Chapter 21: Compressors
21.1 Purpose and Types
- The compressor's function is to take in air from the intake, compress it to a high pressure, and
- deliver it to the combustion chamber. There are two main types:
- Centrifugal Flow Compressor:
- Components: An impeller (a high-speed rotating rotor) and a diffuser (a set of stationary
- vanes).
- Operation: The impeller spins at high speed, flinging air outwards by centrifugal force. The
- air is accelerated and gains some pressure. It then passes into the diffuser, which is a
- divergent duct that slows the air down, converting its velocity into high pressure.
- Characteristics: Very robust and simple, but has a large frontal area and a limited
- compression ratio per stage (approx. 4:1).
- Axial Flow Compressor:
- Components: Consists of multiple stages, with each stage comprising a row of rotating
- blades (rotors) followed by a row of stationary blades (stators).
- Operation: Air flows parallel to the axis of the engine. The rotors accelerate the air, and the
- stators (which form divergent passages) slow it down, increasing its pressure. Each stage
- provides a small pressure increase (approx. 1.2:1), so many stages are needed to achieve a
- high overall compression ratio.
- Characteristics: Can handle a very large mass of air and achieve very high compression
- ratios (30:1 or more). This is the standard type for all modern transport aircraft engines.
21.2 Compressor Stall and Surge
- A compressor blade is an aerofoil and can stall just like a wing if its angle of attack becomes
- too high.
- Compressor Stall: Occurs when there is an imbalance between the incoming air's axial
- velocity and the compressor's rotational speed. This can cause one or more stages of the
- compressor to stall.
- Indications: Engine vibration and a rise in Exhaust Gas Temperature (EGT).
- Surge: A severe, complete breakdown of airflow through the compressor. The high-pressure
- air in the rear of the engine can flow violently forward and out of the intake, often with a loud
- bang. This causes a significant loss of thrust and a rapid rise in EGT.
- Prevention Systems: To prevent stall and surge, especially during acceleration or at low
- RPM, high-compression engines use:
- Variable Inlet Guide Vanes (VIGVs) and Variable Stator Vanes: These vanes
- automatically pivot to adjust the angle at which the air strikes the subsequent rotor blades,
- maintaining an optimal angle of attack.
- Compressor Bleed Valves: These valves are opened automatically at low RPM to bleed off
- excess air from the middle stages of the compressor, increasing the axial velocity in the front
- stages and preventing stalls.
- Multi-Spool Compressors: The compressor is split into two or three sections (Low Pressure,
- Intermediate Pressure, High Pressure), each driven by its own turbine at its own optimal
- speed. This allows the engine to operate efficiently over a wide range of RPM without
- stalling.