Handbook of Turbomachinery Second Edition Revised - Ventech!

frequency and its higher harmonics [12]. Furthermore, because of its
sensitivity to flow conditions, vortex shedding may well couple with other
oscillatory acoustic or blade structural dynamic modes, and act as an
excitation source.
For passage shock oscillations, the flow physical mechanism is
typically associated with shock/boundary-layer interaction, resulting in a
thickened or separating boundary layer at the foot of passage shock wave.
The oscillating shock patterns (frequencies) are very often suspected to be
associated with acoustical modes within blade passages, similar to those
found in transonic ducts (e.g., [13]). An oscillating shock wave produces a
higher time-averaged loss than its steady counterpart. The existence of
shock oscillation can be easily identified from ‘‘steady’’ (time-averaged)
experimental data showing a ‘‘smeared’’ shock wave. The range of smearing
is normally a pretty good indication of the magnitude of the shock
oscillation.
Rotating Stall. Rotating stall is a circumferential flow instability, which is
normally regarded as a precursor of compressor and engine surge. Apart
from seriously detrimental effects on aerodynamic performances, it should
also be noted that stall onset generates severe blade vibration problems. The
vortical and reversal flow pattern produces transient aerodynamic loading
of a considerable magnitude, causing blades to be overstressed. There is a
need to estimate the maximum stress level in this fairly hostile environment,
especially if the stability boundary can only be identified by crossing the
boundary during experimental rig tests.
There has been a considerable amount of work recently resulting in
enhanced understanding of stall inception mechanisms based on experimental
observations, e.g., [14]. It should also be mentioned that full-scale
CFD simulations of stall inception have started to emerge which can help to
understand complex physical mechanisms involved and identify relevant
influencing parameters. For instance, computational studies using an
unsteady Navier–Stokes time-domain flow solver for a compressor stage
indicate that the initial stall inception pattern (number of cells, circumferential
wavelength, and rotating speed) corresponds to that set by rotor–
stator blade counts [15]. Figure 7shows the results at an initial stage of stall
inception for the rotor and stator rows with blade numbers of 10 and 12,
respectively. With these blade counts, the rotor and stator disturbances beat
circumferentially twice per rotor revolution, resulting in an interference
disturbance with a wavelength of half an annulus. This should explain why a
two-cell structure is triggered as shown by a snapshot of entropy contours at
the inception [Fig. 7(b)]. Figure 7(a) shows the time traces of axial velocities
from four circumferential stationary positions upstream of the rotor row.
Copyright © 2003 Marcel Dekker, Inc.