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

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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