Page 75 - Fluid Mechanics and Thermodynamics of Turbomachinery

56 Fluid Mechanics, Thermodynamics of Turbomachinery

FIG. 3.1. Compressor cascade wind tunnels. (a) Conventional low-speed, continuous
running cascade tunnel (adapted from Carter et al. 1950). (b) Transonic/supersonic
cascade tunnel (adapted from Sieverding 1985).

of twist along their length, the amount depending upon the sort of “vortex design”
chosen (see Chapter 6). However, data obtained from two-dimensional cascades can
still be of value to a designer requiring the performance at discrete blade sections
of such blade rows.

Cascade nomenclature

A cascade blade profile can be conceived as a curved camber line upon which a
profile thickness distribution is symmetrically superimposed. Referring to Figure 3.2
the camber line y.x/ and profile thickness t.x/ are shown as functions of the distance
x along the blade chord l. In British practice the shape of the camber line is usually
either a circular arc or a parabolic arc defined by the maximum camber b located at
distance a from the leading edge of the blade. The profile thickness distribution may
be that of a standard aerofoil section but, more usually, is one of the sections specif-
ically developed by the various research establishments for compressor or turbine
applications. Blade camber and thickness distributions are generally presented as
tables of y/l and t/l against x/l. Some examples of these tables are quoted by
Horlock (1958, 1966). Summarising, the useful parameters for describing a cascade
blade are: camber line shape, b/l, a/l, type of thickness distribution and maximum
thickness to chord ratio, t max /l.