مكائن توربنيه 7

3.1.3 The Runner
The runner is circular disc and has the blades fixed on one side. In high speed runners in which the blades are longer a circular band may be used around the blades to keep them in position. The shape of the runner depends on the specific speed of the unit. These are classified as (a) slow runner (b) medium speed runner (c) high speed runner and (d) very high speed runner.
The shape of the runner and the corresponding velocity triangles are shown in figure 3.3. The development of mixed flow runners was necessitated by the limited power capacity of the purely radial flow runner. A larger exit flow area is made possible by the change of shape from radial to axial flow shape. This reduces the outlet velocity and thus increases efficiency. As seen in the figure the velocity triangles are of different shape for different runners.
It is seen from the velocity triangles that the blade inlet angle ?1 changes from acute to obtuse as the speed increases. The guide vane outlet angle ?1 also increases from about 15° to higher values as speed increases.