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Axial Flow Compressors and Centrifugal Compressors
Although centrifugal compressors are more robust, easier and cheaper, it has some disadvantages over an axial one;
- Comparing the two with the same frontal area, the axial flow compressor can consume much more air
- Axial flow compressor also attains much bigger compression ratios
Principles of the centrifugal flow compressor
- Turbine rotates the impeller of the compressor at high speed
- Air is continuously introduced into the eye of the impeller and flows outwards to the tip
- The shape of the vanes in the impeller are divergent, increasing the pressure
Principles of the axial flow compressor
- Several rows of rotating blades and rows of stationary blades
- 1 row of rotor blades followed by 1 row of stator bladed make up '1 stage'
- The spaces between these blades form divergent passages
- In the rotor: both pressure and velocity increase
- In the stator: pressure increases, velocity decreases
To maintain the axial velocity of the airflow (air is compressed into smaller volumes so velocity decreases), the air annulus (space between rotor drum and outer casing) is made convergent to increase the velocity.
Compressor Stall
When the aircraft slows down the compression ratio also decreases. The air is not being compressed as much and it can take up a larger volume. At the high pressure and of the compressor, this will make it difficult for the increased volume of air to pass through the space which can cause choking and turbulence.
Compressor Surge
If the situation keeps getting worse, eventually it'll cause a complete breakdown of the airflow through the engine. This can also cause reversal of the gases in the engine, being expelled through the intake with a loud bang.
If this occurs, the throttle of the affected engine must be closed slowly. There will also be a rise in EGT and a yawing moment due to the loss of thrust in one engine.
Prevention of stall and surge
- Variable Inlet Guide Vanes (VIGV)
The vanes are fitted just before the first rotor stage, they can be automatically pivoted around their axis to vary the airflow's path going into the compressor. It thus maintains the proper relationship between compressor rotational speed and airflow in the front stages.
- Variable Stator Vanes
Placed further down into the compressor and can also pivot automatically so that they maintain an acceptable angle of attack onto the following rotor blades.
- Compressor Bleeds
Can be opened at low RPM or during engine acceleration to allow some of the excess volume of air to escape. Opening the bleeds causes a drop in thrust however because the mass of air going through the compressor decreases which both increases specific fuel consumption and EGT (drop in air available for cooling).
- Multi-spool Compressors
A spool is the unit including the compressor, shaft and turbine. An engine can have multiple spools, each one increases in speed (towards the HP Compressor).
Upon closing the throttle, the speed of the LP spool falls more rapidly than the HP spools which causes the angle of attack to be maintained over a much wider range.
- Active Clearance Control
The size of the air annulus at the HP end of the compressor is varied. By cooling the compressor casing, we can cause it to shrink and so achieve the desired clearance between it and the blade tips.
Rotor and Stator blades
- Both are aerofoil shaped
- Rotor blades are attached to the drum while stator blades are attached to the outer casing
- Method of fixing the rotor blades is the dovetail system where they just "slide" them in and use no screw/bolts , the centrifugal forces will force them into their places
Although centrifugal compressors are more robust, easier and cheaper, it has some disadvantages over an axial one;
- Comparing the two with the same frontal area, the axial flow compressor can consume much more air
- Axial flow compressor also attains much bigger compression ratios
Principles of the centrifugal flow compressor
- Turbine rotates the impeller of the compressor at high speed
- Air is continuously introduced into the eye of the impeller and flows outwards to the tip
- The shape of the vanes in the impeller are divergent, increasing the pressure
Principles of the axial flow compressor
- Several rows of rotating blades and rows of stationary blades
- 1 row of rotor blades followed by 1 row of stator bladed make up '1 stage'
- The spaces between these blades form divergent passages
- In the rotor: both pressure and velocity increase
- In the stator: pressure increases, velocity decreases
To maintain the axial velocity of the airflow (air is compressed into smaller volumes so velocity decreases), the air annulus (space between rotor drum and outer casing) is made convergent to increase the velocity.
Compressor Stall
When the aircraft slows down the compression ratio also decreases. The air is not being compressed as much and it can take up a larger volume. At the high pressure and of the compressor, this will make it difficult for the increased volume of air to pass through the space which can cause choking and turbulence.
Compressor Surge
If the situation keeps getting worse, eventually it'll cause a complete breakdown of the airflow through the engine. This can also cause reversal of the gases in the engine, being expelled through the intake with a loud bang.
If this occurs, the throttle of the affected engine must be closed slowly. There will also be a rise in EGT and a yawing moment due to the loss of thrust in one engine.
Prevention of stall and surge
- Variable Inlet Guide Vanes (VIGV)
The vanes are fitted just before the first rotor stage, they can be automatically pivoted around their axis to vary the airflow's path going into the compressor. It thus maintains the proper relationship between compressor rotational speed and airflow in the front stages.
- Variable Stator Vanes
Placed further down into the compressor and can also pivot automatically so that they maintain an acceptable angle of attack onto the following rotor blades.
- Compressor Bleeds
Can be opened at low RPM or during engine acceleration to allow some of the excess volume of air to escape. Opening the bleeds causes a drop in thrust however because the mass of air going through the compressor decreases which both increases specific fuel consumption and EGT (drop in air available for cooling).
- Multi-spool Compressors
A spool is the unit including the compressor, shaft and turbine. An engine can have multiple spools, each one increases in speed (towards the HP Compressor).
Upon closing the throttle, the speed of the LP spool falls more rapidly than the HP spools which causes the angle of attack to be maintained over a much wider range.
- Active Clearance Control
The size of the air annulus at the HP end of the compressor is varied. By cooling the compressor casing, we can cause it to shrink and so achieve the desired clearance between it and the blade tips.
Rotor and Stator blades
- Both are aerofoil shaped
- Rotor blades are attached to the drum while stator blades are attached to the outer casing
- Method of fixing the rotor blades is the dovetail system where they just "slide" them in and use no screw/bolts , the centrifugal forces will force them into their places
