In a dynamic compressor operating under normal, stable flow conditions, the flow through the compressor is essentially uniform around the annulus, i.e. it is axisymmetric, and the annulus-averaged flow rate is steady. Generally, if the compressor is operated too close to the peak pressure rise on the compressor pressure rise versus mass flow, constant speed performance map, disturbances acting on the compressor may cause it to encounter a region on the performance map in which fluid dynamic instabilities, known as rotating stall and/or surge, develop. This region is bounded on the compressor performance map by the surge/stall line. The instabilities degrade the performance of the compressor and may lead to permanent damage, and are thus to be avoided.
Rotating stall can be viewed as a two-dimensional phenomena that results in a localized region of reduced or reversed flow through the compressor which rotates around the annulus of the flow path. The region is termed "stall cell" and typically extends axially through the compressor. Rotating stall results in reduced output (as measured in annulusaveraged pressure rise and mass flow) from the compressor. In addition, as the stall cell rotates around the annulus it loads and unloads the compressor blades and may induce fatigue failure. Surge is a one-dimensional phenomena defined by oscillations in the annulus-averaged flow through the compressor. Under severe surge conditions, reversal of the flow through the compressor may occur. Both types of instabilities should be avoided, particularly in aircraft applications.
In practical applications, the closer the operating point is to the peak pressure rise, the less the compression system can tolerate a given disturbance level without entering rotating stall and/or surge. Triggering rotating stall results in a sudden jump (within 1-3 rotor revolutions) from a state of high pressure rise, efficient, axisymmetric operation to a reduced pressure rise, inefficient, non-axisymmetric operation. Returning the compressor to axisymmetric operation (i.e., eliminating the rotating stall region) requires lowering the operating line on the compressor performance map to a point well below the point at which the stall occurred. In practical applications, the compressor may have to be shut down and restarted to eliminate (or recover from) the stall. This is referred to as stall hysteresis. Triggering surge results in a similar degradation of performance and operability.
As a result of the potential instabilities, compressors are typically operated with a "stall margin". Stall margin is a measure of the ratio between peak pressure rise, i.e. pressure rise at stall, and the pressure ratio on the operating line of the compressor for the current flow rate. In theory, the greater the stall margin, the larger the disturbance that the compression system can tolerate before entering stall and/or surge. Thus, the design objective is to incorporate enough stall margin to avoid operating in a condition in which an expected disturbance is likely to trigger stall and/or surge. In gas turbine engines used to i power aircraft, stall margins of fifteen to thirty percent are common. Since operating the compressor at less than peak pressure rise carries with it a reduction in operating efficiency and performance, there is a trade-off between stall margin and performance.