Fluid Mechanics and Thermodynamics of Turbomachinery, 5e

278 Fluid Mechanics, Thermodynamics of Turbomachinery
FIG. 8.16. Sections of radial turbines of maximum static efficiency (adapted from
Rohlik 1968).
FIG. 8.17. Variation in blade speed–spouting velocity ratio (U2/c0) and nozzle blade
height/rotor inlet diameter (b 2/D 2) corresponding to maximum total-to-static efficiency
with specific speed (adapted from Rohlik 1968).
ing ratio of flow to specific work. At low W s all friction losses are relatively large
because of the high ratios of surface area to flow area. At high Ws the high velocities
at turbine exit cause the kinetic energy leaving loss to predominate. Figure 8.16 shows
several meridional plane sections at three values of specific speed corresponding to the
curve of maximum total-to-static efficiency. The ratio of nozzle exit height to rotor
diameter, b2/D 2, is shown in Figure 8.17, the general rise of this ratio with increasing
Ws reflecting the increase in nozzle flow area* accompanying the larger flow rates of
higher specific speed. Figure 8.17 also shows the variation of U2/c0 with Ws along the
curve of maximum total-to-static efficiency.
Clearance and windage losses
A clearance gap must exist between the rotor vanes and the shroud. Because of the
pressure difference between the pressure and suction surfaces of a vane, a leakage flow
*The ratio b 2/D 2 is also affected by the pressure ratio and this has not been shown.