Handbook of Turbomachinery Second Edition Revised - Ventech!

that circumferential gradients measured by a stationary observer produce
time-unsteady flows relative to a rotor. This condition at the inlet is thus a
source of unsteady flow in the rotors. Wakes or pressure fields generated by
stator blades and other structures within the flow passages also cause rotors
to be exposed to time-unsteady flow.
Time-unsteady flows in stators are similarly produced by circumferential
gradients in rotors. The magnitude of the unsteadiness produced by
stators on rotors or rotors on stators decreases as the distance between them
increases.
Axial-flow compressors, which depend on adverse pressure gradients
for all the increases in pressure, are quite sensitive to nonuniform, or
distorted, inlet flow. Centrifugal and mixed-flow compressors are more
tolerant, probably because centrifugal force plays a large part in the
development of pressure ratio.
Rotating stall obviously causes unsteady flow in both rotors and
stators. Surging, which is primarily a pulsation of the axial component of
velocity, is another source of unsteady flow in both rotors and stators.
Time-unsteady flow has adverse aerodynamic and mechanical
consequences. There is some evidence, however, that performance improves
slightly when the unsteady flow caused by blade wakes is augmented by
placing rotors and stators close together. This benefit is usually small in
comparison to the danger of destructive vibrations that may be induced by
the increased unsteady forces on the blades. Noise generated within a
compressor is also increased when rotor and stator blades are close
together.
Surging is manifested by either pulsating flow or an abrupt
deterioration of pressure ratio and efficiency. The pulsations may be either
gentle or violent. Sustained operation during surge is to be avoided.
MECHANICAL DESIGN CONSIDERATIONS
This chapter is primarily devoted to the aerodynamic design of compressors.
However, mechanical problems can curtail operating life or even cause
structural failure. Either event limits the freedom of aerodynamic design.
Centrifugal force causes tensile stresses in rotor blades and disks that could
rupture them if proper provisions were not made in the design. Calculating
the expected steady-state and vibratory stresses, together with the thermal
growth of the various parts, offers the biggest challenge to the mechanical
designer. The placement of structural elements such as struts within the flow
passages also causes aerodynamic problems that must be recognized.
Copyright © 2003 Marcel Dekker, Inc.