Kazuhiko Ogawa

In this study, the measurement of the number and size of cavitation bubbles in the biased velocity distribution were performed at nine positions on the downstream side of a butterfly valve. In practical engineering, more than two valves are installed in a series or bends are installed just ahead of a valve to save space. In such cases, the cavitation occurrence will be affected by the biased velocity distribution in front of a valve. In the biased velocity distribution in which the velocity was larger on the nozzle side, there were very small differences in the number of the cavitation bubbles at each position. If a cavitation number was small to some extent, the number of cavitation bubbles at the orifice side also increased due to vena contracta. Moreover, the cavitation bubble occurrence on the nozzle side was more intense than in the biased velocity distribution in which the velocity was larger on the orifice side. Therefore, the installation of a valve where the flow velocity on the nozzle side becomes large should be avoided in actual piping.

The cavitation around a butterfly valve occurs because of the large flow separation behind the valve body and the interference of the flow from the nozzle side and the flow from the orifice side. To avoid this cavitation, the installation of perforated plates behind the valve are proposed in this study. From the experimental results, the cavitation noise can be reduced by the perforated plates. It was confirmed by the numerical analysis that the holes of the plates were helpful for pressure recovery behind the valve body. Butterfly valves are widely used in industry because of their compactness and simple construction. Many kinds of anti-cavitation valves have been proposed, however, those are relatively expensive and the structures are complicated. The installations of perforated plates are very easy and very effective in practical use.

従来キャビテーション気泡の大きさと個数の分布は明らかでなく、定量的な考察もなかったが、本研究では初めてこれを明らかにした。弁付近に発生するキャビテーション気泡の直径は、弁開度45度の場合20μmから500μmの範囲であった。弁前縁でのキャビテーション気泡の直径は、弁下流側1dia.での気泡の直径の数倍であり、縮流による前縁での発生が支配的であること、下流へと流れるにしたがって気泡径が分裂、縮小により増加することが明らかになった。