非能动余热排出换热器运行初始阶段换热特性研究

摘要

Aiming at the temperature rising in the secondary side of the passive residual heat removal heat exchanger (PRHR HX) at initial operating stage, experiments on the heat transfer of vertical tube bundle immerged in an elevated tank during the heating up period were performed. The results show that in the early stage of experiments, heat is transferred by single-phase natural convection due to the large subcooling of water. The water temperature rises gradually which leads to a thermal stratification phenomenon in the tank and the heat transfer capability decreases with the water temperature increasing. As the subcooling decreasing, the heat transfer mechanism transforms from single-phase convection to subcooling boiling gradually. After water reaches the saturation temperature, saturated pool boiling is the primary mechanism of heat transfer. Adop-ting Churchill &? Chu correlation, the natural convection heat transfer coefficient wasseparated from the total heat transfer coefficient. The proportion of single-phase naturalconvection and subcooling boiling at different heat transfer stages was analyzed. Thiswork provides certain directive significance to the design of PRHR HX.%以非能动余热排出换热器运行初始阶段二次侧水箱水的升温过程为原型,通过实验研究了高位水箱内竖直换热管束在主流水温达到饱和前的换热特性.结果表明,换热管束运行初期热量依靠水的单相自然对流带走,水箱竖直方向上出现温度分层,换热量随主流的升温而下降.随着主流欠热度的减小,从管束上端开始换热机理逐渐向欠热沸腾转变；之后,主流水温逐渐达到饱和,沸腾成为换热的主要手段.在实验研究基础上,利用Churchill&Chu公式从管外平均换热系数中分离出自然对流换热系数,分析了不同阶段自然对流和欠热沸腾在管外换热系数中所占的比例.本文的研究对非能动余热排出换热器的设计有一定的指导意义.