Fluid Mechanics and Thermodynamics of Turbomachinery, 5e

P S P S
c 2
Direction
of rotation
(a)
(b)
Radial Flow Gas Turbines 259
a 2 b 2
U 2
FIG. 8.5. Optimum flow condition at inlet to the rotor. (a) Streamline flow at rotor inlet;
p is for pressure surface, s is for suction surface. (b) Velocity diagram for the
pitchwise averaged flow.
Figure 8.5b indicates the average relative velocity w2, entering the rotor at angle b2
and giving optimum flow conditions at the vane leading edge. As the rotor vanes in
IFR turbines are assumed to be radial, the angle b2 is an angle of incidence, and as
drawn it is numerically positive. Depending upon the number of rotor vanes this angle
may be between 20 and 40deg. The static pressure gradient across the passage causes
a streamline shift of the flow towards the suction surface. Streamfunction analyses of
this flow condition show that the streamline pattern properly locates the inlet stagnation
point on the vane leading edge so that this streamline is approximately radial (see
Figure 8.5a). It is reasoned that only at this flow condition will the fluid move smoothly
into the rotor passage. Thus, it is the averaged relative flow that is at an angle of incidence
b2 to the vane. Whitfield and Baines (1990) have comprehensively reviewed
computational methods used in determining turbomachinery flows, including
streamfunction methods.
Wilson and Jansen (1965) appear to have been the first to note that the optimum
angle of incidence was virtually identical to the angle of “slip” of the flow leaving the
w 2