基于PIV的稠密棒束间超临界CO2湍流交混的实验与理论研究

In the thermal design of Supercritical Water-cooled Reactor, promoting the turbulent mixing inside the rod bundle is one of the effective ways to lower the fuel-rod cladding temperature and enhance the system safety. However, the feature of turbulent mixing is closely associated with the density and viscosity variations of the fluid, the large-scale flow-pulsation theory and predicting models developed based on conventional constant-property methods are not applicable to supercritical fluids which undergo strong property variations. Therefore, this project takes supercritical CO2 as the research objective and systematically investigates the turbulent mixing characteristics inside a tight rod bundle. In the experimental aspect, the velocity distributions on the plane inside the rod bundle will be observed using PIV method and the turbulent mixing coefficient could be obtained. Afterwards, the effects of the sudden drop of fluid density and viscosity in the pseudo-critical region on turbulent mixing coefficient will be revealed. In the theoretical aspect, the flow characteristics owing to the combined actions of reduced turbulent shear stress and increased axial velocity gradient that resulted from the variable properties will be analyzed. Furthermore, the mechanism of large-scale flow pulsation to the turbulent mixing of supercritical fluids will be clarified. In the model-development aspect, a property factor, which characterizes the feature of turbulent mixing of supercritical fluid, could be proposed through the nondimensionalization to the drastically varied fluid density and viscosity. Finally, a predicting model of turbulent mixing will be established. Research of this fundamental project aims at exploring the turbulent mixing theory and modelling method under variable-property conditions, and also clarifying the general law of turbulent mixing for supercritical fluids, which shows an important scientific research value.

在超临界水冷堆的热工设计中，增强棒束间冷却剂的湍流交混是降低燃料包壳温度、提高系统安全裕度的有效方式之一。然而，湍流交混规律与流体密度和黏度等热物性的变化紧密相关，传统常物性研究方法得到的大尺度流量脉动理论和预测模型并不完全适用于物性剧烈变化的超临界流体。为此，本项目以超临界CO2为研究对象，开展稠密棒束间湍流交混的系统性研究。实验方面，采用PIV观测棒束间的平面速度分布，确定湍流交混系数，揭示拟临界区流体密度和黏度的骤降对交混系数的影响。理论研究方面，分析变物性导致的湍流剪应力减小和轴向速度梯度增大叠加作用下的流动特征，阐明超临界流体湍流交混的大尺度流量脉动机理。模型研究方面，引入表征交混特殊性的物性因子，将剧烈变化的密度和黏度无量纲化，建立交混系数的预测模型。本项目的基础研究工作旨在探索变物性作用下的交混理论和建模方法，揭示超临界流体湍流交混的一般规律，具有重要的科学研究价值。

在超临界水冷堆热工设计中，增强棒束间冷却剂的湍流交混是降低燃料包壳温度、提高系统安全裕度的有效方式之一。然而，超临界流体的热物性在拟临界点附近复杂而剧烈的变化，直接影响了湍流交混的特征和规律，使得传统常物性研究方法得到的大尺度流量脉动理论和预测模型并不完全适用。为此，本项目以超临界水冷堆堆芯燃料组件为研究对象，开展稠密棒束间湍流交混的系统性研究。首先，通过PIV流场可视化测量得到棒束间的平面速度分布，进而确定湍流交混系数。为此，设计了3×3稠密可视化棒束实验段，透明段采用FEP管制作，其折射率与水高度匹配。实验测量发现了棒束间存在的准周期性大尺度流量脉动现象，脉动半波长随雷诺数的增加而增大，但脉动频率几乎不变。在此基础上，应用子通道横向均方根流速建立湍流交混系数，研究其随雷诺数等参数的变化规律。其次，采用大涡模拟这一先进的数值模拟方法，对稠密棒束内的湍流流动和交混情况进行分析，得到瞬时速度、统计时均速度以及微观湍流变量，并与实验测量数据进行对比，验证大涡模拟的可靠性，深入分析了稠密棒束内的湍流交混特性。最后，对超临界压力下流体模化准则进行适用性分析，找出现阶段模化方法的局限并进行相应优化，拓展模化准则的应用范围，目的是将本项目超临界二氧化碳的研究结果外推至超临界水的对应参数。本课题取得的相关成果将为稠密棒束内冷却剂湍流交混的工程计算提供理论基础和技术支撑。