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

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。

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.