The rotating stall and surge in axial flow compressors are among the most serious problems in turbomachinery aerodynamics. They directly affect the thrust necessary to propel an aircraft at an adequate speed or to drive a generator in AC generation. Both phenomena have to do with the axial flow, and the flow in a multi-stage axial flow compressor is complex in nature because of the proximity of moving blade rows, the build up of endwall boundary layers, and the presence of leakage and secondary flows.
The rotating stall is a flow breakdown at one or more compressor blades. Therefore, it is a stagnated region of air which moves in the circumferential direction of rotor rotation. However, it does so at a fraction of the rotor speed. So, at any given throttle setting, it does not move axially in either direction, but it may cause pressure waves to move upstream (compression waves) or downstream (rarefaction waves). One can distinguish two types of stall; 1) blade stall, which is a two dimensional type of stall, where a significant portion of the blade has large wakes due to substantial thickening or separation of the suction surface boundary layer; 2) wall stall, which is an endwall boundary layer separation.
Surge is the response of the entire engine and it is characterized by a flow stoppage or reversal in the compression system. Upon surge, a compression component will unload by allowing the compressed fluid in downstream stages to expand in the upstream direction, thus forming a more or less planar wave, which at high speeds often leads to flow reversal. The compressor can recover and begin again to pump flow. However, if the surge-inducing cause is not removed, the compressor will surge again.
