Page 177 - Fluid Mechanics and Thermodynamics of Turbomachinery

158 Fluid Mechanics, Thermodynamics of Turbomachinery

FIG. 5.14. Stability of operation of a compressor (adapted from Horlock 1958).

It is possible to distinguish between surge and propagating stall by the unsteadi-
ness, or otherwise, of the total mass flow. The characteristic of stall propagation is
that the flow passing through the annulus, summed over the whole area, is steady
with time; the stall cells merely redistribute the flow over the annulus. Surge, on the
other hand, involves an axial oscillation of the total mass flow, a condition highly
detrimental to efficient compressor operation.
The conditions determining the point of surge of a compressor have not yet been
completely determined satisfactorily. One physical explanation of this breakdown
of the flow is given by Horlock (1958).
Figure 5.14 shows a constant rotor speed compressor characteristic .C/ of pres-
sure ratio plotted against flow coefficient. A second set of curves (T 1 , T 2 , etc.) are
superimposed on this figure showing the pressure loss characteristics of the throttle
for various fixed throttle positions. The intersection of curves T with compressor
curve C denotes the various operating points of the combination. A state of flow
stability exists if the throttle curves at the point of intersection have a greater
(positive) slope than the compressor curve. That this is so may be illustrated as
follows. Consider the operating point at the intersection of T 2 with C. If a small
reduction of flow should momentarily occur, the compressor will produce a greater
pressure rise and the throttle resistance will fall. The flow rate must, of necessity,
increase so that the original operating point is restored. A similar argument holds
if the flow is temporarily augmented, so that the flow is completely stable at this
operating condition.
If, now, the operating point is at point U, unstable operation is possible. A small
reduction in flow will cause a greater reduction in compressor pressure ratio than the
corresponding pressure ratio across the throttle. As a consequence of the increased
resistance of the throttle, the flow will decrease even further and the operating point
U is clearly unstable. By inference, neutral stability exists when the slopes of the
throttle pressure loss curves equal the compressor pressure rise curve.
Tests on low pressure ratio compressors appear to substantiate this explanation
of instability. However, for high rotational speed multistage compressors the above
argument does not seem sufficient to describe surging. With high speeds no stable