**7.2.2 Diameters and numbers of cavitation bubbles**

Fig. 14. Cavitation Bubbles of the normal valve ( 45

Fig.15 shows the diameters and numbers of cavitation bubbles of the normal valve under the normal velocity distribution. The diameter of the bubbles are shown at 8 stages at every 30*m* . For example, a bubble with a diameter between 20 *m* and 50 *m* is expressed as 35 *m* , and a bubble with a diameter between 50 *m* and 80 *m* is expressed as 65 *m* . Bubbles with diameters were smaller than 20 *m* could not be visualized in our experiments. Such size nuclei are presumed to be contained even in water under noncavitation conditions, and will not affect cavitation noise greatly. The bubbles with diameters larger than 230 *m* did not occur in our experiments. In Fig.14, the cavitation condition reached the flashing condition at 24.2 . As for the flashing condition, the number of bubbles is shown as zero because the individual cavitation bubbles were not able to be photographed.

The inception condition was at 48 and the flashing condition was at 25 . Under the normal velocity distribution, the diameters ranged mainly from 20 *m* to 100 *m* as shown in Fig.15(a). However, under the biased velocity distribution (A), the cavitation bubbles with diameters over 100 *m* increased at 35.9 as shown in Fig.15(b). Under this condition, the cavitation condition was in the growth stage and the cavitation bubbles occurred numerously behind the valve body. Though it looks as if the amount of the bubbles decreased compared with Fig.8, this is due to the fact that the photographing was performed at one position. Compared with the results of Fig.15(a), the cavitation noise in Fig.15(b) had already become more intense at the same cavitation number.

Noise Reduction in Butterfly Valve Cavitation

*m* to 125

Fig. 17. The details of Photographing positions.

Position x y

Table 1. Photographing Positions.

larger in the left side of the photograph.

bubbles range from 20 m to 200 m .

flowing from the orifice and nozzle side flow together.

*m* .

**7.3 The aspect of cavitation bubbles** 

increased greatly at 29.7

diameters from 35

by Semicircular Fins and Visualization of Cavitation Flow 497

In the case of Type-B as shown in Fig.16, the cavitation numbers were less than the case of the normal valve. Though the cavitation bubbles with diameters larger than 200

bubbles was suppressed compared with the case of the normal valve in the range of

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

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

A -0.2 dia. 0.36 dia. 0.6×5.0 20000 B 0.3 dia. 0.3 dia. 1.7×6.8 10000 C 1.3 dia. 0 dia. 1.7×6.8 10000

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

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

was on the centerline of the pipe and near the end of the cavitation clouds.

, this condition was just before the flashing and the number of

Photographed Area a×b(mm)

Frame (per second) *m*

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

Fig.15(c) shows the diameters and numbers of cavitation bubbles under the biased velocity distribution (B). At 34.4 , the numbers of the cavitation bubbles with diameters between 65 *m* and 125 *m* increased remarkably comparing with the results of Fig.15(a). From the results of Fig.15(a),(b) and (c), when the bubbles with diameters from 20m to 30m increased, the cavitation noise tends to increase.

Fig. 16. Diameters and numbers of cavitation bubbles of the Type-B valve under the normal velocity distribution ( 45 ).

In the case of Type-B as shown in Fig.16, the cavitation numbers were less than the case of the normal valve. Though the cavitation bubbles with diameters larger than 200 *m* increased greatly at 29.7 , this condition was just before the flashing and the number of bubbles was suppressed compared with the case of the normal valve in the range of diameters from 35*m* to 125*m* .
