Page 96 - Fluid Mechanics and Thermodynamics of Turbomachinery

Two-dimensional Cascades 77

FIG. 3.19. The off-design performance of a compressor cascade (Howell 1942). (By
courtesy of the Controller of H.M.S.O., Crown copyright reserved).

hence

3
D 0.017/0.719 D 0.0458.
The tangential lift force coefficient, eqn. (3.9), is

1 2
C f D .p 2 p 1 //. c / D 2.tan ˛ 1 tan ˛ 2 / D 1.596.
x
2
The diffuser efficiency, eqn. (3.25), is
D D 1 /.C f tan ˛ m / D 1 0.0458/.1.596 ð 0.969/ D 97%.

It is worth nothing, from the representative data contained in the above exercise,
that the validity of the approximation in eqn. (3.20) is amply justified.
Howell’s correlation, clearly, is a simple and fairly direct method of assessing
the performance of a given cascade for a range of inlet flow angles. The data can
also be used for solving the more complex inverse problem, namely, the selection