Page 312 - Fluid Mechanics and Thermodynamics of Turbomachinery

Hydraulic Turbines 293

U 2 U 2

W 2 W 2
C 2 C 2

U 2 U 2

W 3
W 3
C 3
C 3
U 3
U 3
Design point - full load operation Part load operation
FIG. 9.13. Comparison of velocity triangles for a Francis turbine for full load and at part
load operation.

100
Kaplan

Pelton
Efficiency, per cent 60
80
Francis

40
20 40 60 80 100
Load ratio, per cent
FIG. 9.14. Variation of hydraulic efficiency for various types of turbine over a range of
loading, at constant speed and constant head.

It is of interest to note the effect that swirling flow has on the performance
of the following diffuser. The results of an extensive experimental investigation
made by McDonald et al. (1971), showed that swirling inlet flow does not affect
the performance of conical diffusers which are well designed and give unsepa-
rated or only slightly separated flow when the flow through them is entirely axial.
Accordingly, part load operation of the turbine is unlikely to give adverse diffuser
performance.