**6.1 Visualization experiments**

Figs. 7 and 8 show the photographs of the cavitation conditions of each valve taken with a high-speed camera. Figure 7 was taken through the upper surface of the transparent pipe (top view). Figure 8 was taken from the side surface (side view). These photographs were not taken simultaneously because only one camera was available to be used. As shown in Fig. 7(a), vortex cavitation clouds were clearly visible on the orifice side of the normal valve and 1 Dia and further downstream from the stem axis. These cavitation clouds were identified as vortices of an intensity which brings about the cavitation damage (Tani et al.,1991). Similar vortex cavitation also occurred in the TYPE-A valve with one fin. It was impossible to suppress flow interference using only a single fin. Moreover, it is obvious from Fig.7 (b) that the cavitation was further intensified since the fin existed in the part of the orifice side where the contraction flow was severe. However, such a vortex cavitation cloud is not clear around the TYPE-B valve as shown in Fig. 7(c). The interference of the flow from the orifice side with the flow from the nozzle side seemed to be suppressed by the fins. Accordingly, it can be said that the cavitation around the valve body in TYPE-B was

 (a) Normal Valve (θ=45°,σ=17,ζ=8) (b) TYPE-A (θ=50°,σ=17,ζ=8) Fig. 7. Top Views of Cavitation Conditions.

Noise Reduction in Butterfly Valve Cavitation

by Semicircular Fins and Visualization of Cavitation Flow 491

(a) Top View (Y=0.3dia) (b) Side View (X=0.3dia) Normal Valve

(c) Top View (Y=0.3dia) (d) Side View (X=0.3dia) TYPE-B

(e) Top View (Y=0.3dia) (f) Side View (X=0.3dia) TYPE-C

in TYPE-C, the flow interference effect was suppressed by the two fins on either side of the valve, but was canceled out by the intensification of cavitation due to the central fin. Therefore, the effect wherein two fins on either side of a valve suppresses flow interference

Side View

Fig. 9. Cavitation Flow around a Butterfly Valve (θ=45°,σ=22).

Top View

Fig. 10. Coordinate System in Numerical Analysis.

( )

X +

is offset by the effect wherein the fin in the center intensifies cavitation.

)

Y(+

Fig. 8. Side Views of Cavitation Conditions.

suppressed by the attachment of the fins. As for the TYPE-C valve with three fins, though the two fins on either side of the valve probably suppressed the cavitation as they did for the TYPE-B valve, it is appropriate to conclude from Fig.7(d) that the cavitation reduction effect was not obtained because the central fin intensified the cavitation as it did for the TYPE-A valve.

The photograph in Fig.8(c) shows that cavitation was moderate for TYPE-B in comparison to the other three valve types. In TYPE-A and TYPE-B, in the vicinity of the upper wall of the position 1 Dia from the valve stem, vortex cavitation was remarkable as mentioned above.
