PONCELET WHEEL 



485 



ART. 124. THE PONCELET WHEEL. 



If the vanes of an undershot wheel, instead of being radial, are inclined 

 backward so as to make an angle /3 at the tips with the tangent to the 

 circumference, and if a be the angle between the direction of the approach 

 stream and this tangent, then by suitable adjustment of a, (3, and the 

 speed of rotation, the loss due to shock at entrance may be prevented, and 

 at the same time the discharge water may be given a backward velocity 







FIG. 192. 



relative to the wheel, thus reducing the absolute velocity of discharge and 

 the loss by rejection of kinetic energy to the tail-race. 



With flat vanes the best results are obtained when J3 is about 30, 

 and efficiencies of up to 55 per cent, may then be obtained. 



In the Poncelet wheel (Fig. 193) these vanes, instead of being flat, form 

 arcs of circles, and with this type of wheel efficiencies of from 60 to 70 

 per cent, are usual. 



To determine the correct vane angles, let 



AB (Fig. 192 a) represent the absolute velocity of water at entrance. 



CB represent the absolute velocity of vanes. 



Then AC represents the velocity of water relative to vanes, and to 

 avoid shock, the vane tips should be parallel to AC. 



Again, at the discharge point, let 



B'C' = velocity of vanes. 



C'A' = velocity of water relative to the vanes. 



Then B'A' = absolute velocity of discharge. 



F F 2 



