超临界水在垂直管内换热及流动不稳定性研究

清华大学核能与新能源技术研究院在建的250 MWt高温气冷堆核电站示范工程(HTR-PM)中蒸汽发生器二回路为亚临界水,由于反应堆能提供750℃的高温氦气,二回路水可提高到超临界压力和温度,采用多堆带一机方案可与超临界蒸汽透平机组匹配,因此研究超临界水在管内的流动、传热以及流动不稳定现象非常重要。本文通过使用RNGk-ε模型耦合强化壁面函数,发现模拟结果与Yamagata等的实验数据符合较好。基于此模型,分析了超临界流体流动时换热系数的变化规律,并采用瞬态计算方法,线性增大加热功率,分析了流动不稳定现象,发现流体一旦进入不稳定区,进出口流量的波动非常严重,甚至出现倒流,应尽可能避免此类现象。

Study on Heat Transfer and Flow Instability of Supercritical Water in Vertical Tube

Water in the steam generator secondary circuit in the 250 MWt pebble-bed module high temperature gas-cooled reactor (HTR-PM) which was built by the Institute of Nuclear and New Energy Technology (INET) of Tsinghua University, is subcritical. Its pressure and temperature can increase to supercritical as the reactor provides high temperature helium up to 750 ℃, then the reactors and the supercritical steam generators can work with the supercritical steam turbine by adapting multiple-reactors with one-turbine unit design concept. Therefore, the flow, heat transfer and the flow instability of supercritical water in tubes are of great importance. RNG k-ε turbulence model coupled with enhanced wall treatment was applied in the simulation, and the numerical calculation results agree well with the experimental results of Yamagata, et al. The heat transfer coefficients of supercritical water in a tube under various heat fluxes were analyzed based on this model. The flow instability was also studied by linearly increasing the heating power with time. The results show that the inlet and outlet flow rates oscillate intensely once the flow enters the unstable zone and even reserve flow can be observed. Such phenomena should be avoided.