Compressors
Axial and centrifugal compressors, rotor/stator action, stall/surge and its control (bleed valves, VSV/VIGV).
Compressors, summary notes
- 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.
Compressor stall & surge
Compressors concept map
Compressors
Compressors quiz
Compressors, quiz
1. Compressor stall or surge is caused by:
The blade angle of attack becoming too great, breaking down the airflowToo much fuel in the tank, raising the aircraft weight and loading the compressor beyond its limitLow oil pressure, letting the compressor shaft run down until the blade tips contact the casing2. Variable inlet guide vanes and bleed valves are fitted to:
Keep the compressor out of stall at low RPM and during accelerationIncrease the exhaust noise, which is accepted as the price of a wider operating rangeReduce the fuel flow to zero during deceleration, so the compressor cannot be over-fuelled3. A compressor has 10 stages, each giving a pressure ratio of 1.3. The overall pressure ratio is approximately:
13.8:113:11.3:14. In the velocity triangle shown at a compressor blade, the resultant is the vector sum of the axial velocity and:
The blade speedThe exhaust gas velocityThe aircraft speed5. A compressor of 12:1 overall pressure ratio is fed at 100 kPa. Its delivery pressure is:
1200 kPa112 kPa833 kPa6. A compressor raises pressure from 100 kPa to 1200 kPa. Its pressure ratio is:
12:11200:111:17. In an axial compressor stage, the rotor blades:
Add energy (speed) to the airOnly straighten the flowReduce the air pressure8. The stator vanes in a compressor act as a:
Diffuser, converting speed into a pressure riseNozzle accelerating the flowFuel injector