(12b)
and, for an axial flow turbine,
Ωo = Ωi = π (r22 − r12).
(12c)
| Fig. 194. |
Relative and Common Velocity of the Water and Wheel.—There is another way of resolving the velocity of the water. Let V be the velocity of the wheel at the point c, fig. 194. Then the velocity of the water may be resolved into a component V, which the water has in common with the wheel, and a component vr, which is the velocity of the water relatively to the wheel.
Velocity of Flow.—It is obvious that the frictional losses of head in the wheel passages will increase as the velocity of flow is greater, that is, the smaller the wheel is made. But if the wheel works under water, the skin friction of the wheel cover increases as the diameter of the wheel is made greater, and in any case the weight of the wheel and consequently the journal friction increase as the wheel is made larger. It is therefore desirable to choose, for the velocity of flow, as large a value as is consistent with the condition that the frictional losses in the wheel passages are a small fraction of the total head.
The values most commonly assumed in practice are these:—
| In axial flow turbines, | uo = ui = 0.15 to 0.2 √(2gH); |
| In outward flow turbines, | ui = 0.25 √2g (H − ɧ), |
| uo = 0.21 to 0.17 √2g (H − ɧ); | |
| In inward flow turbines, | uo = ui = 0.125 √(2gH). |
§ 191. Speed of the Wheel.—The best speed of the wheel depends partly on the frictional losses, which the ordinary theory of turbines disregards. It is best, therefore, to assume for Vo and Vi values which experiment has shown to be most advantageous.