环状流是一种重要的气液两相流型，在石油天然气开采输运、核反应堆热工水力等工业场景中扮演着十分重要的角色。环状流的形态特征包括附着在管壁的液膜、占据管道中心的气核以及气核中夹带的液滴，这三种结构之间又存在着复杂的质量传递、动量传递甚至能量传递行为，因此环状流的流动特征和力学特性非常复杂，需要建立可靠的数理模型预测环状流的运动规律与关键参数。根据环状流的模化方式，可以将预测模型分为两场模型和三场模型两类：两场模型是指对液膜和气核写出守恒方程并求解的方法，而三场模型是同时写出液膜、气核与液滴三套控制方程并耦合求解的一种数值方法。目前环状流的两场模型和三场模型都存在一定局限性，模型的预测性能和模拟效果有待提升，这限制了预测模型的进一步推广和应用。

本文通过理论推导、室内实验和数值模拟相结合的方法开展研究工作，对环状流的预测模型进行了系统的构建、优化和验证工作，阐明了模型的建立流程，从物理机制出发改善了模型性能，有效提高了模型的准确性，进一步拓展了模型的普适性。

在两场模型方面，构建了一套基础的两场分相模型，将预测模型与液膜反转的临界气速实验数据对比，发现当前的两场模型在垂直管和倾斜管中都存在一定缺陷：(1). 针对垂直管中出现的问题，建立了一套考虑液膜速度分布和液滴夹带的优化模型，并且基于稳定性分析得到了新的临界气速的预测公式。为验证模型的预测性能，开展了垂直管的液膜反转室内实验，通过捕捉涟漪波的运动轨迹获得了液膜反转的临界气速。通过大量实验数据对比发现，新模型预测结果的相对误差绝对值为9.31%，与前人模型相比降低了30.65%，因此具有更高的准确性。此外，模型讨论结果显示考虑液膜速度分布对模型优化的贡献更大，而液滴夹带对模型性能的提升幅度较小，据此建立了忽略液滴夹带的临界气体流速无量纲图版，以方便现场推广使用。(2). 针对倾斜管中出现的问题开展了液膜厚度预测研究，基于扰动波的扩散机制建立了非均匀液膜的力学模型。该模型认为由扰动波引起的“泵送效应”会导致环向上出现压力梯度，同时提供了液膜沿管壁爬升的动力，所以模型中将“泵送效应”以环向应力梯度的形式体现。结合数学推导与实验数据得到了环向应力梯度的表达式，并将其以封闭关系的形式引入力学分析，建立了新的非均匀液膜厚度预测模型。新模型以无量纲解析解的形式表达，可简单直接且准确地计算任意环向位置的液膜厚度。通过与大量实验数据对比发现，新模型的相对误差绝对值为15.83%，相比前人模型降低了57.16%，证实了提出的新模型能更准确地计算环向液膜厚度和气液界面位置。

在三场模型方面，使用VOF(Volume of Fluid)-TFM(Two Fluid Model)耦合框架构建了一套基础的环状流三场数值模型，模型中选用了已被广泛使用的封闭关系描述相间质量传递和动量传递，实现了在一个求解域内同时求解液膜、气核、液滴以及气液界面波。通过室内实验对模拟结果进行了定性和定量评估，结果显示三场模拟所呈现的界面波生成演化、运动发展过程与实验观测一致，液膜含率在时间域和频率域内的AVE(平均值，Average Value)、STD(标准偏差，Standard Deviation)、PDF(概率密度函数，Possibility Density Function)、PSD(功率谱密度，Power Spectral Density)等统计学参量与真实数据的吻合度较高，这些结果表明了本文三场模拟工作的有效性。然而，评估结果也显示液滴运动的定性和定量评估都与真实流动严重不符，这说明液滴夹带模型的性能亟需改进。针对该缺陷，本文基于液滴夹带的剪切机理，通过将液滴夹带速率与无量纲韦伯数关联，建立了一套由局部流动参数表示的液滴夹带新模型；在OpenFOAM平台内将新模型以源项(汇项)的形式植入液滴(液膜)的质量守恒方程，并通过实验数据对模型进行了评估。评估结果显示新模型能更真实地反映液滴的运动过程：模拟中的液滴夹带现象与界面波的剪切滚动过程紧密关联，夹带位置主要出现在界面剪切作用最显著的扰动波尖端区域，生成的液滴被卷吸至管道中心；此外，使用新模型后得到的液滴夹带组分以及液膜含率等参量的AVE、STD、PDF、PSD等统计学数据也更符合实验测试值，证实本文建立的液滴夹带新模型在定性和定量模拟两个方面都具有更好的性能。

本文开展的两场模型和三场模型优化工作推进了环状流预测模型的进一步发展，提高了预测模型的精度，有助于理论工作的现场推广应用，改善环状流管道的相关工艺流程。

Annular flow is an important gas and liquid flow regime, which plays a significant role in many industrial fields, including oil and gas exploitation wells, transportation process and nuclear reactor thermo-hydraulics. Annular flow is characterized by the continuous liquid film on the pipe wall, the gas core in the pipe center and the entrained droplets in the gas core. Notably, there are mass transfer, momentum transfer and energy transfer among the three phases, so that annular flow is very difficult to be modelled. Mechanistic models should be developed to estimate the flow characters and parameters in annular flow. Basically, prediction models of annular flow can be divided into two categories: the two fields model and the three fields model. The two fields model is based on the conservative equations of liquid film and gas core, while the three fields model is developed by coupling the conservative equations of liquid film, gas core and droplets. However, the literature review revealed that both two fields model and three fields model have limitations that impede the model application.

