Fluid Mechanics and Thermodynamics of Turbomachinery, 5e

72 Fluid Mechanics, Thermodynamics of Turbomachinery
FIG. 3.13. Losses in a compressor stage (Howell 1945a and 1945b). (Courtesy of the
Institution of Mechanical Engineers).
Turbine cascade performance
Figure 3.14 shows results obtained by Ainley (1948) from two sets of turbine cascade
blades, impulse and “reaction”. The term reaction is used here to denote, in a qualitative
sense, that the fluid accelerates through the blade row and thus experiences a pressure
drop during its passage. There is no pressure change across an impulse blade row.
The performance is expressed in the form l =Dpo/(p o2 - p 2) and a 2 against incidence.
From these results it is observed that
(i) the reaction blades have a much wider range of low loss performance than the
impulse blades, a result to be expected as the blade boundary layers are subjected
to a favourable pressure gradient,
(ii) the fluid outlet angle a2 remains relatively constant over the whole range of
incidence in contrast with the compressor cascade results.
For turbine cascade blades, a method of correlation is given by Ainley and Mathieson
(1951) which enables the performance of a gas turbine to be predicted with an estimated
tolerance of within 2% on peak efficiency. In Chapter 4 a rather different
approach, using a method attributed to Soderberg, is outlined. While being possibly
slightly less accurate than Ainley’s correlation, Soderberg’s method employs fewer
parameters and is rather easier to apply.
Compressor cascade correlations
Many experimental investigations have confirmed that the efficient performance of
compressor cascade blades is limited by the growth and separation of the blade surface
boundary layers. One of the aims of cascade research is to establish the generalised
loss characteristics and stall limits of conventional blades. This task is made difficult
because of the large number of factors which can influence the growth of the blade