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National Advisory Committee for Aeronautics, Technical Notes - Smoke Study of Nozzle Secondary Flows in a Low Speed Turbine

Smoke was used to visualize boundary-layer and wake secondary-flow
phenomena in the nozzle passages of a low—speed turbine, and the flow
patterns were recorded in both still and motion pictures. The two annu—
lar cascades of turbine nozzles which were used Were designed for con-
stant discharge angle from hub to tip, but they differed in blade shape
and suction-surface velocity distribution. Cross-channel secondary
flows were similar for both cascades, but radial-flow patterns and
outer-shroud vortex formation differed. This flow behavior at low air-
speed (7 ft/sec) agreed with that previously indicated for the same
blades by pressure and flow-angle measurements near sonic speed.

The effect of a downstream rotor on nozzle secondary flows was also
studied. Motion of the rotor blade row disturbed nozzle trailing—edge
radial flows at low rotor speeds and produced pulsations in the radial
flow from the outer shroud. At increased rotor speed the amplitude of
the radial—flow pulsations decreased. I

The motion pictures prepared as a supplement to this report may be
obtained on loan from NACA Headquarters, Washington, D.C.

When fluid in an annular cascade is turned, the resulting mainstream
pressure gradients are imposed on the boundary layers of lowemomentum
fluid on the walls and blades. Boundary-layer turning equal to the free-
stream turning is not sufficient to balance the imposed pressure gradients
and the centrifugal forces associated with motion along a curved path.
Therefore, more than free-stream turning of the low—momentum boundary-
layer fluid results.

Experimental investigations of these deviations of flow direction
in boundary layers and wakes, herein referred to as secondary flows, are
reported in references 1 and 2. The object of these studies was to
clarify the nature and causes of such flows and to present information
that permits an estimate of the extent of their influence on cascade and
turbine performance. In reference 1, secondary flows of low-speed air
in two-dimensional cascades were mapped by such visualization techniques
as smoke filaments and chemical traces. In reference 2, which extended
the work to turbine nozzles in annular cascades at Operational airspeeds,
similar patterns were obtained, with radial flows added in these three—
dimensional cases.