Fluid Mechanics and Thermodynamics of Turbomachinery, 5e

where B = 0.5 for a plain tip clearance, 0.25 for shrouded tips.
Reynolds number correction
(3.51)
Ainley and Mathieson (1951) obtained their data for a mean Reynolds number of
2 ¥ 10 5 based on the mean chord and exit flow conditions from the turbine state.
They recommended for lower Reynolds numbers, down to 5 ¥ 10 4 , that a correction be
made to stage efficiency according to the rough rule
Dunham and Came (1970) gave an optional correction which is applied directly to the
sum of the profile and secondary loss coefficients for a blade row using the Reynolds
number appropriate to that row. The rule is
Flow outlet angle from a turbine cascade
Two-dimensional Cascades 87
It was pointed out by Ainley (1948) that the method of defining deviation angle as
adopted in several well-known compressor cascade correlations had proved to be
impracticable for turbine blade cascade. In order to predict fluid outlet angle a2, steam
turbine designers had made much use of the simple empirical rule that
(3.52a)
where Q is the opening at the throat, depicted in Figure 3.26, and s is the pitch. This
widely used rule gives a very good approximation to measured pitchwise averaged flow
angles when the outlet Mach number is at or close to unity. However, at low Mach
numbers substantial variations have been found between the rule and observed flow
angles. Ainley and Mathieson (1951) recommended that for low outlet Mach numbers
0 < M2 0.5, the following rule be used:
FIG. 3.26. Details near turbine cascade exit showing “throat” and suction-surface
curvature parameters.