556 HYDRAULICS AND ITS APPLICATIONS 



In this type of turbine / 2 is generally given a value about *25 *J 2 g H', 

 while 7/2 varies from ['56 to '62] /</ 2 g H'. In general y lies between 

 20 and 30, and in high-class turbines a lies between 20 and 25. 



ART. 147. AXIAL FLOW PRESSURE TURBINE. 



Here the general theory is the same as for the inward and outward 

 radial flow types of turbine, and the same demonstrations hold good 

 if it be remembered that ? f 2 = r 3 = r ; 1/2 = t 8 = o> r ; w = 1. 



If, as assumed in the previous cases, the plane of the wheel is hori- 

 zontal, the equation of energy now becomes : 



2 f 2 



+ |L J + |L' + { W ork done in wheel + losses from (2) to 



(3) - h } per Ib. 

 where h = depth of wheel. 



This assumes no velocity of whirl at exit, so that/ 3 = i? a . 

 Also, from equation (16), p. 539, 



p 2 ps _/ 3 2 cosec 2 y / 2 2 cosec 2 /3 _ , 



s _ 3 2 



W 



the term ~ (l --- u expressing the centrifugal effect, now vanishing. 



Here / 2 generally equals from ['175 to '225] */ 2 g (H r h), while w a 

 varies from ['62 to '68] V 2 g (H' h). a varies from 16 to 22, while 

 y varies from 15 to 25. 



Evidently with a given value of a and of ft, at only one point in the 

 radius of the wheel for any given speed, will the conditions be suit- 

 able for entrance without shock. For entrance without shock at all 

 radii, the value of fi should change continuously, so as to suit the corre- 

 sponding peripheral speed. This construction is seldom carried out in 

 practice, it being usual to have helical vanes, with radial inlet and outlet 

 edges, both for guides and wheel, and to give a and (3 their correct 

 values at the mean radius. With such vanes, having a constant pitch, 

 the angle of inclination diminishes as the radius increases, and while 

 this is an advantage in the case of the guides, it is the reverse in the case 

 of the wheel, where the value of (3 for no shock should increase with 

 the peripheral velocity and therefore with the radius. Better results 

 would be obtained by giving the wheel vanes the same angle at all radii 

 although if the width of the buckets is small not exceeding T X Q the whee 

 diameter the loss from shock is trifling. 



