Page 101 - Fluid Mechanics and Thermodynamics of Turbomachinery

82 Fluid Mechanics, Thermodynamics of Turbomachinery
the total pressure loss and gas efflux angle for each row of a turbine stage is
determined at a single reference diameter and under a wide range of inlet condi-
tions. This reference diameter was taken as the arithmetic mean of the rotor and
stator rows inner and outer diameters. Dunham and Came (1970) gathered together
details of several improvements to the method of Ainley and Mathieson which
gave better performance prediction for small turbines than did the original method.
When the blading is competently designed the revised method appears to give reli-
able predictions of efficiency to within 2% over a wide range of designs, sizes and
operating conditions.

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.
p 01 p 02
Y p D . (3.45)
p 02 p 2
In the Ainley method, profile loss is determined initially at zero incidence .i D 0/.
At any other incidence the profile loss ratio Y p /Y p.iD0/ is assumed to be defined by
a unique function of the incidence ratio i/i s (Figure 3.24), where i s is the stalling
incidence. This is defined as the incidence at which Y p /Y p.iD0/ D 2.0.
Ainley and Mathieson correlated the profile losses of turbine blade rows against
space/chord ratio s/l, fluid outlet angle ˛ 2 , blade maximum thickness/chord ratio t/l

FIG. 3.24. Variation of profile loss with incidence for typical turbine blading (adapted
from Ainley and Mathieson 1951).