湿气管道积液临界气速预测的新模型

气田开发经常采用湿气集输方案。针对湿气输送管道出现的积液问题,基于分层流最小界面剪切应力准则,利用气液平界面分层流液膜区的速度分布规律,建立了求解积液临界气速的新机理模型。由分层流液膜区的流场描述和气相动量方程得到气液界面剪切应力的表达式;利用界面剪切应力函数曲线特性,可以通过界面剪切应力关于持液率求导获得临界气速。以不同文献中收集的临界气速实验数据,对新模型和其他具有代表性的湿气管道积液模型进行验证对比,表明新模型的预测精度要优于其他模型。     

水平管气液两相分层流界面剪切预测研究进展

水平管气液两相分层流虽流型简单,但由于界面存在复杂的动量和能量传递,分层流的界面剪切预测至今没有一致的结论。本文从理论模型、实验模型、数值计算3个角度出发,详细阐述水平管气液两相分层流界面剪切预测的研究现状,得出不同研究方法的优势和缺陷。针对3种研究方法,指出理论模型通过模型简化和经验关联式来建立封闭模型,实验模型则在封闭关系上修正经验关联式,但由于简化假设和实验条件的限制,使得这两种研究方法对界面剪切应力的预测具有一定的局限性;数值计算能够弥补机理模型在流场细节等方面的不足,但能够提供界面剪切预测或封闭关系的工作很少。此外,对比了5种不同形式的已有模型对气液两相分层流持液率和压降预测的结果。最后展望了水平管气液两相分层流界面剪切预测的研究趋势,提出理论和实验研究需要提出更详细的局部模型,并考虑工程实际工况进行研究,发展针对气液界面计算的新方法,并为分层流提供封闭关系则是数值计算研究面临的挑战。     

竖直管外气液逆流环状降膜速度与温度分布

建立了竖直管外环状降膜气液逆流传热传质条件下稳态层流降膜一维速度分布和二维温度分布模型,以及膜厚和降膜表面热通量的数值计算方法。表面热通量的模型计算值与实验值在气体Reynolds数Re_g<1200的范围内吻合较好,表明基于界面摩擦因子求解模型的方法在两相均为层流条件下是可靠的。模型显示了降膜速度分布和温度分布的非线性特征,降膜表面附近陡降的温度梯度表明,减小膜厚是强化降膜传热传质过程的有效途径。     

Local Measurement of Gas-Liquid Bubbly Flow with a Double-Sensor Probe

A double-sensor probe was used to measure local interfacial parameters of a gas-liquid bubbly flow in a horizontal tube. The parameters included void fraction, interfacial concentration, bubble size distribution, bubble frequency and bubble interface velocity. The authors paid special attention to the probe design and construction for minimizing measurement errors. Measures were also taken in the design of sensor ends for preventing corrosions in the flow. This is an effort to improve the current double-sensor probe technique to meet the ever-increasing needs to local varameter measurements in gas-liquid two-phase flows.     

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.     

湿天然气在上倾管道的气液两相流动特性研究

利用VOF (Volume of Fluid)模型模拟了湿天然气在上倾管道的气液两相流动特性。讨论了管道内气体入口流速对气液两相流动特性的影响。结果表明,当管道内气体入口流速较低时,由于气液界面的切应力小于液相在上倾段的重力分力,因此气体无法将液体携带至管道上倾段。随着气速的增加,气液界面的切应力逐渐增大,气液界面开始出现波纹,气体逐渐将液体携带并完全平铺于管道上倾段。     

INTERFACIAL FRICTION FACTORS IN COUNTERCURRENT STRATIFIED TWO-PHASE FLOW

Interfacial shear stresses have been determined for a countercurrent stratified flow of air-water in a nearly-horizontal rectangular channel based upon measurements of pressure drops, gas velocity profiles and mean film thicknesses. The interfacial friction factors of air-water flow were of the same order of magnitude as those of steam-water flow in the three-dimensional wave regime. A new dimensionless intensity of wave height fluctuation, which may be regarded as an effective roughness of the interface, was proposed. It showed that this dimensionless parameter has a unique relationship with the equivalent roughness for the gas-liquid interface calculated using the Nikuradse equation for both air-water and steam-water data. In addition, an empirical correlation of the interfacial friction factor for air-water and steam-water flows has been developed for practical application.     

