Handbook of Turbomachinery Second Edition Revised - Ventech!

etween the rotating assembly and the support structure (stator) accompany
the flow of the fluid through the turbomachine. Usually, these forces are
large enough to significantly impact the rotordynamic characteristics of the
system. Examples of these fluid forces are associated with aerodynamic cross
coupling, impeller–diffuser interaction, propeller whirl, turbomachinery
whirl, and fluid entrapment. Fluid dynamicists have been quite successful in
modeling the fluid flow through turbomachinery using computational fluid
dynamics and/or empirical techniques and are able to provide a reasonable
prediction of the fluid forces. This information can then be used in a
structural dynamics design/analysis to predict the rotordynamic characteristics
of the machine.
The Rotating Assembly
A rotating assembly for a high-speed rotor dynamic system is a straight
axisymmetric structural group. Rotating asymmetries tend to introduce
destabilizing conditions that should be avoided in rotor designs, e.g., Tondi
[20]. A rotor consists of many possible components, depending on the
application, and includes compressor wheels, impellers, turbine disks, and
shaft segments (constant or variable cross section). For analysis and design
purposes, rotating assemblies are usually approximated by a set of
interconnected idealized components that include rigid or flexible disks
and rigid or flexible shaft segments. For example, the generalized Laval–
Jeffcott rotor system of Fig. 21 and the straddle-mounted rotor system of
Fig. 25 include rigid disks and uniform flexible shaft segments. To
demonstrate this concept more clearly, a suggested physical model is shown
for the rotating assembly of the gas turbine illustrated in Fig. 31.
Rigid Disk
A rigid disk, as used in the generalized Laval–Jeffcott rotor system of
Fig. 21, is afour-degree-of-freedom component whose position is defined by
two transverse translational displacements ðu,vÞ and two small-order
transverse rotational displacements ðb,gÞ as illustrated in Fig. 21 and also
in Fig. 32(a). The properties of the disk are defined by its mass, mass center
location ðcgxÞ, diametral moment of inertia, and polar moment of inertia.
The disk is referred to as ‘‘thin’’ if the axial extent of the disk, at the
connection point, is negligible compared to the overall length of the rotating
assembly. If the disk occupies a significant axial space, as part of the
rotating assembly, the disk is usually modeled as a thick rigid disk, as
illustrated in Fig. 32(b). In this situation, arigid-body constraint exists
Copyright © 2003 Marcel Dekker, Inc.