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Handbook of Turbomachinery Second Edition Revised - Ventech!

Handbook of Turbomachinery Second Edition Revised - Ventech!

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The blade velocity u1 is determined by using the speed ratio f, which is<br />

defined as the ratio <strong>of</strong> blade velocity to fluid velocity V1:<br />

f ¼ u1=V1<br />

ð12Þ<br />

Its values range from 0.6 to 0.9 for Ns ranging from 25 to 85. The knowledge<br />

<strong>of</strong> the blade velocity can be used in computing the runner diameter. The next<br />

step in the design process is to size the gates and the runner. The most<br />

important dimension in this regard is the height <strong>of</strong> the gates, which in a<br />

sense also determines the height <strong>of</strong> the runner. This can be easily done since<br />

the flow rate, the velocity <strong>of</strong> water at entrance, and the angle a1 are known.<br />

Thus, the height <strong>of</strong> the gates B is given by<br />

B ¼ Q=ðV1pCD sin a1Þ ð13Þ<br />

In this above equation, C is a coefficient that allows for the thickness <strong>of</strong> the<br />

gates. The value <strong>of</strong> C is usually about 0.95. The relative velocity can now be<br />

computed using the law <strong>of</strong> cosines as follows (see Fig. 10):<br />

w1 ¼ðu 2 1 þ V2 1<br />

2u1V1 cos a1Þ 1=2<br />

Figure 10 Velocity triangle at entrance <strong>of</strong> Francis runner.<br />

Copyright © 2003 Marcel Dekker, Inc.<br />

ð14Þ

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