加,液塞长度波动的混沌程度增加;而液相折算速度较小时,随着混合速度的增加,液塞长度波动的混沌程度减小.管线下倾角越大,液塞长度波动的长程相关性越好.     

Energy Dissipation at the Slug Nose and the Modeling of Solids Transport in Intermittent Flow

It is shown, by means of Lagrangian momentum and power balances across the slug nose in slug flow, that the power dissipated due to sudden expansion of the fluid in the film immediately downstream of the slug nose is greater than the power lost due to viscous action at the pipe wall but that it may not be large enough to materially aid particle transport. This has the implication that sand transport in intermittent flow may be considered as transport in a hybrid flow of hydraulic conveying and stratified flow when the length of the mixing section at the slug nose is short compared to the length of the slug body.     

气液两相垂直管流中弹状流型数学模型

本文对气液两相垂直管流的弹状流型的流动机理进行了分析,建立了数学模型。模型的特点是采用了以弹状气泡速度运动的相对座标系,在弹状气泡周围下落液膜流动、液体段空隙率计算和压降计算等方面部提出了新的观点。在两种管径中用空气-水系统进行了实验以验证模型,与一些其他作者的实验数据和理论模型也进行了比较,结果都表明这个机理模型计算的弹状流型的各种流体力学参数值与实验值吻合良好,也优于其他作者提出的模型。     

垂直上升管中的液弹长度分布

根据气液相不同的电导性质,通过实验测得了气液二相弹状流中气液弹的长度及速度。利用统计方法研究了在垂直上升管内不同位置以及不同气液表观速度下的液弹长度分布。结果表明该分布能够与对数正态分布关系式符合得很好,气液表观速度对液弹长度影响并不显著。同时还研究了气泡上升速度与前方液弹长度的关系,该关系可以用R.van Hout得到的经验关系式来描绘。     

Hydrodynamics and local mass transfer characterization under gas–liquid–liquid slug flow in a rectangular microchannel

Gas–liquid–liquid three-phase slug flow was generated in a glass microreactor with rectangular microchannel, where aqueous slugs were distinguished by relative positions to air bubbles and organic droplets. Oxygen from bubbles reacted with resazurin in slugs, leading to prominent color changes, which was used to quantify mass transfer performance. The development of slug length indicated a film flow through the corner between bubbles and the channel wall, where the aqueous phase was saturated with oxygen transferred from bubble body. This film flow results in the highest equivalent oxygen concentration within the slug led by a bubble and followed by a droplet. The three-phase slug flow subregime with alternate bubble and droplet was found to benefit the overall mass transfer performance most. These results provide insights into a precise manipulation of gas–liquid–liquid slug flow in microreactors and the relevant mass transfer behavior thereof.     

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.     

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。     

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.     

水平管段塞流气弹区液膜特性研究

为了研究气液段塞流相界面的结构特征,采用了双平行电导探针技术对水平管内段塞流气弹区的液膜特性进行了实验测量,并基于一维双流体模型导出液膜厚度的控制方程,该方程对液膜厚度的预测结果与实验结果吻合良好。结合液膜厚度计算模型提出了计算段塞流的机理模型,该段塞流机理模型的计算结果表明对于液相表观速度较高而气相表观速度较低的段塞流,机理模型中忽略液膜非平衡性时得到的平均液膜厚度和气弹区长度明显偏低。     

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.     

Pressure drop in monolith reactors

A theoretical model for the computation of pressure drop in bubble-train flow inside capillaries of square cross-section was developed. The model is based on three contributions: hydrostatics, viscous pressure drop, and capillary pressure drop. Capillary pressure drop is related to the shape of the fronts and ends of the bubbles. The model does not include entrance or exit effects, has no adjustable parameters, and agrees very well with available experimental data.

For a given set of flow parameters, bubble velocity and liquid slug average velocity are computed as a function of gas and liquid superficial velocities. The length of the unit cell determines the number of bubbles inside the capillary for a given flow situation. The model requires experimental information of average bubble lengths to compute the length of a unit cell consisting of a bubble and a liquid slug.

The three pressure contributions for a unit capillary length are linear functions of the number of bubbles inside the capillary. The length of the bubbles in bubble-train flows is a critical parameter in the computation of pressure drop.     

注气孔位置对文丘里管式微气泡发生器成泡特性的影响分析

尽管文丘里管式微气泡发生器的注气口位置会对气泡在文丘里流道内的碎化特征产生直接影响,但迄今缺乏针对性的深入研究。通过可视化实验方法,对比分析了注气口分别位于喉管处（结构1型）和进水管处（结构2型）时的气液流型、气泡破碎特征以及成泡特性。实验表明,气、液相流量对结构1型微气泡发生器内的气液流型影响显著,初始成泡区域随液相流量增加,环状流或泡状流向弹状流转变,而随气相流量增加则由泡状流或弹状流向环状流转变;结构2型微气泡发生器则在此过程中始终为泡状流,其对操作工况的适应范围大于结构1型。在相同工况下,结构1型微气泡发生器的成泡Sauter平均粒径小于结构2型,但随着液相Reynolds数的增大,二者间的成泡平均粒径差值随之减小。分析原因是由于弹状流流型下,延伸至扩张段区域的弹型泡的表面积更大,能量转化率更高,气泡界面失稳碎化的程度更显著。随着液相Reynolds数的增大,初始成泡体积减小,湍流破碎机理作用占据主导,掩盖了由于界面失稳引起的气泡破碎。结构1型微气泡发生器的成泡能耗高于结构2型,并且随液相Reynolds数的增大,两者之间的差值随之增大。综合来看,结构2型微气泡发生器能够在低能耗下实现高效成泡,面向工程应用将更具优势。     

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。