Fluid Mechanics and Thermodynamics of Turbomachinery, 5e

84 Fluid Mechanics, Thermodynamics of Turbomachinery
Total pressure loss correlations
The overall total pressure loss is composed of three parts, viz. (i) profile loss, (ii)
secondary loss, and (iii) tip clearance loss.
(i) A profile loss coefficient is defined as the loss in stagnation pressure across the
blade row or cascade, divided by the difference between stagnation and static pressures
at blade outlet; i.e.
(3.45)
In the Ainley and Mathieson method, profile loss is determined initially at zero incidence
(i = 0). At any other incidence the profile loss ratio Yp/Y p(i=0) is assumed to be
defined by a unique function of the incidence ratio i/is (Figure 3.24), where is is the
stalling incidence. This is defined as the incidence at which Yp/Y p(i=0) = 2.0.
Ainley and Mathieson correlated the profile losses of turbine blade rows against
space–chord ratio s/l, fluid outlet angle a2, blade maximum thickness–chord ratio t/l
and blade inlet angle. The variation of Yp(i=0) against s/l is shown in Figure 3.25 for
nozzles and impulse blading at various flow outlet angles. The sign convention used
for flow angles in a turbine cascade is indicated in Figure 3.27. For other types of
blading intermediate between nozzle blades and impulse blades the following expression
is employed:
(3.46)
FIG. 3.24. Variation of profile loss with incidence for typical turbine blading (adapted
from Ainley and Mathieson 1951).