弹状流液弹区含气率分布的试验研究

采用光纤探针法和高速摄影法对垂直上升气液两相弹状流的液弹区含气率分布进行了试验研究,测试管径为15 mm。一种基于机器学习的图像处理技术用来识别气液两相界面,通过搭建气泡边界提取的神经网路系统,使用构建的气泡边界数据库对模型进行多次迭代训练,该方法可以有效地识别多种复杂类型的气泡边界。试验得到了弹状流液弹区的径向含气率分布曲线,结果表明,壁峰分布是液弹区含气率分布的主要形式,其中泰勒气泡的尾迹效应对分布形式有重要影响,尾迹的旋涡中心和含气率分布的峰值相对应。针对弹状流液弹区径向含气率分布的两个主要特征——中心局部含气率和壁峰位置,分别提出了相应的预测公式,且与本文的试验数据吻合良好。     

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.     

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.     

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 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.     

矩形截面螺旋通道内气液两相流局部含气率分布实验研究

应用电导探针测量技术,对矩形截面螺旋通道内气液两相流局部含气率进行实验研究。在不同的气相折算速度下,应用电导探针测量了弹状流弹单元的长度,并与可视化方法进行对比,验证了电导探针的可靠性,并为信号处理选择合适的阈值。分别在泡状流、弹状流及环状流三种流型的条件下,分析了气相与液相折算速度对局部含气率分布的影响。实验结果发现,螺旋通道气液两相局部含气率呈非对称的抛物线形分布,这种非对称性受流型和液相折算速度的影响。     

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。     

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.     

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。     

基于段塞流的通用气液两相流模型的建立与验证

气液两相混输流动中,由于段塞流与其他流型均有联系,从段塞流出发建立模型可以使不同流型下的计算模型得到统一。基于段塞流建立气液两相流动量方程和连续性方程,完善模型对气泡和液滴夹带的处理。建立模型的闭合关系式,对关键参数(壁面及气液相间水力摩阻系数、液塞平移速度、平均液塞长度)的计算闭合关系式进行优选,得到适用于模型的闭合关系式,同时,对液滴和气泡的夹带给出夹带条件及相关参数计算式,最终建立基于段塞流的通用气液两相流模型。使用3组不同来源的实验数据验证了模型计算压降和持液率的准确性,实验数据分别来自中国石油大学(华东)、大庆油田实验基地的气液两相流实验以及国外研究人员的实验研究,包含各个气液流型。模型具有较高的计算精度,优于未经关系式完善和优化的原始模型,大部分压降及持液率参数的计算误差在±15%以内。     

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.     

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。     

垂直管内通气空泡掺气过程仿真分析简

A semi-empirical gas entrainment model was proposed for the ventilated cavity in vertical pipe, based on which, a complete numerical scheme was established by coupling with the Eulerian-Eulerian two-fluid model to predict the multiscale flow field created by ventilated cavity. Model predictions were validated against experimental measurements on void fraction and bubble size distributions. Simulations were carried out to explore the effect of ventilation rate and inlet turbulence intensity on the macroscale cavity shape and the bubbly flow downstream of the ventilated cavity. As the ventilation rate increasing, a reverse trend was observed for the void fraction and bub- ble size distributions. It is concluded that the average void fraction in the pipe flow region is determined by the volumetric ratio between liquid and gas. However, the bubble size evolution is dominated by the breakage effect induced by turbulence in the vortex region. Furthermore, simulations were conducted to analyze geometric scale effect based upon Froude similitude. The results imply that the velocity distributions were properly scaled. Slight scale effect was seen for the void fraction caused by faster dispersion of bubbles in the larger size model. The com- paratively greater bubble size was predicted in the smaller model, implying significant scale effects in terms of tur- bulence and surface tension effect. It reveals that empirical correlations valid in wide range are required for the ex- trapolation from small-size laboratory models.     

