Fluid Mechanics and Thermodynamics of Turbomachinery, 5e

Two-dimensional Cascades 65
applied
FIG. 3.7. Streamline flow through cascades (adapted from Carter et al. 1950).
from the tunnel wall boundary layers interacting with the blades. In particular, and as
illustrated in Figure 3.7a, the tunnel wall boundary layer mingles with the end blade
boundary layer and, as a consequence, this blade stalls resulting in a non-uniform flow
field.
Stalling of the end blade may be delayed by applying a controlled amount of suction
to a slit just upstream of the blade, and sufficient to remove the tunnel wall boundary
layer (Figure 3.7b). Without such boundary-layer removal the effects of flow interference
can be quite pronounced. They become most pronounced near the cascade “stalling
point” (defined later) when any small disturbance of the upstream flow field precipitates
stall on blades adjacent to the end blade. Instability of this type has been observed
in compressor cascades and can affect every blade of the cascade. It is usually characterised
by regular, periodic “cells” of stall crossing rapidly from blade to blade; the
term propagating stall is often applied to the phenomenon. Some discussion of the
mechanism of propagating stall is given in Chapter 6.
The boundary layers on the walls to which the blade roots are attached generate secondary
vorticity in passing through the blades which may produce substantial secondary
flows. The mechanism of this phenomenon has been discussed at some length by
Carter (1948), Horlock (1958) and many others and a brief explanation is included in
Chapter 6.
In a compressor cascade the rapid increase in pressure across the blades causes a
marked thickening of the wall boundary layers and produces an effective contraction
of the flow, as depicted in Figure 3.8. A contraction coefficient, used as a measure of