超临界流体并联管流动不稳定性及传热特性研究

The strong variation of the thermal-physical properties of supercritical fluids in the vicinity of the pseudo-critical line results in challenging tasks in thermal-hydraulic design of Supercritical water cooled reactor (SCWR). One of the main challenging tasks is to understand and to predict the flow instability and heat transfer oscillation among the parallel assemblies in SCWR,which is rarely reported. The present study will focus on the supercritical fuild flow instability and heat transfer performance in parallel heating tubes which share the similar geometry features with the parallel assemblies.Based on the mass, momentum and energy conservation equations, nondimensional parameters and novel numerical models will be developed for predicting the onset of flow instability of supercritical fluids in the parallel heated tubes.Meanwhile, experimental study will be carried out on the parallel two tubes and three tubes.We will mainly use the supercitical freon R134a which has a lower supercritical pressure and temperature than the supercirical water, and use the supercritical water as a supplement.The transient feature of flow instability will be analyzed with the suport of detailed experimental results of supercritical freon and numerical simulation results. The effect of flow instability on the heat transfer performance will be further stuided. Numerical model to predict the heat transfer to supercritical fluids will be established, as well as the critireon for the onset of the heat transfer deterioration. Through the above mentioned work,we will get to know the basic laws and provide numerical models for the flow instability and heat tranfer ossilation in the SCWR.

超临界流体在拟临界区域奇异的物性变化给超临界水冷堆的热工水力设计带来各种挑战，其一是堆芯并联燃料组件内超临界流体的流动不稳定性及其引起的传热变化认识和预测是关键的问题之一。目前针对超临界流体不稳定性研究极少，仅限于单管的理论研究和自然循环单管实验研究。本项目以加热并联管内超临界流体流动不稳定现象为研究对象，建立描述超临界流体流动不稳定性的无量纲准则数和数学模型，获得无量纲准则不稳定边界,以超临界氟利昂实验数据为主、超临界水实验数据为辅验证无量纲边界对不同物性的普遍适用。基于氟利昂实验数据和流动数学模型认识并联管内超临界流体不稳定流动时域特性，进一步建立传热数学模型，揭示流动不稳定下以传热恶化为代表的异常超临界流体传热的发展规律。本项目研究旨在为超临界水冷堆堆芯的流动传热不稳定性分析提供理论基础和数学模型。

超临界流体在拟临界区域奇异的物性变化会导致堆芯发生传热恶化和流动不稳定性问题，给超临界水冷堆的热工水力设计和安全运行带来极大的挑战。本项目针对超临界传热恶化和流动不稳定问题，采用实验和数值方法揭示和分析了超临界管内传热强化、传热恶化、并联通道流动不稳定性发生规律。研究内容包括：超临界流动不稳定性无量纲分析模型和频域模型建立和分析、超临界并联通道流动不稳定时域模型的建立与分析、超临界并联通道流动传热特性和不稳定性实验研究。通过研究：（1）提出了基于临界温度的过冷数和膨胀数两个无量纲准则数来描述超临界流动不稳定性，建立了适用不同超临界流体的频域数学模型，开发了基于频域模型的计算程序，对单通道、并联通道和多通道内超临界流动不稳定发生区域进行了预测；（2）建立了并联通道流动不稳定的数值求解模型，开发了相应的时域程序，实现对超临界压力下流量飘移型和密度波型流动不稳定的数值模拟，通过基准算例和实验数据验证程序具有准确的预测能力，并开展了超临界单通道和并联通道流动不稳定性的建模方法研究，进行了过冷度、压降、流量等系统参数和长度、倾斜度、节流系数等结构参数的对不稳定性的敏感性分析。（3）开展了并联管超临界氟利昂流动不稳定性实验研究，获得了不同流体超临界传热丰富的实验数据，研究发现大管径下传热恶化的现象比流动不稳定现象更易发生，分析了热流密度、质量流速、压力等参数对超临界传热强化的影响规律和作用机理，，获得了压力瞬态变化对传热系数的影响以及跨临界泄压后发生CHF临界现象。（4）以临界压力建立了压力模化关系，以流体温度和拟临界温度的结合建立了温度模化关系，改进了现有的超临界传热模化准则，提出了不需要迭代、结构简单、基于明确物理意义无量纲数的超临界传热关联式。