THEORY OF TURBINE DESIGN 



The following table gives some of Fliegner's results : 



545 



It follows that with angles designed for entry without shock, the most 

 efficient speed will be less than the theoretical for values of ft less than 105, 

 and will be greater for greater values of ,8, a conclusion which is verified 

 by the results of experiment, and which is indicated by the values of w 2 

 given in the table on p. 542. 



Fliegner's results led him to the conclusion that the best value for /3 is 



, where /3 and y 



given by the formula ft 1 = 90 + ^^ 30 ^- 



O ks% 



are the values of inlet and discharge angles calculated in the usual way 

 for maximum efficiency, and where 6 2 and b 3 are the bucket widths at 

 entrance and exit. 



Again, in the pressure turbine, the assumption that the water is com- 

 pletely guided by the vanes is probably not even approximately true, 

 only a small proportion of the water being directly guided, and that only 

 on one side of the stream. Other parts of the stream may follow very 

 different paths. Moreover, unless the guide passag.es are parallel for some 

 short distance before the point of exit, the issuing stream may (as at p 

 in Fig. 257 c) fail to occupy the total exit area, so that, even apart from 

 the effect of the vane thickness, which will be considered later, the 

 effective area of flow is less than 2 TT r 2 Z> 2 . 



It is usual to allow for this effect approximately by making the calcula- 

 tions on the assumption that the coefficient of contraction is '95. The 

 same reasoning applies at the point of discharge. 



Further, we have the impossibility of taking full account of the various 

 frictional and eddy formation losses in the turbine, except when guided 

 by the results of experiment on the particular type of wheel under 

 consideration. 



The true value of the theory thus lies in its power of indicating the 

 relative influence of the different details of design on the efficiency, and 

 in its possibility of giving a preliminary design which may afterwards be 

 slightly modified by the results of experiment on some similar type of 

 machine, so as to approximate more nearly to that of the perfect turbine. 



H.A. N N 



