Compressor surge

A compressor stall is a local disruption of the airflow in a gas turbine or turbocharger compressor. It is related to compressor surge which is a complete disruption of the flow through the compressor. Stalls range in severity from a momentary power drop (occurring so quickly it is barely registered on engine instruments) to a complete loss of compression (surge) necessitating a reduction in the fuel flow to the engine.

Stall was a common problem on early jet engines with simple aerodynamics and manual or mechanical fuel control units, but has been virtually eliminated by better design and the use of hydromechanical and electronic control systems such as Full Authority Digital Engine Controls. Modern compressors are carefully designed and controlled to avoid or limit stall within an engine's operating range.

There are two types of compressor stall:

Rotating stall is a local disruption of airflow within the compressor which continues to provide compressed air but with reduced effectiveness. Rotating stall arises when a small proportion of airfoils experience airfoil stall disrupting the local airflow without destabilising the compressor. The stalled airfoils create pockets of relatively stagnant air (referred to as stall cells) which, rather than moving in the flow direction, rotate around the circumference of the compressor. The stall cells rotate with the rotor blades but at 50 to 70% of their speed, affecting subsequent airfoils around the rotor as each encounters the stall cell. Propagation of the instability around the flow path annulus is driven by stall cell blockage causing an incidence spike on the adjacent blade. The adjacent blade stalls as a result of the incidence spike, thus causing stall cell "rotation" around the rotor. Stable local stalls can also occur which are axi-symmetric, covering the complete circumference of the compressor disc but only a portion of its radial plane, with the remainder of the face of the compressor continuing to pass normal flow.

A rotational stall may be momentary, resulting from an external disturbance, or may be steady as the compressor finds a working equilibrium between stalled and unstalled areas. Local stalls substantially reduce the efficiency of the compressor and increase the structural loads on the airfoils encountering stall cells in the region affected. In many cases however, the compressor airfoils are critically loaded without capacity to absorb the disturbance to normal airflow such that the original stall cells affect neighbouring regions and the stalled region rapidly grows to become a complete compressor stall.

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Wikipedia