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。     

微通道内伴有化学吸收气液两相流的空隙率与压力降

微通道内气液两相流空隙率与压力降对微反应器的热质传递性能有显著影响,是微反应器的重要设计参数。采用高速摄像仪和压力测量系统分别对矩形微通道内单乙醇胺水溶液化学吸收CO₂过程的空隙率和压力降进行了研究,考察了弹状流下气液两相流量与化学反应速率对空隙率及压力降的影响。结果表明:当液相流量一定时,微通道内空隙率和压力降均随着气相流量的增大而增大,空隙率随化学反应速率的增大而减小,压力降随化学反应速率的增大而增大;当气相流量一定时,随着液相流量和化学反应速率的上升,微通道内空隙率下降,而压力降上升。提出了微通道内伴有化学吸收的空隙率和压力降的半理论预测模型,模型平均误差分别为15.79%和11.12%,显示了良好的预测性能。     

Wall shear stress and mass transfer in vertical two-component flow

Mass transfer rates in vertical gas-liquid flow were measured by an electrochemical technique. The flow regimes studied were slug, churn and annular. Average mass transfer coefficients in gas-liquid flow could be correlated by expressions similar to those for single phase flow. Fluctuations in local mass transfer coefficient could be used to indicate flow regimes. Average wall shear stress determined from the average mass transfer coefficients agreed with values calculated from measurements of pressure drop, void fraction and flow rate, provided flow reversals did not occur. The results indicate that the electrochemical technique could be used to measure wall shear stress in accelerating gas-liquid flows, such as critical flow.     

Shear stress distribution in adhesive layers of a double-lap joint with void or bond separation

In this work, stress distribution in adhesive layers of a double-lap joint subjected to tension and suffering from a void or a partial debond at the adhesive–adherend interface is examined. For symmetric voids, the deduced equilibrium equations are decoupled for better application of boundary conditions at the extreme ends of each adhesive layer. The proposed method of solution has resulted in better estimates on peak shear stress developed in the adhesive layers. The results based on analytical solution are compared with those of finite element findings. Very good agreement is observed between the two. The major difference between stresses stemming from debonds and voids occurs at the edge of the large size defects. For small central defects, it is hardly discernible by the stresses to distinguish the type of defect. Moreover, there appears to be an optimum length to thickness ratio for each adhesive layer which produces minimum peak interfacial shear stress. This value seems to be a function of defect size and location. A double-lap joint shows to experience smaller interfacial shear stresses due to a single void or debond in comparison with a single-lap joint with a similar defect. The peak interfacial shear stress in a double-lap joint suffering from symmetric voids or debonds is still lower than that of a single-lap joint with a single defect of the same size and location.     

An interface model for the prediction of Young's modulus of layered silicate-elastomer nanocomposites

Recent experiments on layered silicate-elastomer nancomposites by Burnside and Giannelis have shown that there is a discrepancy between theoretical modulus predictions and experimental modulus measurements. A theory is proposed to explain this discrepancy. We hypothesize that the discrepancy is due to imperfect bonding between the matrix/inclusion interface which effectively reduces the aspect ratio and the volume fraction of the inclusion. We use a simple interface model to quantify the imperfect interfacial bonding. From this model, we introduce the concept of the effective aspect ratio and effective volume fraction of the inclusions. These effective quantities depends on a single material parameter, namely, the constant interfacial shear stress, τ. The interfacial shear stress for the elastomer-silicate nanocomposites is found by fitting the theory to the experimentally measured modulus of Burnside and Giannelis. The interfacial shear stress is in the range of thousands of Pascals. For the elastomer-silicate nanocomposite systems considered here, the interfacial shear stress can be decomposed into two parts; intrinsic shear stress τ_i and frictional shear stress τ_f. The intrinsic interfacial shear stress τ_i depends only on the volume fraction of inclusions and decreases with increasing volume fraction of inclusions. On the other hand, the frictional shear stress τ_f is found to increase linearly with the applied strain. Since the mean stress is also proportional to the applied strain, this gives rise to an effective coefficient of friction, which is found to be 0.0932 for the nanocomposite system considered here.     

Phenomenological model of interfacial stress transfer in carbon nanotube reinforced composites with van der Waals effects

A pullout model is presented to analyze interfacial stress transfer in the double‐walled carbon nanotube (DWCNT) reinforced composites. The effects of the van der Waals (vdW) interaction between two layers of DWCNT and the Poisson's effects of DWCNT and matrix are taken into account in the model. Based on the equilibrium of the interfacial shear stress and the DWCNT axial stress as well the continuous condition of the displacement and stress on the interface of DWCNT and matrix, normalized interfacial shear stress, DWCNT axial stress and matrix axial stress are derived, respectively. Moreover, the effects of DWCNT aspect ratio, DWCNT volume fraction and relative modulus between the DWCNT and matrix are analyzed in details. Finally, the maximum normalized interfacial shear stress with vdW effect is compared with that without vdW effects. POLYM. COMPOS., 2011. © 2011 Society of Plastics Engineers     

Liquid wall friction in two-phase turbulent gas laminar liquid stratified pipe flow

