560 



HYPBAULICS AND ITS APPLICATIONS 



Evidently the whole series of angles depends entirely on the value to 

 be given to/ 2 . 



If the relative velocity of water and vane is unaltered by its passage 



through the wheel, we have 



sin /3 / a 1 



/ 2 cosec/?=/ 3 cosecy /. -j =- = -. 



Again, the effective sectional area of the wheel passages at inlet 

 = 2 TT ?- 2 6 2 sin (3, and at outlet = 2 TT r 3 b 3 sin y (neglecting the thick- 

 ness of the vanes), and since the effective area should increase rather 



FlG. 261. Girard Turbine with Outward Radial Flow. 



than diminish towards the outlet, in order to give free deviation of the 

 jet, we must have 



2 TT r 3 b 3 sin y = 



2 TT 7*2 &2 sin ft > 



In an axial flow machine this makes ~^. \ ( k ^ } equal to or greater! 



& 2 sin p \ & 2 / 



than 1, so that b a must be equal to or greater than -p This necessitates 



K 



the buckets being splayed out towards the exit as illustrated in Fig. 261 , 

 The buckets should not be more than *9 full at entrance, and *75 fulj 



at outlet. The pitch of the guides should then be less than that of th 



wheel vanes, the breadth & 2 being calculated to give the necessar J 



proportion full at entrance. 



The pitch of the vanes should be small to avoid excessive loss by shocj 



at entrance when the tip of the wheel vanes has passed the correspondinj 



guide vane. 



