Handbook of Turbomachinery Second Edition Revised - Ventech!

The two-cell pattern with a relatively short circumferential wavelength
rotates in the absolute frame of reference at a faster speed (60% of rotation
speed) before merging to a more common single-cell pattern rotating at a
slower speed (48% of rotation speed). However, if the rotor–stator blade
counts are such that the interference wavelength is the whole annulus, a
single-cell pattern will be triggered directly, as shown in Fig. 8, where the
rotor and stator blade numbers are 10 and 9, respectively. The overall
observation is that although a single-cell pattern rotating at about 50% rotor
speed is the most common one, a multiple-cell structure might occur at an
initial stage of stall inception if the stalling blade row is subject a distortion
disturbance of the same circumferential wave pattern. A numerical analysis
of 3D unsteady CFD results for a fan rotor at subsonic and relatively
supersonic flow conditions [16] suggests that the basic stall inception
mechanisms in high-speed transonic flows might be qualitatively different
from that at low speeds due to the difference between propagating
information upstream in subsonic flow and in supersonic flow.
So far, most of the research activities are largely aimed at providing
detailed (even qualitative) information to help understanding, motivated by
the prospect of active control/depression of rotating stall. We must
appreciate that true prediction of exact stall onset conditions for
determining suitable stall margins during designs is still extremely difficult,
depending strongly on numerical resolution, turbulence modeling, and
boundary condition treatment of truncated computational domains.
Aeroelastic Instability (Flutter)
Blade flutter is a self-excited aeroelastic instability phenomenon. When
working blades are disturbed aerodynamically or mechanically, they tend to
vibrate in their natural modes with small amplitudes, which will in turn
induce unsteady aerodynamic forces. At certain conditions, the unsteady
forces resulting from the initial blade vibration will do a net work (energy
input) to the blade in each period of vibration. As a result, the initially small
vibration will be amplified and the instability (flutter) will occur, often
leading to blade failure if there is not enough mechanical damping to
dissipate the energy.
Flutter can be a serious problem for frontal stages of compressors, in
particular fans of aero-engines, and needs to be addressed during a design
process. Figure 9shows typical blade flutter boundaries in an axial-flow
compressor/fan performance map. Corresponding aerodynamic conditions
of the instabilities are indicated by the names given, which clearly suggest
that steady and unsteady aerodynamics play an essential part in the
aeroelastic instability. At low off-design speeds, danger exists under high
Copyright © 2003 Marcel Dekker, Inc.