Fluid Mechanics and Thermodynamics of Turbomachinery, 5e

222 Fluid Mechanics, Thermodynamics of Turbomachinery
Incidence angle, i deg
20
10
0.4 0.6 0.8 1.0
Slip factor
Introduction
n = 0
n = 1
n = 2
n = -1
(a) Radius ratio, r/r 1s
0
0.4 0.6 0.8 1.0
Even under ideal (frictionless) conditions the relative flow leaving the impeller of a
compressor or pump will receive less than perfect guidance from the vanes and the flow
is said to slip. If the impeller could be imagined as being made with an infinite number
of infinitesimally thin vanes, then an ideal flow would be perfectly guided by the vanes
and would leave the impeller at the vane angle. Figure 7.7 compares the relative flow
angle, b2, obtained with a finite number of vanes, with the vane angle, b¢2. A slip factor
may be defined as
Absolute flow angle, a 1 deg
40
20
n = 0
n = -1
n = 1
n = 2
(b) Radius ratio, r/r 1s
FIG. 7.6. Effect of prewhirl vanes on flow angle and incidence for a1s = 30deg, b1s =
60deg and b ¢ 1s = 60deg. (a) Incidence angle; (b) Inducer flow angle.
C r2
C qs
b¢ 2
W 2
b 2
U 2
C ¢ q2
C 2
C q2
b 2¢ is the vane angle
b 2 is the average
relative flow angle
FIG. 7.7. Actual and hypothetical velocity diagrams at exit from an impeller with back
swept vanes.