重力注水流动不稳定现象关键影响因素实验研究

Experimental Study on Influence Factors of Flow Instabilityduring Gravity-Driven Reflooding

重力注水过程由于流量较小,可能导致流动不稳定现象等问题,对核反应堆安全性有着重要的影响。因此,基于由高位水箱、实验本体、出入口阻力调节阀和冷却水箱组成的实验装置开展了实验,研究了实验本体入口形阻、加热功率、系统压力和冷却水过冷度对流动不稳定现象的影响。研究结果表明,根据冷却水注入流量的变化,重力注水流动不稳定现象可以分为3个阶段:冷却水初次注入阶段、冷却水逐出阶段和冷却水再注入阶段。在一定的加热棒初始温度、实验本体出口形阻和高位水箱液位的情况下,增大实验本体入口形阻减小了流动不稳定现象的发生次数和持续时间,同时也降低了冷却水注入流量,并最终导致一段时间内冷却水注入出现了停滞。增大加热功率加快了冷却水的沸腾,缩短了单相流动的时间,降低了系统的稳定性。提高系统压力减小了冷却水和蒸汽的密度差,提高了冷却水的吸热能力,抑制了冷却水的沸腾,提高了系统的稳定性。增大冷却水过冷度提高了冷却水的吸热能力,降低了空泡系数,延长了压力震荡的周期,提高了系统的稳定性。相关结果可以为核反应堆非能动安全系统的评估提供参考。

Gravity driven water injection with relatively low mass flow rate would lead to unstable flow,which may have effect on the safety of the nuclear reactor.Therefore,the effects of inlet resistance,heat flux,system pressure and inlet subcooling on the flow instability need to be investigated.The gravity injection loop is established,which consists of a cooling water storage tank,an experimental vessel,inlet and outlet valves and a condensate tank.The results show that,based on the changing characteristics of the cooling water flow,the instability of the gravity driven water injection process can be divided into three stages:the initial injection,the ejection,and the re-injection of the cooling water.Under certain initial temperature of the heating rods,outlet resistance and cooling water level,the increasing of the inlet resistance reduces the times and duration of cooling water expulsion as well as the mass flow of cooling water,which leads to the stagnation of cooling water injection during an interval of time ultimately.Increasing the heat flux leads to the rising of the evaporation rate of the cooling water,accelerates the boiling of the cooling water,shortens the single-phase flow section,and thus degrades the stability of the system.Amplifying the system pressure stabilizes the system by reducing the density difference of the gas-liquid two-phase flow and restraining flow boiling which is achieved through the increasing in the thermal power required for the system to transit from a single-phase steady flow to a two-phase oscillating flow.Increasing the inlet subcooling is also proved to upgrade the stability of the system and increase the oscillation period through the improving of the heat storage capacity of cooling water and the reduction of the void fraction of cooling water.These results may provide reference for the safety assessment of passive systems in nuclear power plants.