**7.3 The aspect of cavitation bubbles**

496 Mechanical Engineering

(a) Normal velocity distribution (b) Biased velocity distribution (A)

(c) Biased velocity distribution (B)

Fig.15(c) shows the diameters and numbers of cavitation bubbles under the biased velocity

results of Fig.15(a),(b) and (c), when the bubbles with diameters from 20m to 30m increased,

Fig. 16. Diameters and numbers of cavitation bubbles of the Type-B valve under the normal

, the numbers of the cavitation bubbles with diameters between

*m* increased remarkably comparing with the results of Fig.15(a). From the

).

Fig. 15. Diameters and numbers of cavitation bubbles of the normal valve ( 45

distribution (B). At 34.4

velocity distribution ( 45

).

*m* and 125

65  the cavitation noise tends to increase.

Figure 17 and Table 1 shows the positions of photographing. Position A was very near the leading edge of the valve body. Position B was in the wake region of the valve. Position C was on the centerline of the pipe and near the end of the cavitation clouds.

Fig. 17. The details of Photographing positions.


Table 1. Photographing Positions.

Fig.18 shows the occurrence of the cavitation bubbles at position A. The five photographs in the image were of a series and photographed at 20000 frames/second. Position A was near the leading edge and the crescent in the central lower of the photograph is the one part of the leading edge of the valve body. It is very clear that the cavitation bubble occurred at the position on the right in the leading edge when photograph (c) is compared with photograph (d). The bubble seems to be oval and the size is about 200 m . The cavitation bubble grows larger in the left side of the photograph.

Fig.19 shows the cavitation bubbles at position B. Oosition B was 0.3dia. downstream of the valve. This position is in the large separation region behind the valve. Though the mainstream is the right direction from the left, the flow circulates in the separation region, and the bubbles of the photograph move from the right to the left. The diameters of the bubbles range from 20 m to 200 m .

Fig.20 shows the cavitation bubbles at position C. Position C was 1.3dia. downstream of the valve. In this figure, the bubbles move from left to right and the number of the bubbles were relatively larger than that at position B. This vicinity is the position where the cavitation cloud ends, and the number of bubbles increases because the bubbles which have been flowing from the orifice and nozzle side flow together.

Noise Reduction in Butterfly Valve Cavitation

**8. Conclusions** 

by Semicircular Fins and Visualization of Cavitation Flow 499

(a) t=1/10000(s)

(b) t=2/10000(s)

(c) t=3/10000(s)

(d) t=4/10000(s)

The ability of fins to reduce the cavitation noise around a butterfly valve was investigated in this study. When two semicircular fins were attached to the downstream side of a valve body (TYPE-B), the experimental results indicated that the inception of cavitation was earlier than that found with a normal valve. This was discovered via comparison of noise given a constant pressure loss coefficient. However, cavitation noise increased gradually after inception and the maximum value of cavitation noise just before flashing was shown to be suppressed for the TYPE-B valve by the fins. Cavitation noise was reduced by about 5 dB when a valve opening of 45degrees was used. The effect of noise reduction around the TYPE-B valve was most remarkable just before flashing. In this study, it was found that fins are an adequate method for the reduction of cavitation noise. The optimum size and

Visualizations created by a high-speed camera showed that intense vortex cavitation clouds were not clear in the TYPE-B valve with two fins. The interference of the flow from the orifice side with the flow from the nozzle side seemed to be suppressed by the fins. This was

As for the visualization of cavaitation bubbles by using high-speed camera, it was found in this study that cavitation bubbles occur at the position before the leading edge and that the cavitation bubbles grow larger just behind the leading edge. It was also found that the maximum diameter occurs near the leading edge and that the bubbles become smaller through the pressure recovery of the flow. In this observation, the maximum diameter of the bubbles was 500 m near the leading edge. The diameter of the bubbles ranged from 20 m

Fig. 20. Cavitation Bubbles at the Position C (valve opening= 45 , 44.6 ).

position of the fins should be investigated in future studies.

also confirmed by numerical analysis.

(e) t=5/20000(s)

Fig. 18. Cavitation Bubbles at the Position A (valve opening= 45 , 48.6 ).

Fig. 20. Cavitation Bubbles at the Position C (valve opening= 45 , 44.6 ).
