Mechanisms of Burnout Generation due to the Instability and the Transient Change of Boiling Two-Phase Flow near a Flow Obstacle simulating a Spacer of a BWR

在垂直环形通道中安装了一个模拟沸水堆中支撑核燃料棒的隔离器的圆柱形流障。通过高速摄像机仔细观察了流动模式，特别是干斑和燃尽的产生。在心脏流量、进口质量、质量流量突变引起的瞬变流量实验条件下，同时测量了在间隔器附近6个轴向不同位置的加热管表面温度的时间变化。研究了流动不稳定性对燃尽产生的影响。结果总结如下：(1)在热流密度急剧增加或质量流量下降引起的流量瞬态变化中，过渡对燃尽产生的影响较小，只有在加热条件下，即变化后的热流密度与稳定运行时的燃尽热流密度相当时才会发生燃尽。在j_L=0.1 m/s的低流量条件下，干斑的产生既不是在隔离器的内部，也不是在隔离器的上游。燃烬不会在垫片的下游产生。(2)热通量的快速减小导致大面积干斑在间隔层内延伸，并延伸至间隔层上游。升温可达20k ~ 30k。(3)进口质量的急剧提高使流动稳定地转变为新的流动状态。另一方面，由于扰动波通过频率的降低，热流密度的急剧减小会在间隔器附近产生烧坏。(4)流速增大、剪应力增大的流型变化是稳定的。另一方面，如果变化引起速度的减小，即使热流密度小于稳态燃尽，也会发生燃尽。(5)本研究发现的一般规律是干扰波的通过频率是导致烧坏产生的控制因素。也就是说，如果扰动波通过的间隔变长，就容易发生烧坏。少

A cylindrical flow obstruction which simulates a spacer supporting nuclear fuel rods in BWR was installed in the vertical annular channel. Flow patterns, especially the generation of drypatch and burnout, were carefully observed by a high speed video camera. The time variations of temperature on the heating tube surface were simultaneously measured near the spacer at six axially different location under the experimental conditions of transiently changing flow caused by the sudden change in the heart flux, the inlet quality, the mass flow rate. The effect of flow instability on the generation of burnout was also investigated. The results are summarized as follow.(1) In the transient change of flow caused by the sharp increase of heat flux or decrease of mass flow rate, the effect of transition on the generation of burnout is small and the burnout was occurred only when the heating condition, i.e., the heat flux after the change is as large as the burnout heat flux in steady operation. I … More n low flow rate condition j_L=0.1 m/s burnout is triggered by the generation of the drypatch in the location neither in or upstream of the spacer. The burnout dose not generate downstream of the spacer.(2) The quick reduction of heat flux causes the large scale drypatch extending in the spacer and also reached to the point upstream of the spacer. The temperature increase reaches up to 20 K〜30 K.(3)The drastic increase in the inlet quality causes the flow stably change to a new flow condition. On the other hand the drastic decrease in the heat flux causes the generation of burnout near the spacer due to decreasing the passing frequency of the disturbance waves.(4)The change in flow pattern, in which the velocity increases and shear stress increases, is stable. On the other hand if the change causes the decrease in the velocity the burnout occurs even if the heat flux is less than the steady burnout.(5)The general rule found in the present research is that the passing frequency of disturbance waves is the controlling factor to couse the generation of burn out. That is If the interbal of the passing of disturbance wave becomes long the burnout is easy to occur. Less