脉动流条件下气液相界面构形及流型演化机理研究

Two-phase flow instability is a complex flow and heat transfer phenomenon, which plays a significant role on the nuclear reactor safety. The coupling between oscillatory velocity field and heat transfer brings a dynamic self-feedback balance. The interface morphology, which reflects the momentum transport and heat transfer mechanisms in two-phase flow, is the foundation of two-phase flow and heat transfer mechanism model development. Focusing on the flow pattern in an oscillatory velocity field, the two-phase flow visualization research is conducted in air-water pulsating flow to separate the complex feedback effects during interface heat transfer and phase change. Advanced flow field measurements using high speed camera and Particle Image Velocimetry (PIV) are carried out to obtain the visualization results in bubble pair coalescence experiment. The numerical calculations by Moving Particle Semi-implicit (MPS) method, which are verified by visualization results, are adopted to analyze the bubble interaction dynamic characteristics. Then, the gas-liquid interface morphology and flow pattern evolution mechanisms in oscillatory velocity field are explored, based on the microanalysis of bubble pair coalescence. The gas-liquid interface morphology and flow pattern evolution mechanisms research in pulsating flow make a scientific step for exploring the two-phase pulsating flow characteristics with heat feedback, and provide phenomenological and theoretical support to the two-phase heat transfer mechanism under complicated transient flow condition.

两相流动不稳定性是核反应堆安全不可忽视的复杂流动传热现象，其周期脉动速度场与传热之间相互耦合形成自反馈的动态平衡。流型的相界面构形反映了不同的动量传递模式和传热模式，是构建两相流动和传热机理模型的重要基础。本研究采用空气-水开展脉动流下的两相流型特性研究，通过分离相界面传热及相变的复杂反馈效应，专注于速度场脉动对流型的影响。研究中从气泡相互作用的动力学特性出发，分别采用高速摄像仪和粒子图像测速（PIV）等先进流场测量手段针对脉动流内气泡融合过程开展可视化实验，同时对比移动粒子半隐式（MPS）无网格数值计算结果，分析速度场脉动下的气泡融合形态演化特性。基于气泡融合特性，对脉动流下的气液相界面构形及流型演化机理进行探索，所得到的速度场脉动下的相界面构形及流型演化规律将为进一步开展流动传热耦合下的流型特性研究奠定科学基础，为认识复杂瞬变条件下两相流动的传热机制提供关键性的基础数据和理论依据。

脉动流是核动力系统中不可忽视的现象，自然循环工况、流动不稳定性的发生以及升潜、摇摆等海洋条件的作用均会使堆芯流量发生不同程度的周期性脉动，进而影响反应堆的安全运行。.本研究通过精确设定脉动流的幅值和周期，开展了正弦流量脉动下两相可视化实验，结合高速摄像仪的数字化图像和丝网探测器的出口截面空泡份额时序曲线，获得了稳定流及脉动流对应不同流型下的气液相界面特性及流型过渡边界。发现大幅的流量脉动将导致泡状流-弹状流过渡边界的前移，使得泡状流在空泡份额达到0.25之前就开始向弹状流转变；在高频流量脉动下泡状流-弹状流过渡边界的前移更为显著。基于MPS-MAFL方法开展了脉动流下气泡上升及融合过程的数值模拟研究。在脉动流下气泡上升与合并的无网格计算中，发现脉动流中速度场及压力场的周期变化带来了气泡速度及体积的同周期波动，其对气泡上升及融合过程的影响，可以用来分析中低含汽率下泡状流和弹状流区间在脉动流条件下的相分布时序不均匀性。基于流场瞬态热工水力特性模型结合微液层蒸干临界机理，完成了脉动流下的临界热流密度预测程序的开发与验证。通过对比验证，所构建的脉动流条件下的临界热流密度预测程序平台对实验工况范围内的稳定流及脉动流下的临界热流密度预测误差可控制在±20%范围内。.本项目研究为认识脉动流下流型特性及过渡准则的提供了重要的基础实验数据和有价值的理论指导。为流动不稳定性下的反应堆安全分析提供了临界热流密度预测工具，具有一定工程应用价值。