两相临界流

由于两相临界流在压水反应堆安全分析中的重要性，在过去的４０年内，两相临界流一直是一个活跃的分支学科。本文对压水堆失水事故常用的两相临界流均匀平衡模型、滑移模型、考虑相间热力学非平衡效应的简化模型及两流体模型做了综述。结论是到目前为止，没有一个数学模型能准确预测各种参数范围、各种通道形状的临界流量。特定的模型只能适用于特定的情况。本文对非平衡两相流中汽泡成核、汽泡生长、相界面间质量、动量、能量交换的机理给予了更多的关注。     

汽液两相流机械密封的研究进展

液体润滑端面机械密封是流程工业用机泵轴端密封的主要形式，且大量应用于航空航天、海洋工程装备和高端制造装备中，实际运行过程中其端面间液体膜常常发生相变，由此产生端面变形、干磨和密封失稳等现象，甚至完全失效，严重影响了有关设备或整个装置的安全可靠与稳定运行，因此开展汽液两相流端面机械密封研究具有重要意义. 本文中综述了近50年来汽液两相端面机械密封的研究现状，归纳出了机械密封的典型热源及其热传播途径，阐述了机械密封的相变原理，总结了端面流体膜参数的测量技术与方法；在试验研究方面，提出机械密封的端面液膜汽化主要与操作工况、几何尺寸和表面形貌或表面织构有关，其中端面液体入口和出口工况、平衡比和端面变形程度等受到关注，但缺少关于端面形貌或织构的深度研究，也缺少端面摩擦副合理配对的系统研究；在理论研究方面，提出了机械密封相变稳定性判据，以及间断沸腾模型、连续沸腾模型和薄膜均相沸腾模型等3种理论模型，比较了3种模型的优缺点并指出其适用范围. 指出建立合理的相变理论模型，实现相变密封的稳定运行，突破相变密封监控关键技术，构建智能型液体润滑机械密封是未来两相流机械密封的研究重点.      

风沙两相流跃移层中沙粒相的速度分布

从单个跃移沙粒在气流中的运动方程出发导出了风沙两相流中沙粒相速度分布函数的Boltzmann方程，对风沙流研究中几种不同的分布函数及其相应的统计平均值等基本概念给出了严密的数学定义，指出了不同分布函数之间的区别和联系，在略去铅垂方向空气阻力的情况下，给出了沙粒相速度分布函数沿铅垂方向的边缘分布，作为风沙流中跃移理论的主要基础之一。利用结果对前人在风沙流研究中发现的某些重要规律和现象进行了解释。     

水平井两相渗流

研究了水平井两相渗流问题.在流线不变的假定下,给出了水驱油饱和度的二维分布和见水时间公式。两相渗流的见水时间比活塞式驱替的时间快 f(s_t)倍,f(s_t)是含水百分数在前沿处的导数值。     

激光多普勒测速技术在气液两相流中的应用

本文采用激光颗粒动态分析仪(PDA)测量了钢包底部喷吹气液两相流中气泡的直径和气泡上升速度的分布;采用激光多普勒测速仪(LDA)测量了气液两相区和液体单相循环区液体速度场的分布。测量结果表明:气泡在脱离喷嘴上浮一定距离后,其大小基本保持不变;在气液两相区中,气泡速度和液体速度的分布均服从高斯分布;液体在单相区作循环流动,在侧壁与底部交接处,存在液体流动的“死区”。     

基于两相紊流模型的非平衡输沙研究

基于雷诺平均的水沙两相流方程, 建立了一个非平衡全沙输移二维数学模型. 模型考虑相间相对运动以及多颗粒之间的相互影响, 通过相间作用力进行两相耦合.和传统的单相流模型以及低浓度两相流模型相比, 该模型摆脱了依赖经验公式给定床面边界条件的局限性. 针对明渠净冲刷问题, 在合理给定水相和泥沙相边界条件的前提下计算了泥沙浓度分布的沿程变化, 并利用物理模型实验的结果和理论解验证了数学模型的正确性,同时也分析了明渠净冲刷问题中紊动扩散和重力沉降现象的特征.      

