190 HYDRAULICS AND ITS APPLICATIONS 



for which the reduction in the eddy loss due to this relative motion may 

 exactly counterbalance the loss due to an increased area of the surface of 

 the casing. As the clearance is further increased the impact loss will 

 diminish until the effect of surface friction becomes the all-important 

 factor in producing the resistance. 



To determine whether, by the use of concentric circular baffles fixed to 

 the casing, the circulating currents might be localised so as to reduce 

 their effect, two series of experiments were carried out with two such baffles 

 in position on each side of the casing ; these baffles consisted of rings 

 respectively 5 inches and 10 inches in diameter and projecting \ inch. 

 The clearance between the casing and the disc was f inch in one series 

 and 1| inch in the other. The results showed that while in the first case 

 the resistance was unaltered by the baffles, in the second case it was 

 reduced by about 2 per cent. 



Effect of Radial Vanes on the Disc. In turbines and centrifugal pumps 

 having impellers of the open-vaned type, or having balancing discs fitted 

 with shallow radial vanes, it has usually been assumed that the increased 

 resistance over that obtaining with plain discs is negligible. To test this 

 a brass disc, 12 inches in diameter, was provided with four radial vanes 

 on each face. Two series of experiments were carried out, one with vanes 

 | inch deep, and the second with \ inch vanes. 



The results of these experiments, as compared with those carried out 

 on plain discs, are given below. 



The surprisingly high value of the resistance with these vanes is doubt- 

 less due to loss of energy in eddy formation behind the trailing edges of 

 the vanes. 



EFFECT OF RADIAL VANES. 



Effect of a Variation in Disc Diameter. As indicated on p. 180, similar 

 discs of different diameters and having clearances proportional to their 

 radii, would have resistances proportional to the (n -j- 3) power of these 

 radii. 



