Fluid Mechanics and Thermodynamics of Turbomachinery, 5e

Rotating stall and surge
Axial-flow Compressors and Fans 165
A salient feature of a compressor performance map, such as Figure 1.10, is the
limit to stable operation known as the surge line. This limit can be reached by reducing
the mass flow (with a throttle valve) whilst the rotational speed is maintained
constant.
When a compressor goes into surge the effects are usually quite dramatic. Generally,
an increase in noise level is experienced, indicative of a pulsation of the air flow and
of mechanical vibration. Commonly, a small number of predominant frequencies are
superimposed on a high background noise. The lowest frequencies are usually associated
with a Helmholtz-type of resonance of the flow through the machine, with the inlet
and/or outlet volumes. The higher frequencies are known to be due to rotating stall and
are of the same order as the rotational speed of the impeller.
Rotating stall is a phenomenon of axial-compressor flow which has been the subject
of many detailed experimental and theoretical investigations and the matter is still not
fully resolved. An early survey of the subject was given by Emmons et al. (1959).
Briefly, when a blade row (usually the rotor of a compressor) reaches the “stall point”,
the blades instead of all stalling together as might be expected, stall in separate patches
and these stall patches, moreover, travel around the compressor annulus (i.e. they
rotate).
That stall patches must propagate from blade to blade has a simple physical explanation.
Consider a portion of a blade row, as illustrated in Figure 5.13 to be affected
by a stall patch. This patch must cause a partial obstruction to the flow which is
deflected on both sides of it. Thus, the incidence of the flow onto the blades on the right
of the stall cell is reduced, but the incidence to the left is increased. As these blades
are already close to stalling, the net effect is for the stall patch to move to the left; the
motion is then self-sustaining.
There is a strong practical reason for the wide interest in rotating stall. Stall patches
travelling around blade rows load and unload each blade at some frequency related
to the speed and number of the patches. This frequency may be close to a natural
frequency of blade vibration and there is clearly a need for accurate prediction of the
FIG. 5.13. Model illustrating mechanism of stall cell propagation: partial blockage due
to stall patch deflects flow, increasing incidence to the left and decreasing incidence
to the right.