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64 Fluid Mechanics, Thermodynamics of Turbomachinery
































                             FIG. 3.7. Streamline flow through cascades (adapted from Carter et al. 1950).

                            In a well-designed cascade tunnel it is most important that the flow near the
                          central region of the cascade blades (where the flow measurements are made) is
                          approximately two-dimensional. This effect could be achieved by employing a large
                          number of long blades, but an excessive amount of power would be required to
                          operate the tunnel. With a tunnel of more reasonable size, aerodynamic difficulties
                          become apparent and arise 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.
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