气泡-水流两相流的激光多普勒法测量

为区分激光多普勒稀态两相流动测量中出现的流体相信号和颗粒(气泡、液滴)相信号,本文提出可见度和基座的复合判别法,并设计了专用的信号预处理器,此预处理器能将原始多普勒信号分为两路,分别代表两相,并可用计数型信号处理器测量每路的速度及其它统计量。使用本方案对矩形管道內低速气泡-水两相流的实验和测量表明,对两相的区分和测量是成功的。文中给出了两相速度、湍流度和气泡滑移速度的测量结果。     

分层两相流曲管的耦合振动建模研究

针对两相流曲管振动的复杂特点,对分层两相流曲管的耦合振动建模进行了专题研究。在前人的基础上,增加了对两相流中气、液两相不同的质量分量和速度的考虑,同时考虑了气相在振动过程中的势能变化,求解出曲管和流体微元的总能量;再通过总能量变分法原理,运用欧拉方程推导出了分层两相流曲管的振动控制微分方程;对微分方程进行量纲归一化处理后,最终得到了简化的分层两相流曲管振动微分方程。采用有限元法对分层两相流曲管振动方程进行求解,并将求解的结果应用到实际算例中,得出了分层两相流曲管的临界流速与曲管直径、管壁厚度、管道内径的关系曲线,还得出了分层两相流曲管的固有频率与流体流速、管壁厚度、管道内径的关系曲线。本文方法可以用于分析曲管中分层两相流、环状流的耦合振动问题。     

风沙两相流PIV测量算法研究

在风沙两相流图像特征的基础上，提出了一种基于模式识别动态聚类方法中$K$-均值 算法的数字面具(Digital Mask)自动生成算法来求解风沙两相流动. 简化了传统生成Digital Mask过程中手动设置参数的操作，减少了人为误差，为批量处理风沙两相流PIV图像提供 了一种安全快捷的方法. 并将该算法应用于风沙两相流的PIV实际测量，分别得到了不同流 动状态下的气流、沙粒以及风沙气固两相流的速度场.     

方管内气固两相流动速度的LDV测量

介绍了气固两相流动的ＬＤＶ测试方法和有关技术，并给出了方管内网栅后气固两相流动的部分测试结果与分析。对于气相，给出了纯净气流（α＝０）、平均粒径ｄｐ＝３５０μｍ的颗粒（α＝０．１２％、０．２１％、０．３３５％）存在时的气流在管内不同断面上时均速度分布情况及不同颗粒浓度时气流速度结构的比较；对于固相，给出了三种不同固相浓度时颗粒在管内不同断面上时均速度的分布情况，并对三种固相浓度时颗粒与气相的速度分布进行了比较。     

Non-conservative pressure-based compressible formulation for multiphase flows with heat and mass transfer

A pressure-based compressible multiphase flow solver has been developed based on non-conservative discretization of the mixture continuity equation. The formulation is an extension of the single phase incompressible pressure-correction approach, such that it can be applied to both two-phase flows using interface resolving methods and general n-phase ensemble-averaged mixture flows. The formulation is currently presented with the single pressure and single temperature assumption, but extension to multiple temperatures is straightforward. A robust treatment of phase change allows the method to model conditions with rapid phase change such as expansion through nozzles and valves. The method has been validated thoroughly using canonical single phase problems such as the shock tube, tank filling and sudden valve closure problems. Multiphase flow validation has been carried out for sound propagation in mixtures using the ensemble-averaged model and pressure wave transmission and reflection across an air-water interface, using the level set interface tracking method. The method has been used to study sound propagation in saturated steam-water systems under thermodynamic non-equilibrium, where the expected drastic reduction in the speed of sound is reproduced. Finally the method is applied to the problem of critical (choked) flow in a nozzle for a saturated steam-water system.     

