Page 236 - Fluid Mechanics and Thermodynamics of Turbomachinery

Centrifugal Pumps, Fans and Compressors 217

FIG. 7.11. Head correction factors for centrifugal impellers (adapted from Csanady
(1960)).

For radial vaned impellers this becomes s D 1 0.63 /Z but is often written for
convenience and initial approximate calculations as s D 1 2/Z.
Ferguson (1963) has usefully compiled values of slip factor found from several
theories for a number of blade angles and blade numbers and compared them with
0
known experimental values. He found that for pumps, with ˇ between 60 deg and
2
70 deg, the Busemann or Stodola slip factors gave fairly good agreement with
experimental results. For radial vaned impellers on the other hand, the Stanitz
expression, eqn. (7.18a) agreed very well with experimental observations. For inter-
0
mediate values of ˇ the Busemann slip factor gave the most consistent agreement
2
with experiment.
Wiesner (1967) reviewed all the available methods for calculating values of slip
factor and compared them with values obtained from tests. He concluded from
all the material presented that Busemann’s procedure was still the most generally
applicable predictor for determining the basic slip factor of centripetal impellers.
Wiesner obtained the following simple empirical expression for the slip velocity:
p
U 2 cosˇ 0 2
c s D . (7.19a/
Z 0.7
and the corresponding slip factor
p 0 0.7
cosˇ /Z
2
w D 1 , (7.19b/
0
.1 2 tan ˇ /
2
which, according to Wiesner, fitted the Busemann results “extremely well over the
whole range of practical blade angles and number of blades”.