To address current problems, we systematically investigated the model development, optimization and validation by the methods of theoretic derivation, laboratory experiment, and numerical simulation. In this thesis, the process of model development has been clarified, the model performance has been advanced on the basis of physical mechanisms, the model accuracy has been improved, and the applicability of the model has been broadened.

For the two-fields model, this work developed a basic two fields separated model for the first step, then the critical gas velocities of film reversal were collected to evaluate the basic model. It was found that the basic model exhibited obvious problems in vertical pipes and inclined pipes. (1). To optimize the model performance in vertical pipes, we developed a two fields model that considers the liquid film velocity distribution and droplet entrainment. Based on this optimized two fields model, a new correlation for predicting the critical gas velocity of film reversal was proposed. Then, laboratory experiments were conducted to measure the critical gas velocity of film reversal by observing the motion trajectories of ripples. By comparing with a large amount of experimental data, it was found that the relative error of the optimized model is 9.31%, which reduces 30.65% compared to previous models, verifying the optimized model has better performance. Besides, the discussion showed that taking the liquid film velocity distribution into account mainly contributes to the model optimization, while the droplet entrainment accounts for less contribution. Hence, we developed a dimensionless diagram that ignores the droplet entrainment to estimate the critical gas velocity conveniently, which is helpful for the field application. (2). An investigation on liquid film thickness was conducted to address the problems that arise in inclined pipes. A mechanical model of non-uniform liquid film was established based on the spreading mechanism of disturbance waves. The model suggests that the "pumping effect" caused by disturbance waves results in a circumferential pressure gradient, providing the driving force for the liquid film to climb along the pipe wall. Therefore, the "pumping effect" can be expressed in the form of radial stress. By combining mathematical derivation and experimental data, the expression for circumferential stress was obtained and incorporated into the mechanical model as a closure relationship, and a new prediction model for non-uniform liquid film thickness was developed. The model is expressed in the form of a dimensionless analytical solution, which can easily, directly, and accurately calculate the liquid film thickness at any circumferential position. Compared with a large amount of experimental data, it was found that the new model has higher accuracy compared to previous models, with a relative error reduction of 57.16% and a final error value of 15.83%, confirming that the new model can more accurately calculate the circumferential liquid film thickness and the position of the gas-liquid interface.

For the three-field model, this paper used the VOF (Volume of Fluid) - TFM (Two Fluid Model) coupling framework to build a basic three fields numerical model for annular flow. The model utilized the widely used closure relationship to describe the mass and momentum transfer between phases, simultaneously simulation liquid film, gas core, droplets, and interfacial waves in a single computational domain. The simulation results were qualitatively and quantitatively evaluated by laboratory experiments, and the results showed that the generation, evolution, and motion development processes of interfacial waves were consistent with experimental observations and physical understanding. The statistical parameters such as AVE (Average Value), STD (Standard Deviation), PDF (Probability Density Function), and PSD (Power Spectral Density) of liquid film holdup in the time and frequency domains were highly consistent with the experimental results. These findings demonstrate the effectiveness of the three fields simulation work in this paper. However, the evaluation results also showed that the qualitative and quantitative evaluation of droplet movement is inconsistent with the experiments, indicating that the performance of the droplet entrainment model needs to be improved. Then, based on the shear mechanism of droplet entrainment, a new droplet entrainment model expressed by local flow parameters was established by correlating the droplet entrainment rate with the dimensionless parameter of Weber number; After that, the new model was implemented into the mass conservation equation of the liquid droplet (liquid film) in the form of a source term (sink term) in the OpenFOAM platform, and the model was evaluated by experimental datasets. The evaluation results show that the new model can more realistically reflect the movement of liquid droplets: the droplet entrainment behavior in the simulation is closely related to the shear rolling process of interface waves, and the entrainment mainly occurs in the tip region of the disturbance wave where the interfacial shear effect is most significant, and the generated liquid droplets are entrained to the pipe center; In addition, the statistical data such as AVE, STD, PDF, and PSD for parameters such as droplet entrainment fraction and liquid film holdup obtained using the new model are also more consistent with experimental tests, confirming that the new model for droplet entrainment established in this work has better performance in both qualitative and quantitative aspects. The optimization work of the two fields model and three fields model in this paper has promoted the development of the annular flow prediction model, contributing to the application of theoretical work.