Handbook of Turbomachinery Second Edition Revised - Ventech!

component that is perpendicular to the main flow. This movement is called
secondary flow. Some secondary flows move through the gas stream in the
same way that steam bubbles rise from the bottom of a steam kettle toward
the surface. This motion is related to boundary-layer stability.
Of interest at this time are two secondary flows. One crawls along the
casing or hub from the high- to the low-pressure side of the passage between
two blades. The other moves up and down the blades. The former originates
near the leading edge of a blade. The boundary layer at the hub or casing
comes under the influence of the stagnation pressure created by the main
stream near the leading edge. The boundary layer cannot overcome the
retarding pressure gradient and rolls up into an eddy. Most of this eddy
moves across the channel to the low-pressure side of the passage; from there
the slowly moving fluid within the eddy proceeds downstream to the trailing
edge (see Fig. 9).
A secondary flow eddy is very stable, and the momentum normally
supplied to a boundary layer by the turbulence of the main stream does not
penetrate it. The retarded flow is thus easily disturbed by the adverse
pressure gradients usually found on the low-pressure side and near the
trailing edge of compressor blades.
The other type of secondary flow is generated along blade surfaces and
in the blade wakes when the circumferential speed of a blade differs from
that of the neighboring gas. The flow is radially inward when the rotating
speed of the gas exceeds that of the blades; otherwise it is radially outward.
Many of the losses generated on the blades thus ultimately appear near the
hub or casing.
Still another source of secondary flow is the leakage of gas through the
clearance between the ends of the rotor or stator blades and the cylindrical
surfaces forming the outer casing or hub. This flow (Fig. 10) is from the
high- to the low-pressure surfaces of a blade, and it forms another slowly
moving stable vortex on the low-pressure side. Flow of gas over the end of a
blade also lowers the pressure difference across it. The useful work delivered
to the gas near a rotor tip is diminished, even though the local gas
temperature is raised by nonconservative forces that increase the entropy.
Smith [13] offers information about this subject.
Although secondary flows contain a large portion of the friction losses
in compressors and turbines, they are still poorly understood. It is important
to appreciate that they concentrate thick and stable boundary layers, in the
form of eddies, on the low-pressure side of blades near the hub and casing.
When an adverse pressure gradient is then imposed by the main stream, the
area occupied by the eddy is considerably enlarged; flow separation may
even be observed. The resulting diffusion losses are aggravated. This
behavior is particularly obvious at rotor tips, and the blade loading that can
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