Effects of Power Extraction on Turbofan Compressor and Turbine Maps
The two diagrams below show the effects of shaft power extraction (shaft power offtake / shaft power off-take for the propulsion of accessories such as fuel-/hydraulic-/oil-pumps, generators etc.) on a high-bypass turbofan engine at a flight altitude of $H = 11km$ and a speed of $Ma = 0.8$, calculated with GasTurb 12:
Effects of Power Extraction on the Operating Line (Blue)
Why are the operating lines (blue) of both the compressor and turbine shifted to the left towards the surge line?
In particular, why does the non-dimensional corrected mass flow decrease, while the pressure ratio stays approximately constant?
The same question from a different angle: Why does the pressure ratio not decrease at a constant mass flow?
Shaft power extraction, no matter if taken from the High-Pressure (HP) shaft or the Low-Pressure (LP) shaft, will reduce the speed of this shaft. This reduces the mass flow [...] and the thrust [...]. Constant thrust regulation applied to the engine (achieved today by the Full Authority Digital Engine Control, FADEC) will primarily result in an increase of fuel flow, increasing the Turbine Entry Temperature, TET. Higher pressure in the combustion chamber and higher turbine load together with a reduced shaft speed will enlarge the angle of attack at the compressor blades and therewith slightly lift the pressure rise achieved at each stage. In this way, shaft power off-takes also result in closer operation to the surge line. [...]
If shaft power is taken from the LP shaft, its speed will reduce. Also, the fan speed is reduced, which decreases the thrust considerably. With the engine controls increasing the TET, the HP shaft (in this case not affected by power off-takes) will even increase its speed and mass flow through the LP turbine and compressor compared to the original situation. [...]
If shaft power is taken from the HP shaft, its speed will reduce and so also the mass flow through the LP compressor and turbine. With the engine controls increasing the TET, the situation is rectified.
Low thrust ratings always mean a high relative power extraction P/T [shaft power off-take P, jet engine thrust T] and high relative specific fuel consumption. At high thrust ratings, the relative power extraction P/T is smaller and the relative specific fuel consumption is less.