低压自然循环流动闪蒸过程机理研究

Flow flashing is a key fundamental physical phenomenon in the low-pressure natural circulations system, and a main factor to cause two-phase flow instability. Many studies have mainly focused on the flow instability and ignored the flow flashing process itself and flow pattern evolution, which lead to some deviation of theoretical calculation results of the dynamic behaviors from the corresponding experimental results. combining some visualized experimental methods such as high-speed camera，and wire-mesh technology, this project aims to analyze the flow flashing phenomenon itself from bubble behavior and the heat and mass transfer process on the gas-liquid interface of flashing generated two-phase flow point of view. Meanwhile, the behaviors characteristics of bubbles generated by the flow flashing, and flow pattern evolution in vertical pipes will be analyzed, as well as the relationship between the flow flashing and the flow behavior of the low-pressure natural circulations are also obtained. The experimental study results of the flow flashing will further be used to model more exactly the flow flashing, and analyze the dynamic flow characteristics of the low-pressure natural circulations. The results of the project will provide a theoretical basis for all the other industrial devices using low pressure natural circulation technology, and also provide some analysis methods for inducing the influence of flashing-induced instability in low-pressure natural circulations.

流动闪蒸是低压下自然循环流动具有共性的关键物理现象，也是引发两相流动不稳定的主要因素。针对目前研究中重流动稳定性边界而轻流动闪蒸过程机理的研究现状，本课题从流动闪蒸汽泡的行为特征、闪蒸过程中两相流型演变两个方面入手，采用可视化观察与高速摄影方法，研究汽泡的运动行为特性，确定闪蒸过程中两相流型类型；结合先进的wire mesh空泡测量技术，建立闪蒸非热力平衡条件下的空泡份额分布方程，分析闪蒸过程中两相流型之间的传热传质行为及演变机理，建立流型转变准则式，绘制适用于竖直上升圆管内闪蒸两相流流型图。在此基础上，分析闪蒸过程对低压自然循环流动特性的影响，并采用合理的热工参数建立闪蒸驱动下自然循环流动动态模型。通过本课题的研究，对低压自然循环的流动闪蒸过程形成较深入的认识，为采用低压自然循环流动方式的系统运行提供理论支持。

流动闪蒸现象是低压自然循环系统运行中常见中一种物理现象。在流动闪蒸过程中，汽相的产生和发展受流体过热度、循环流速等因素的影响。本项目的主要研究内容是对流动闪蒸过程中气泡的行为及流型演变进行分析，确定闪蒸起始点及闪蒸过程汽相分布规律，绘制闪蒸两相流流型图并构建汽相分布方程。然后，在此基础上分析闪蒸过程中低压自然循环流动特性的影响机制。在闪蒸起始点的确定方面，考虑闪蒸诱发机理，通过化学势理论，采用非均相核化系数推导给出闪蒸起始点计算模型。在汽相分布的研究方面，借助高速摄影、wire-mesh网格传感器和电导探针等测量技术，给出了流动闪蒸过程中汽相的径向和轴向分布规律，并引入过热度的影响，建立轴向空泡份额计算关系式。在流型图的研究方面，利用可视化观察，确定闪蒸过程中两相流的流型特征，采用液相和汽相折算速度作为横纵坐标，结合数据分析给出了适用于竖直上升管内闪蒸两相流流型图。在低压自然循环系统流动特性研究方面，分析了流动闪蒸过程中热工参数变化对流动特性的影响规律，分析了回路驱动力和阻力的耦和关系，给出了计算驱动力和阻力的计算模型并编制计算程序。基于本课题的研究，所得结论可为采用低压自然循环流动方式的系统设计及运行提供理论支持。