THEORY OF TURBINE DESIGN 



543 



Remarks on the Theory of Turbine Design. 



Although the preceding theory is extremely valuable, its limitations 

 are important and must be kept in view. In the first place, even with 

 vanes designed so as (theoretically) to prevent shock at entrance, experi- 

 ment shows that a certain contraction of section of the stream, as 

 indicated in Fig. 257 a and b, takes place, and also indicates that this con- 



FIG. 257. 



traction and tha subsequent re-expansion is greater with the theoretically 

 correct entrant angle than with one slightly different. When working at 

 part gate the ingoing stream may, if sufficiently reduced in width, not fill 

 the buckets at all, in which case the wheel runs as an impulse turbine, 

 the pressure at the exit from the guides falls to that in the discharge pipe, 

 and the outflow per unit area is largely increased. Since the wheel 

 speed is now altogether unsuited to the velocity of influx, the efficiency is 



