Handbook of Turbomachinery Second Edition Revised - Ventech!

Nastran, which was developed by NASA. They can then more accurately
forecast steady-state stresses and thermal growth.
The combination of centrifugal forces and thermal expansions distorts
the shape of centrifugal and mixed-flow units. This event alone can be a
possible cause of structural failure. Moreover, the clearance between the
rotor blades and the casing can be decreased in one area and increased in
another, resulting in a failure by rubbing in one region of the component
even though there is excess clearance in another region. Growth analysis
using computer programs is almost mandatory in this case.
Blade Flutter and Impact Stresses
Rotating stall and blade flutter instigate blade vibrations and cause fatigue
failure in both rotors and stators. Failures have also resulted from impacts
between rotor blades and foreign objects, such as tools, rags, or birds.
Computer programs such as Nastran can analyze and identify the natural
frequencies of the bending and torsional modes of blade vibration. Other
programs have been developed within the industry for calculating thermal
growth and the effects of internal pressures. Note that these phenomena are
functions of the rotating blade speed. Similar programs have been developed
that can estimate the damage to be expected from impacts with foreign
objects. These analyses are preferably done by someone experienced in stress
and vibration calculations for compressors.
Carta [40] offers a good overview of the mechanism of flutter.
Contending with vibration problems triggered by rotating stall unfortunately
requires some experience, because neither the frequency nor the
magnitude of the forcing functions can be accurately defined in advance. We
know only that the stall zones probably excite blades at frequencies
corresponding to multiples of 40 to 60% of rotor speed. The lack of accurate
calculation procedures is the main reason why the design and development
of new compressors can sometimes be a time-consuming and expensive
venture.
Other Design Considerations
It might seem convenient to design compressors with hollow struts
immediately behind the last row of blades, or close to the inlet blading.
These struts would enclose accessory items such as tubes for delivering oil to
and from the bearings. They could also be part of the structural support of
the bearings. This arrangement should be avoided, particularly in axial
compressors where the close axial spacing of the blades inhibits
circumferential movement of gas between successive rows of blades. An
Copyright © 2003 Marcel Dekker, Inc.