Kazuhiko Ogawa

In this study, the measurement of the number and size of cavitation bubbles around a butterfly valve was performed at three positions; at the leading edge, at the position near the rear face of the valve, and at the position which was 1 diameter downstream of the valve. From the visualization results, it was found that cavitation bubble diameters range from about 20 micromters to about 500 micrometers, at the valve opening of 45 degrees. On the contrary, at the position near the rear face of the valve and at a position 1 diameter downstream of the valve, the maximum diameter of cavitation bubbles was about 200 micrometers, at the same valve opening. The number of cavitation bubbles at the leading edge was over a few times the number which was 1 diameter downstream of the valve. Consequently, it is considered that cavitation occurrence is dominant at the leading edge by the contraction flow and that the bubbles which occurr at the leading edge become smaller size bubbles through pressure recovery.

The effect of an upstream biased velocity distribution of a butterfly valve on the cavitation noise was examined in this study. The cavitation noise of the valve with semi-circular fins was a few dB smaller than that of a conventional valve under biased velocity cinditions. The effect of semi-circular fins was clear also under the biased velocity distribution condition.From the visualization of cavitation bubbles by a Hi-speed camera, it was clarified that the intense vortex cavitation clouds were suppressed on the back surface of the valve body by the two fins. The diameters of the cavitation bubbles ranged from about 20μm to about 200μm . It was very clear that the interference of the flow from the orifice side and the flow from the nozzle side was suppressed by the fins under not only the normal velocity distribution but also the biased velocity distribution and that the numbers of cavitation bubbles in the valve with semi-circular fins was less than that of the normal valve.

The purpose of this study is to reduce cavitation noise occurring around a butterfly valve using a simple method. This study proposes the attachment of fins to the valve body in order to reduce cavitation noise. The occurrence of cavitation around butterfly valves is due to the interference of the flow from the nozzle side with the flow from the orifice side. To avoid this interference, semicircular fins were attached to the valve body. The effect of the fins was investigated based on both experimental results and numerical analysis. The results of the experiment, wherein the two semicircular fins were attached to the downstream side of the valve body, showed that the inception of cavitation around the valve with two fins was earlier than that around a normal valve, with the fins under the constant pressure loss coefficient, and that the maximum cavitation noise just before flashing condition could be suppressed by the fins. This is because the interference of the flow from the orifice side with the flow from the nozzle side is suppressed by the fins. Therefore, the attachment of the fins is an effective method for reducing cavitation noise.

バタフライ弁は産業プラントで汎用バルブとして多数使用されるが、キャビテーションによる騒音と壊食のためメンテナンスの面から大きな問題となっている。著者は配管形状と弁体形状の改善によりキャビテーション現象の抑制法について研究してきており、弁体の前縁側と後縁側からの流れの干渉を抑制することにより、騒音を5-10dB低下させることができた。実測による検証と流れの可視化、数値解析を援用してその騒音低減の効果について検討したので報告する。

キャビテーション現象は、流れの圧力が低下して液体が蒸発し、液体の中に含まれる気泡核が成長し、気泡へと成長する現象である。この気泡はすぐに崩壊して、周囲に大きな衝撃圧を及ぼし、騒音や壊食現象を引き起こす。したがって、船舶のスクリュー、ポンプ等の流体機械や、産業用プラントにおける流量制御弁や流体計測機器にも種々の障害をもたらす。これらの現象の基礎的な解説を初心者向けに平易に述べた。またこれらの現象の対策や、キャビテーションを加工技術や環境問題などへの利用についても、最近の研究の動向を概説した。

"The purpose of this study is to reduce cavitation noise occurring around a butterfly-valve with a simple method. In this paper, the attachment of fins to the valve body and the enlargement of the pipe diameter is proposed to reduce cavitation noise further. The cavitation around the butterfly valve occurs because of the interference of the flow from the nozzle side and the flow from the orifice side. To avoid this interference, semicircular fins were attached to the valve body. From the experimental results, it was confirmed that the attachment of the fins was very effective in reducing the cavitation noise and that the combination of the fins and the enlargement of the pipe diameter was also effective for the reduction of the cavitation noise."

A method of reduction of cavitation noise around a butterfiy valve was proposed in this study. A butterfiy valve with riblets was tested and it was confirmed that the valve was effective for reduction of cavitation noise. Moreover, the two-diwensinal bubbly cavitating flow was aralyzed by numerical simulation. From the calculation result, it was suggested that the numder of density of bubbles more affected the bubble growth than the size of bubbes.