Fundamental equations of turbulent two-phase flow

A new set of Reynolds equations for predicting turbulent two-phase flows has been developed by means of Reynolds averaging method on the unsteady laminar equations of two-phase flow. These equations involve average terms of products of turbulent fluctuations in some physical parameters in a large degree. The interaction forces between two phases, the pressures for dispersed phase, extra stresses except for pressure and the expressions for energy interchange between two phases have been discussed.     

An analytic solution of dense two-phase flow in a vertical pipeline

According to a mathematical model for dense two-phase flows presented in theprevious paper,a dense two-phase flow in a vertical pipeline is analytically solved,and theanalytic expressions of velocity of each continuous phase and dispersed phase arerespectively derived The results show that when the drag force between two phases dependslinearly on their relative velocity,the relative velocity profile in the pipeline coincides withDarcy’s law except for the thin layer region near the pipeline wall,and that the theoreticalassumptions in the dense two-phase flow theory mentioned are reasonable.     

下倾管-立管水气严重段塞流数值模拟

针对海洋油气传输中常见的下倾管-立管系统, 采用Brackbill模型模拟气液相界面间表面张力, VOF方法追踪气液两相运动界面, 提出了管内气液两相流数值模拟方法. 在低气液相进口折算速度下, 数值模拟了该种管型下的严重段塞流动现象, 分析了相关物理参数的变化特性. 结果表明, 在严重段塞流下, 管内流型流态、压力、液塞运动速度、立管出口气液相平均速度、下倾管及立管内含气率等均具有明显周期性特征, 而且一个周期内严重段塞流可分为4个阶段, 进而给出了各阶段中相关参数的变化特性. 数值模拟结果与相关文献中的实验结果吻合良好,表明了该数值模拟方法的有效性.      

Gas-Dynamic Processes in Two-Phase Flows in MHD Generators

A plane problem of a two-phase monodisperse flow of combustion products of plasma-forming composite solid propellants in the duct of a Faraday's MHD generator with continuous electrodes, including an accelerating nozzle, MHD channel, and diffuser, is considered. An algorithm based on the pseudo-transient method is developed to solve the system of equations describing the two-phase flow. Gas-dynamic processes in the channels of the Pamir-1 setup are numerically studied. It is shown that shock-free deceleration of a supersonic flow to velocities close to the equilibrium velocity of sound in a two-phase mixture and significantly lower than the velocity of sound in the gas is possible in two-phase flows.     

Pressure calculations in disperse and continuous multiphase flows

In many models for disperse two-phase flows, the pressure of the disperse phase is often assumed to be the same as that of the continuous phase, or differ only by an amount caused by the surface tension. This type of model is referred to as an equilibrium pressure model. Recent research indicates that the stress difference between the phases caused by dynamics of the motion can be significantly important in the modeling of disperse two-phase flows. Although this difference is still ignored in most calculations of disperse multiphase flows for various reasons, when an equilibrium pressure model is applied to continuous multiphase flows, a conceptual difficulty arises. For instance, the equilibrium pressure model cannot be used to study the tensile break of a sponge with interconnected pores, because the air in the pores can never go into tension while the sponge material does not break without tension.     

A Three-Dimensional Model of Two-Phase Flows in a Porous Medium Accounting for Motion of the Liquid–Liquid Interface

