Compressors
Axial and centrifugal compressors, rotor/stator action, stall/surge and its control (bleed valves, VSV/VIGV).
Compressors — summary notes
Free- Two compressor types: centrifugal (an impeller flings air outward, robust and simple, good pressure ratio per stage but large frontal area — small engines/APUs) and axial (many rotor/stator stages, small frontal area, high efficiency and pressure ratio — most large engines).
- In an axial stage the rotor blades add energy (speed) to the air and the following stator vanes (a diffuser) convert that speed into a pressure rise, straightening the flow for the next rotor.
- Compressor stall/surge happens when the blade angle of attack becomes too high and the flow breaks down — from too-high fuel flow (rapid acceleration), damage, or distorted intake flow. Surge is a violent flow reversal that can be heard as a bang and damage the engine.
- Surge is controlled by bleed valves (dumping excess compressor air at low RPM), variable inlet guide vanes (VIGVs) and variable stator vanes (VSVs) that keep the blade angle of attack correct across the speed range, and by multi-spool designs (separate N1/N2 shafts).
- Overall pressure ratio (OPR) is the ratio of compressor-delivery to inlet pressure; higher OPR improves thermal efficiency. Bleed air is tapped from the compressor for cabin, anti-ice and pneumatic services.
- ⚠ Exam trap: in the compressor the ROTOR speeds the air up (adds energy) and the STATOR (diffuser) turns that into a pressure rise; bleed valves and variable vanes exist to prevent surge, especially during acceleration and at low RPM.
- Pressure ratio
- OPR = P(compressor delivery) / P(inlet)
During a rapid slam-acceleration a technician hears a loud bang and sees a rise in EGT. What has happened and what design features prevent it?
Too much fuel too quickly over-pressurised the combustor relative to the compressor, raising blade angle of attack until the flow stalled and reversed — a compressor surge, which spikes EGT. It is prevented by scheduling fuel (acceleration control/FADEC), bleed valves that offload the compressor at low RPM, and variable stator vanes/VIGVs keeping the correct blade angle.
Compressors concept map
FreeCompressors
Compressors quiz
ProThis quiz is part of Engineer
Upgrade to unlock every quiz, mock exam and advanced simulation across all modules. Notes and mind maps stay free.