配管入口部に発生する液中渦キャビテーションに粘性が与える影響についての研究

摘要

Cavitation is a highly important issue in various fluid machineries. In the design of an advanced loop-type sodium-cooled fast reactor in Japan, vortex cavitation is also a significant issue for the integrity of the reactor structure. Thus, an evaluation method for vortex cavitation is required. In this study, vortex cavitation at a single suction pipe inlet was studied under several different viscosity conditions including its transient behavior. The intermittent occurrence behaviors of vortex cavitation were grasped by visualization measurements. The experimental results showed that the influence of the kinematic viscosity was obvious under a hign kinematic viscosity. However, the influence became smaller with decreasing kinematic viscosity. From these results, the non-dimensional circulation, which was defined as the ratio of the local circulation to the kinematic viscosity, was deduced as an evaluation parameter to estimate the influence of the kinematic viscosity. Cavitation factors at transition points from continuous occurrence to intermittent occurrences were also evaluated as representative points where vortex cavitation occurs. Then, the occurrences of vortex cavitation were expressed as a relation between the cavitation factor at transition points and the non-dimensional circulation. As a result, it was clarified that the cavitation factor at transition points increased linearly in relatively small non-dimensional circulation, while it was nearly constant in relatively large non-dimensional circulation.%流体機械において広くみられるキヤビテーションは，流体中の減圧に起因して生じる相変化現象である。一般に，キヤビテーションが発生すると，振動•騒音といった現象の原因となることや，発生したキャビティが構造材近傍で崩壊する場合に材料の壊食などを引き起こす可能があることなど，様々な問題を引き起こすことが知られている。大型ナトリウム冷却高速炉においても，熱出力の増加と冷却系の2ループ化の結果として，ホットレグ配管(以下，H/L)内の断面平均流速が約9m/sと高流速となることから，H/L吸込部に形成される渦流れによる局所減圧により，渦型のキャビテーション(以下，液中渦キャビテーションとする)が発生する可能性がある。このため，液中渦キャビテーションの発生状況を評価することが，構造材の健全性を担保し，安全性を確保する観点から重要である。