Compressor map
A compressor map is a chart created for a compressor in a gas turbine engine. Complete maps are based on compressor rig test results or predicted by a special computer program. Alternatively the map of a similar compressor can be suitably scaled.
Compressor maps are an integral part of predicting the performance of a gas turbine engine, both at design and off-design conditions. Fans and turbines also have operating maps, although the latter are significantly different in appearance to that of compressors.
The x-axis is usually some function of compressor entry mass flow, usually corrected flow or non-dimensional flow, as opposed to real flow. This axis can be considered a rough measure of the axial Mach number of the flow through the device.
Normally the y-axis is pressure ratio (Pexit/Pinlet), where P is stagnation (or total head) pressure.
ΔT/T (or similar), where T is stagnation (or total head) temperature, is also used.
The slightly kinked diagonal line on the main (i.e. lower) part of the map is known as the surge (or stall) line. Above this line is a region of unstable flow, which is an area best avoided.
A compressor surge typically causes an abrupt reversal of the airflow through the unit, as the pumping action of the airfoils stall (akin to an aircraft wing stalling).
As the name suggests, surge margin provides a measure of how close an operating point is to surge. Unfortunately, there are a number of different definitions of surge margin. A popular one in use is defined as follows:
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