Experimental study and CFD modelling of a two-phase slug flow for an airlift tubular membrane

The aim of the present study was to develop a computational fluid dynamics (CFD) model to study the effect of slug flow on the surface shear stress in a vertical tubular membrane. The model was validated using: (1) surface shear stresses, measured using an electrochemical shear probe and (2) gas slug (Taylor bubble) rising velocities, measured using a high speed camera. The length of the gas slugs and, therefore, the duration of a shear event, was observed to vary substantially due to the coalescing of gas slugs as they travelled up the tube. However, the magnitude of the peak surface shear stress during a shear event was not observed to vary significantly. The experimental conditions significantly affected the extent to which the gas slugs coalesced. More coalescing between gas slugs was typically observed for the experiments performed with higher gas flow rates and lower liquid flow rates. Therefore, the results imply that the frequency of shear events decreases at higher gas flow rates and lower liquid flow rates.Shear stress histograms (SSH) were used as a simple approach to compare the different experimental conditions investigated. All conditions resulted in bi-modal distributions: a positive surface shear peak, caused by the liquid slug, and a negative shear peak caused by the gas slugs. At high gas flow rates and at low liquid flow rates, the frequency of the shear stresses in both the negative and positive peaks were more evenly distributed. For all cases, increasing the liquid flow rate and decreasing the gas flow rate tends to result in a predominant positive peak. These results are of importance since conditions that promote evenly distributed positive and negative peaks, are likely to promote better fouling control in membrane system. At high liquid and low gas flow rates, the frequencies obtained numerically and experimentally were found to be similar, deviating by less than approximately 10%. However, at high gas and low liquid flow rates, the differences were slightly higher, exceeding 20%. Under these conditions, the CFD model simulations over predicted the shear stresses induced by gas slugs. Nonetheless, the results indicate that the CFD model was able to accurately simulate shear stresses induced by gas slugs for conditions of high liquid and low gas flow rates.     

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.     

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。     

CFD modeling and validation for two-phase medium viscosity oil-air flow in horizontal pipes

This study presents the results of a Computational Fluid Dynamics (CFD) simulation of two-phase medium viscosity oil-air flow in a 50.8?mm internal diameter horizontal pipe. Void fraction and pressure gradient predictions were validated using experimental data for four different oil viscosities (0.039, 0.06, 0.108 and 0.166?Pa s) and different flow rates varying from 0.1 to 2.9?m/s for the gas phase and from 0.01 to 2.95?m/s for the liquid phase, where four flow patterns were predicted (stratified, dispersed bubble, bubble elongated and slug flow). The obtained results of void fraction and pressure gradient presented a mean relative error of 30.04 and 21.38%, respectively. Furthermore, the CFD results were compared against 66 empirical correlations and predictions from OLGA. It was found that between the three studied methods (CFD, OLGA and empirical correlations) the CFD model outperformed the other two methods regarding the predicted flow patterns, pressure gradients and void fractions on most cases.     

Method for calculating the void fraction and relative length of bubbles under slug flow conditions in capillaries

A method for calculating the void fraction and relative size of bubbles at the known flow rates of phases is constructed using the mathematical model of a gas—liquid slug flow in capillaries that was developed earlier. The results of the calculations are in good agreement with the experimental data of other authors. The boundedness of linear approximations such as the Armand formula by the small values of the capillary numbers is revealed. It is shown that the void fraction depends not only on the dynamic gas holdup, but also on the capillary number and the Weber number, as well as on the direction of the flow. It is found that the ratio of the dynamic gas holdup to the void fraction varies from 1 to 2.5 as the capillary number increases. A tenfold error in the experimental determination of the length of liquid slugs by a simplified procedure is revealed. A simple calculation relationship that relates the dynamic gas holdup to the void fraction is derived from the Liu—Vandu—Krishna approximation. The theoretical explanation of the causes of the abnormal dependence of the void fraction on the dynamic gas holdup in microchannels with sizes of less than 100 fum is given. The specific features of a slug flow in microchannels that are caused by the disintegration of a film into drops are explained. The developed calculation method can also be applied to liquid—liquid systems.     

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。     

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。