The method of reduction of cavitation noise around a butterfly-valve is proposed and cavitating flow by numerical simulation is discussed. For the reduction of cavitation, a ditch was set up behind the valve. We experimented in the duct of circular cross section to confirm whether the method is effective. From the experimental result, it was confirmed that the ditch was effective for the reduction of cavitation noise in an actual pipe without losing the function of a butterfly-valve. Next the two-dimensional bubbly cavitating flow was analyzed by numerical simulation and it was confirmed that the bubble periodically grew up and collapsed near a comer of the wall.

The purpose of this study is to reduce cavitation noise generated behind a butterfly-valve set in a straight pipe. The main reason of cavitation occurrence is an abrupt decrease of the pressure behind the valve. In our experiments, the duct was partly enlarged to recover the pressure behind the valve and to reduce cavitation occurrence. It was confirmed that this method is effective to reduce cavitation through the experiments using a pipe with a circular cross section. Moreover, we carried out the experiments using butterfly-valve with aileron. And we confirmed that the effect of the aileron on the noise was not so large.

The purpose of this study is to reduce cavitation noise generated behind a butterfly-valve set in a straight pipe. The main reason of cavitation occurrence is an abrupt decrease of the pressure behind the valve. In our experiments, the duct was partly enlarged to recover the pressure behind the valve and to reduce cavitation occurrence. It was confirmed that this method is effective to reduce cavitation through the experiments using a pipe with a circular cross section. Moreover, we carried out the experiments on the flow with the difference in top and bottom of the pipe in flow velocity, it was confinmed that the noise reduction method by the ditches was effective when using a butterfly-valve in the flow with the sloped velocity distribution.

The cavitation noise in and around piping is very severe and not desirable in working environment. The reduction of cavitation noise is a very important research theme. The purpose of this study is to reduce cavitation noise occurring around a butterfly valve. The main cause of cavitation occurrence is an abrupt decrease of the fluid pressure behind a valve. To recover the pressure behind the valve in our experiment, the pipe was partly enlarged from just ahead of the valve shaft forward downstream. The enlarged part of the pipe is called a "ditch" in this study. To observe the cavitation around the butterfly valve easily, experiments were conducted by the use of a square valve body and a duct having a square cross section. From the results of the noise measurement and visualization, the size of the ditch was determined and it was found that the effect of noise reduction was greatest when the length and the depth of the ditch were 1 and 0.2 times the width of the duct, respectively. In our study, the noise measurement was performed under the condition that the magnitude of the Reynolds number was 100000. With no cavitation, the sound pressure level was about 40 dB but the sound pressure level increased to 60 dB with severe cavitation. By the effect of the ditch, the cavitation noise was reduced by 8 dB to 10 dB at the maximum in the case of the square valve body. The effect of the ditch was very clear in the experiments using an actual valve and piping: It was confirmed that the sound pressure level can be reduced by about 10 dB even with severe cavitation. In the actual operation of the butterfly valve, when a partly enlarged pipe is used, the valve opening must be smaller to obtain the same flow rate as in the case where a conventional pipe and a valve are used, because the pressure loss is smaller when using a partly enlarged pipe. However, even when taking into account the difference of the valve opening, the effect of noise reduction was very remarkable. When a partly enlarged pipe was used, secondary cavitation occurred from the edge of the ditch. From the experimental results, it was found that the effect of the noise of secondary cavitation was about 2 dB to 3dB. However, this noise could be eliminated by changing the shape of the ditch. Thus, partial enlargement of the pipe is very effective for reduction of cavitation noise and it can be realized more easily than changing the shape of the valve body.

The purpose of this study is to reduce cavitation noise generated behind a butterfly-valve set in a straight pipe. The main reason of cavitation occurrence is abrupt decrease of the pressure behind a valve, In our experiment, the duct which has circular cross section was partly enlarged to recover the pressure behind the valve and to reduce cavitation occurrence. It was confirmed that this method is effective to reduce cavitation and that the cavitation decreased by examining the relation between discharge coefficient and valve opening of no ditch pipe and standard ditch.

The purpose of this study is to reduce cavitation noise generated behind a butterfly-valve set in a straight pipe. The main reason of cavitation occurrence is abrupt decrease of the pressure behind a valve. In our experiment, the duct of a circular cross section was partly enlarged to recover the pressure behind the valve and to reduce cavitation occurrence. It wasconfirmed that this method was effective to reduce cavitation noise by comparing the relation between discharge coefficient and valve opening of a pipe with no ditch and a pipe with a standard ditch.