The main result of the present work is an analytic expression for the mean liquid wall shear stress in two-phase turbulent gas/laminar liquid stratified pipe flow. The Navier-Stokes equations are solved assuming a flat fluid-fluid interface subject to a constant interfacial shear — approximating the interfacial drag exerted by the gas. The effect of a pipe inclination is accounted for, thereby retaining the interesting two-phase phenomenon of backflow in upwardly inclined pipes. The corresponding expression for the wall shear stress distribution is obtained by formal differentiation. Its limiting behaviour in the triple points, where the fluid-fluid interface meets the pipe wall, is determined by residue calculus. The expression for the mean wall shear stress is given by integration. It is found to be a linear combination of two terms. The first term accounts for the free surface liquid flow in the absence of the gas. The corresponding approximate hydraulic diameter model is found to be in surprisingly good agreement with this term. The second term represents the shear flow associated with the interfacial drag exerted by the gas (not accounted for by the hydraulic diameter approximation). The shear flow increases the flow rate near the interface on behalf of the flow rate near the pipe wall, thus reducing the wall shear stress below the free surface flow value. Expedient evaluation of the expression for the mean wall shear stress, suitable for use in a 1-D multiphase pipe flow simulator, is facilitated by replacing certain one-parameter integrals with highly accurate rational approximations.     

Investigation of the wall region of gas-liquid flow using an electrodiffusional technique

Measurements of the hydrodynamic characteristics of upward gas-liquid flow in an inclined channel were performed. Experiments were made by an electrodiffusional method using microprobes for wall shear stress and liquid velocity measurements. Special attention was paid to the study of two-phase flow structure in the vicinity of the wall. A strong effect of the channel orientation on the characteristics of the flow was demonstrated. The results show that maximum wall shear stress values correspond to an intermediate channel inclination. High values of near-wall void fraction result in the reduction of liquid velocity fluctuations in horizontal and near-horizontal channel positions. This paper was presented at the International Workshop on Electrodiffusion Diagnostics of Flows held in Dourdan, France, May 1993.     

Numerical simulation of churn flow in a vertical pipe

A numerical simulation of the churn flow regime of air-water and R134a vapour-liquid mixtures by means of the volume of fluid (VOF) method is presented. The focus of the paper is on the inlet region of a vertical pipe. An axisymmetrical domain is used, reproducing the region next to the porous wall liquid injector of a typical test section for the investigation of vertical gas-liquid flows.A simplified model of the levitation process of the ring-type waves typical in churn flow is proposed. The influence of the gas Froude number on the waves amplitude is shown by means of the simplified model and used to explain the numerical results.A comparison of the numerical results with experimental wave frequency data and visualizations available in the literature is performed. The velocity field in the forming wave region and the pressure and shear stress variations along the interface are shown.Simulations have been performed at different liquid and gas superficial velocities and pipe diameters and the influence of these parameters on the gas-liquid interface is discussed.     

底吹钢包两相区两段模型

在考虑雷诺应力的基础上，建立了底吹钢包中气液两相区流体流动的两段模型。这一模型可用于计算近喷嘴处和浮羽流区各截面处两相流的平均速度、速度分布、流股直径和平均含气率等．数值计算表明：理论含气率与实验值符合很好．计算还表明：平均含气率和平均速度随高度的增大而减小，但流股直径随高度的增大而增大．另一方面，平均速度随初始供气量的增大而增大，而抽引比则随供气量增大而减小．通过两段模型可很好地描述喷嘴处气液流的行为．     

Stratified turbulent-turbulent gas-liquid flow in horizontal and inclined pipes

A two-dimensional model for stratified turbulent-turbulent gas-liquid flow in inclined pipes is proposed. The gas phase is treated as bulk flow, but an exact solution is carried out for the liquid phase, applying the eddy viscosity theory to model the turbulent viscosity. The interfacial structure is taken into consideration using appropriate correlations for the interfacial shear stress. The model is capable of predicting the liquid velocity field, holdup and pressure drop given gas and liquid flow rates, physical properties, pipe size, and angle of inclination. The results are substantially better than the prediction of Lockhart and Martinelli (1949) correlation and better than the Taitel and Dukler (1976) model for stratified flow.     

Modeling the pressure drop of wet gas in horizontal pipe

A model for gas-liquid annular and stratified flow through a horizontal pipe is investigated,using the two-phase hydrokinetics theory.Taking into consideration the flow factors including the void fraction,the friction between the two phases and the entrainment in the gas core,the one-dimensional momentum equation for gas has been solved.The differential pressure of the wet gas between the two tapings in the straight pipe has been modeled in the pressure range of 0.1-0.8 MPa.In addition a more objective iteration approach to determine the local void fraction is proposed.Compared with the experimental data,more than 83％ deviation of the test data distributed evenly within the band of ± 10％.Since the model is less dependent on the specific empirical apparatus and data,it forms the foundation for further establishing a flow measurement model of wet gas which will produce fewer biases in results when it is extrapolated.