Compressor map

Compressor maps are an integral part of predicting the performance of gas turbine and turbocharged engines, both at design and off-design conditions.

Satisfactory operation of the compressor relies on controlling the angle at which the gas approaches rotating and stationary blades to within an acceptable range.

Deviating from the optimum first results in increased losses/reduced efficiency then either stalling or sonic velocity/choking which occur in the blade passages at opposite ends of an axial compressor at the same time.

A NACA[clarification needed] report[6] illustrates pictorially the difference in contraction required at the design condition and at low speed.

[8] Rows of variable stators or split compressors, which allowed the front stages to speed up and the rear to slow down relative to each other, would also be introduced for the same reason.

[citation needed] Compressor bleed up to this point had been only necessary for starting and accelerating beyond low corrected speeds where its loss to thrust production, from dumping overboard, was not important.

A split compressor with this bypass arrangement allowed the highest pressure ratio of any Rolls-Royce engine, at that time, without the need for variable inlet guide vanes or interstage bleed.

The map may be produced by driving the compressor with an electric motor with the flow resistance selected artificially using a variable area throttle valve.

Parameter groups which are used as the basis for gas turbine engine compressor maps are total-pressure ratio (Pexit/Pinlet),

A final step is to give the pseudo-non-dimensional parameters standard units for mass flow and speed and more recognizable numerical values by applying pressure and temperature ratio correction factors, also derived as part of the dimensional analysis.

, because non-dimensional airflow is a form of fluid Mach number while fuel is flow of an incompressible energy source.

Fuel flow is also shown on a compressor map, but in the form of its effect, ie turbine inlet temperature.

Grandcoing[16] shows the constant temperature lines crossed as a helicopter compressor goes from no-load to full-load with increasing fuel flow.

Since the 'day' conditions are those at entry to the compressor an extremely 'hot' day is produced artificially by the ram temperature rise at high Mach numbers.

[24][25] The same corrected operating point required the same solution to prevent stalling and increase efficiency which was to bleed air from the 4th compressor stage.

[29] Hiereth et al.[30] shows a turbocharger compressor full-load, or maximum fuelling, curve runs up close to the surge line.

[31] Excess power is available to inadvertently take the compressor beyond the overload limit to a hazardous condition on cold days if it is driven by a gas turbine.

Single-shaft engines which drive an electric generator or helicopter rotor/aircraft propeller run with the compressor at no-load while accelerating to operating speed.

Grandcoing[37] shows the Turbomeca Artouste helicopter engine constant speed operation from no-load idle to maximum power.

Grandcoing[37] also shows the effect of a rapid load increase where the speed droops before regaining its required setting.

Variable vane angles and flow areas (bleed valves) in the compressor don't change the running line at a particular operating point because the angles and valve positions are unique for a corrected speed, that is they are controlled according to a schedule against corrected speed.

[39] Low-speed rear-stage turbining[40][41] occurs with excessive negative incidence leading to a pressure ratio less than one and the compressor stage absorbing power from the airflow.

Two examples where crossing the surge line prevented accelerating to high speed occurred with the first designs of the Rolls-Royce Avon[42] and the IAE V2500[43] and required major compressor redesigns.

Rotating stall at low corrected speeds caused blade failures on early axial compressors.

If driven by a constant speed electric motor it may be controlled with variable inlet guide vanes or suction and discharge throttling.

During a slam-acceleration from a mid-throttle setting, the compressor working line will move rapidly towards surge and then slowly approach the steady state operating point, further up the map.

Owing to the nature of the constraints involved, the fan working lines of a mixed turbofan are somewhat steeper than those of the equivalent unmixed engine.

Consequently, even at cruise flight speeds, the cold (or mixed final) propelling nozzle may only be choked at high throttle settings.

The fan working lines become more curved and migrate quickly towards surge as flight Mach number decreases.

Increasing the nozzle area at low flight speeds brings the fan working line away from surge.