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.