超临界二氧化碳临界流动机理可视化研究

Supercritical carbon dioxide Brayton cycle is one of the strategic booming technologies in the research field of nuclear energy, fossil energy, and solar energy. The focus of its application is the seal design for turbo machinery with high backpressure and the prevention of loss of fluid accident. The scientific base to solve these issues is to establish a clear understanding on the mechanisms of critical flow for supercritical carbon dioxide. Based on the lack of mechanism discussion for two-phase critical flow and experimental observation supporting for model, a visualized investigation on critical flow for supercritical carbon dioxide is proposed. The aim of this investigation is to observe the phenomena of fluid flashing and its mass transfer process, and thus to reveal the mechanism of two-phase critical flow and its parameter trends through the effect of phase-change on pressure wave speed. The boundary condition and scientific trends of critical flow will be established. A theoretical model, which can accurately describe the experimental phenomena and agree with experimental data, will be proposed through the mechanism based mathematical relation between critical flow rate and the parameters of thermohydraulics and geometry. The outcome of this work can provide a phenomenological and theoretical supporting to two-phase critical flow mechnism, and thus promote the development of two-phase critical flow research to theoretical and modelling stage. It also can provide a scientific base for the application of supercritical carbon dioxide Brayton cycle.

超临界二氧化碳布雷顿循环技术是近年来核能、化石能、太阳能等领域的战略新兴方向之一。高背压叶轮机械的密封设计及破口事故的预防缓解是其应用关注的焦点，而解决该问题的科学前提是必须对超临界二氧化碳的临界流动机理建立起清晰认知。本项目针对当前两相临界流动机理探索不足、模型缺乏实验现象支持的现状，通过可视化手段研究超临界二氧化碳的临界流动特性，观察临界流动状态下管内液体闪蒸及相变传质过程的基本现象，从工质相变影响其内部压力波传递速度的角度探索两相临界流动及其参数作用的内在机理，掌握超临界流体临界流动形成的边界条件和科学规律，揭示临界流量与热工滞止参数、几何结构参数之间的关联机制，建立可准确描述实验现象且与实验数据相吻合的理论预测模型，为探索两相临界流动机理、推动两相临界流动研究向理论化与模型化方向发展提供直接的实验现象和理论支撑，也为超临界二氧化碳布雷顿循环技术的应用提供科学基础。

超临界二氧化碳布雷顿循环是第四代反应堆重要发展方向之一。由于系统内工质处于超临界态，当破口事故发生时，管道内外巨大的压力差会导致破口位置出现临界流动现象。冷却剂的泄漏会对反应堆安全运行造成巨大威胁。因此，临界流现象是超临界二氧化碳布雷顿循环重要安全问题之一。.本课题瞄准临界流动现象中质量流量问题，对临界流动特性以及流动速率的影响参数问题开展研究。具体内容包括：基于简化破口几何参数，开展不同上游滞止压力、上游滞止温度和实验段长径比的稳态喷放实验，获得了超临界二氧化碳临界流动时的可视化现象，测得了不同热工参数和几何参数下超临界CO2临界流质量流量。分析了不同参数对质量流量的影响规律，采用CFD方法对喷管内的超临界二氧化碳临界流动特性进行了模拟，明确了临界流在管内的基本流动特性，建立了超临界二氧化碳临界流动模型，可为超临界二氧化碳循环系统泄漏速率的预测提供计算分析工具。