非能动余热排出热交换器半液位换热性能研究

非能动余热排出热交换器（PRHR HX）是先进反应堆非能动堆芯余热排出系统的关键设备之一，置于内置换料水箱中。水箱内水蒸发或水箱泄漏等故障失水后液位降低，PRHR HX与管外水传热面积和管外流场均发生变化，导致堆芯非能动余热排出能力下降。为分析半液位下PRHR HX换热性能，建立半液位PRHR HX实验装置和数值计算模型，获得稳态和瞬态升温过程的换热量、温度分布和流场特性。实验结果表明，稳态沸腾换热阶段半液位结构的总换热量较全液位时的下降12%～22%，仍具有较强的换热能力。瞬态升温过程中管外换热模式从纯单相对流换热开始，先后逐渐出现局部过冷沸腾和局部饱和沸腾，并最终迅速由局部饱和沸腾转变为全管长饱和沸腾。水箱内逐渐升温过程中出现显著的热分层现象，结合数值模拟分析发现此时管外流体在高度上呈现多层回流流动结构。

Investigation on Heat Transfer Performance of Passive Residual Heat Removal Heat Exchanger in Half Water Level Tank

Passive residual heat removal heat exchanger (PRHR HX) is one of the key equipments of a passive core residual heat removal system of advanced nuclear reactors, which is placed in a in-containment refueling water storage tank (IRWST). The water level of the tank decreases due to the water evaporation under the accident condition or the water leakage by accident, and then the heat transfer area of PRHR HX and water outside the tubes and the external flow field of the tubes change, resulting in the decrease of the passive residual heat removal ability of the core. The experimental device and numerical model of half water level PRHR HX were established to analysis the heat transfer performance of PRHR HX in half water level condition. And the heat transfer rate, temperature distribution and flow field characteristics were obtained both in transient and steady states. The experimental results show that the heat transfer rate is reduced only 12%-22% from that in a full tank to that in half water level tank, the passive residual heat removal system still has higher heat transfer capacity. During the transient heating process, the heat transfer mode outside the tube starts from the pure single phase convective flow heat transfer, gradually appears local subcooled boiling and local saturated boiling. Finally, the heat transfer mode rapidly changes from local to the whole tube length saturated boiling. Moreover, a significant thermal stratification phenomenon still exists in the process of gradually heating up in the water tank. The local flow circulation at different regions with different heights is found in numerical simulation results.