Page 178 - Fluid Mechanics and Thermodynamics of Turbomachinery

Axial-flow Compressors and Fans 159
operation appears possible on constant speed curves of positive slope and surge
appears to occur when this slope is zero or even a little negative. A more complete
understanding of surge in multistage compressors is only possible from a detailed
study of the individual stages performance and their interaction with one another.

Control of flow instabilities
Important and dramatic advances have been made in recent years in the under-
standing and controlling of surge and rotating stall. Both phenomena are now
regarded as the mature forms of the natural oscillatory modes of the compression
system (see Moore and Greizer 1986). The flow model they considered predicts
that an initial disturbance starts with a very small amplitude but quickly grows
into a large amplitude form. Thus, the stability of the compressor is equivalent to
the stability of these small amplitude waves that exist just prior to stall or surge
(Haynes et al. 1994). Only a very brief outline can be given of the advances in the
understanding of these unstable flows and the means now available for controlling
them. Likewise only a few of the many papers written on these topics are cited.
Epstein et al. (1989) first suggested that surge and rotating stall could be prevented
by using active feedback control to damp the hydrodynamic disturbances while they
were still of small amplitude. Active suppression of surge was subsequently demon-
strated on a centrifugal compressor by Ffowcs Williams and Huang (1989), also
by Pinsley et al. (1991) and on an axial compressor by Day (1993). Shortly after
this Paduano et al. (1993) demonstrated active suppression of rotating stall in a
single-stage low-speed axial compressor. By damping the small amplitude waves
rotating about the annulus prior to stall, they increased the stable flow range of
the compressor by 25%. The control scheme adopted comprised a circumferential
array of hot wires just upstream of the compressor and a set of 12 individually
actuated vanes upstream of the rotor used to generate the rotating disturbance struc-
ture required for control. Haynes et al. (1994), using the same control scheme as
Paduano et al., actively stabilised a three-stage, low-speed axial compressor and
obtained an 8% increase in the operating flow range.
Gysling and Greitzer (1995) employed a different strategy using aeromechanical
feedback to suppress the onset of rotating stall in a low-speed axial compressor.
Figure 5.15 shows a schematic of the aeromechanical feedback system they used.
An auxiliary injection plenum chamber is fed by a high pressure source so that high
momentum air is injected upsteam towards the compressor rotor. The amount of
air injected at a given circumferential position is governed by an array of locally
reacting reed valves able to respond to perturbations in the static pressure upstream
of the compressor. The reeds valves, which were modelled as mass-spring-dampers,
regulated the amount of high-pressure air injected into the face of the compressor.
The cantilevered reeds were designed to deflect upward to allow an increase of the
injected flow, whereas a downward deflection decreases the injection.
A qualitative explanation of the stabilising mechanism has been given by Gysling
and Greitzer (1995):

Consider a disturbance to an initally steady, axisymmetric flow, which causes a
small decrease in axial velocity in one region of the compressor annulus. In this