In a jet engine, there are two modes of compressor operation which have to be considered: 1) stationary mode, where engine parameters do not change (cruising flight); 2) transient mode, where engine parameters quickly change, with the engine accelerating to maximum thrust.
Stationary mode, or steady-state, operation is depicted by the equilibrium operating line, which implies balance of power between compressor and turbine. This means that the turbine provides as much power as the compressor demands. Aircraft flight operations require the engine to be able to intercept any point of the operating line quickly.
The most critical phase is when the engine is accelerated (transient mode). Accelerating the compressor can be accomplished only by having the turbine produce more power that the compressor is able to absorb. This can simply be done by injecting and burning additional fuel, much like opening the throttle in a car engine. As a result, turbine inlet temperature will rise, which has the effect that engine components downstream of the compressor (combustor, turbine, nozzle) are momentarily less accepting the flow.
In order to minimize the risk of a compressor surging, a surge margin (between operating line and surge line) is provided, usually by setting maximum operating line pressure ratios 20% below those of the surge line. However, a large surge margin is generally impossible to maintain over the entire operating regime of the compressor. At low-corrected engine speeds, for example, if no corrective action is taken at the compressor, the steady-state operating line will unavoidably approach the surge line, with the risk of blade vibration.
To prevent the steady-state operating line from nearing the surge line, compressors employ either of two methods: 1) bleeding air at mid-compressor stage to adjust mass flow rate to the turbine´s demands; 2) modulating compressor air flow through variable stator vanes.
Below, a schematic picture showing the surge in compressor operation.