A new three-dimensional hydrodynamic model for unsteady two-phase flows in a porous medium, accounting for the motion of the interface between the flowing liquids, is developed. In a minimum number of interpretable geometrical assumptions, a complete system of macroscale flow equations is derived by averaging the microscale equations for viscous flow. The macroscale flow velocities of the phases may be non-parallel, while the interface between them is, on average, inclined to the directions of the phase velocities, as well as to the direction of the saturation gradient. The last gradient plays a specific role in the determination of the flow geometry. The resulting system of flow equations is a far generalization of the classical Buckley–Leverett model, explicitly describing the motion of the interface and velocity of the liquid close to it. Apart from propagation of the two liquid volumes, their expansion or contraction is also described, while rotation has been proven negligible. A detailed comparison with the previous studies for the two-phase flows accounting for propagation of the interface on micro- and macroscale has been carried out. A numerical algorithm has been developed allowing for solution of the system of flow equations in multiple dimensions. Sample computations demonstrate that the new model results in sharpening the displacement front and a more piston-like character of displacement. It is also demonstrated that the velocities of the flowing phases may indeed be non-collinear, especially at the zone of intersection of the displacement front and a zone of sharp permeability variation.     

Analysis of two-phase flow of compressible immiscible fluids through nondeformable porous media using moving finite elements

A system of evolutionary partial differential equations (PDEs) describing the two-phase flow of immiscible fluids in one dimension is developed. In this formulation, the wetting and nonwetting phases are treated to be incompressible and compressible, respectively. This treatment is indeed necessary when a compressible nonwetting phase is subjected to compression during confinement. The system of PDEs consists of an evolution equation for the wetting-phase saturation and an evolution equation for the pressure in the nonwetting phase. This system is applied to the problem of unsaturated flows to assess the importance of air-phase compressibility. For those situations where air can move freely within the medium and ultimately escape through the boundaries without experiencing any compression, it is then reasonable to treat air as an incompressible phase so that the total volumetric flux becomes spatially invariant. As shown by Morel-Seytoux and Billica, this leads to a coupled evolution equation for water saturation and an integral expression for total volumetric flux. In the event that an air phase is subjected to confinement in some manner, the total volumetric flux cannot be assumed to be spatially invariant as did Morel-Seytouxet al.The system of evolutionary PDEs developed in the present paper are precise and uniformly valid in time and space and, more importantly, smoothly accommodate a nonwetting phase whose state may change from unconfined to confined during the course of the flow process and vice-versa. Consequently, the complete system of PDEs may be used to analyze unsaturated flows in a straightforward manner.Depending on the initial and boundary conditions, the solutions to the system of PDEs may develop steep gradients near the wetting front. For this reason, the moving finite element (MFE) method introduced by Miller and Miller in conjunction with Gear's implicit stiff temporal solver provides an automatic and powerful scheme suitable for the initial-boundary value-problem (IBVP) developed herein.     

A first order relaxation model for the prediction of the local interfacial area density in two-phase flows

Many energy production and chemical processes involve vapor/liquid two-phase flows. Mass and energy are often exchanged between the vapor and the liquid phases, and the fluid mechanics of the two-phase system is strongly influenced by the exchange of momentum between each phase. Significantly, the transport of mass, energy and momentum between the phases takes place across interfaces. Therefore the interfacial area density (i.e. the interfacial area per unit volume) has to be accurately known in order to make reliable predictions of the interfacial transfers. Indeed, the interfacial area density must be known for both steady and transient two-phase flows. It is the purpose of this paper to present a first order relaxation model which is derived from the Boltzmann transport equation, and which accurately describes the evolution of interfacial area density for bubbly flows. In particular, the local, instantaneous interfacial area densities and volume fractions are predicted for vertical flow of a vapor/liquid bubbly flow involving both bubble clusters and individual bubbles.     

Speed of sound in two-phase vapor-liquid systems

The principal theorems of thermodynamics of irreversible processes are applied to the process of propagation of acoustic waves in a two-phase medium. Expressions are derived which determine the dependence of the sound speed in a vapor-liquid medium on the degree of dryness and the degree of nonequilibrium of different relaxation processes accompanying the propagation of acoustic waves. In the limiting case of equilibrium these expressions reduce to the well-known formulas obtained in equilibrium thermodynamics.Translated from Zhurnal Prikladnoi Mekhaniki i Tekhnicheskoi Fiziki, No. 5, pp. 78–85, September–October, 1970.