Fluid Mechanics and Thermodynamics of Turbomachinery, 5e

Hydraulic Turbines 317
using Thoma’s coefficient and the data shown in Figure 9.19, determine whether
cavitation is likely to occur. Also using the data of Wislicenus verify the result.
Solution. From tables of fluid properties, e.g. Rogers and Mayhew (1995), or using
the data of Figure 9.20, the vapour pressure for water corresponding to a temperature
of 25°C is 0.03166 bar. From the definition of NPSH, eqn. (9.24), we obtain
Thus, Thoma’s coefficient is, s = HS/H E = 8.003/150 = 0.05336.
At the value of WSP = 0.8 given as data, the value of the critical Thoma coefficient
sc corresponding to this is 0.09 from Figure 9.19. From the fact that s < sc, then the
turbine will cavitate.
From the definition of the suction specific speed
Cavitation coefficient, s
4.0
2.0
1.0
0.4
0.2
0.1
0.04
No cavitation
region
Severe
cavitation
region
Francis turbinesKaplan turbines
0.02
0.1 0.2 0.4 0.6 1.0 2.0 4.0 6 8 10
Power specific speed, Wsp (rad)
FIG. 9.19. Variation of critical cavitation coefficient with non-dimensional specific
speed for Francis and Kaplan turbines (adapted from Moody and Zowski 1969).