Handbook of Turbomachinery Second Edition Revised - Ventech!

allowable static pressure ratio of 1.4 seems to be a better guide for this event
when the surface is curved.
Friction Losses
Losses of this type are listed in published data concerning the flow in pipes
and other ducts. These losses are relatively low. Friction is also involved in
the actual mechanism of ideal shock losses. The principal action of friction
in compressors, however, is to create boundary layers, which are a zone
between the part of the fluid not retarded by friction and the solid
boundaries confining the flow. The relative speed of a fluid within the
boundary layers ranges from 0 at a bounding surface to that attained by the
flow particles undisturbed by friction. McDonald [8] summarizes some of
the basic concepts about boundary layers, which are the source of mixing
losses and secondary flows.
Mixing Losses
The low speeds of the fluid in boundary layers effectively block part of the
available passage area; continuity then requires the speed of the undisturbed
flow to increase and the local static pressures to be less than their ideal value.
However, if the static pressure is required to increase in the direction of flow,
as it does in diffusers, the flow in the boundary layer becomes even more
retarded and the thickness of the layer increases even further. This adverse
pressure gradient can completely stop part of the flow in a boundary layer
and even reverse it—a phenomenon known as flow separation.
At this point we must recognize the importance of other pressure
gradients that are at once perpendicular to both the bounding surfaces and
the direction of the flow itself. We also need to distinguish situations in
which the principal through-flow component is axial from those in which it
is radial. Pressure gradients perpendicular, or normal, to the fluid velocity
are created by curvature of the flow passages and by Coriolis accelerations.
These gradients have important effects in both radial and axial flows. The
Coriolis accelerations, however, which predominate in radial flows when the
blades are rotating, are more potent by an order of magnitude. Observe that
the normal pressure gradients associated with curved flow are proportional
to V2 s =R; the gradients caused by Coriolis accelerations are proportional to
2oVr. It can be shown that boundary layers are unstable when the scalar
product of the normal pressure gradient with the normal entropy gradient is
positive. They become unstable and thin on the high-pressure side of a
blade, especially in centrifugal compressors. Conversely, they are stable and
relatively thick on the low-pressure side; local turbulence can be so
Copyright © 2003 Marcel Dekker, Inc.