Turbine Engine Compressors Flashcards
What are the advantages of a centrifugal compressor over an axial flow compressor?
- Cheaper and easier to manufacture.
- More robust.
- Better able to handle FOD ingestion.
- Less prone to stalling and surging.
- Relatively lower in weight.
What is an impellor?
The internal rotating component of a centrifugal compressor, which takes inlet/intake air at its centre and compresses it by centrifugal force. Sometimes known as the rotor.
What is the purpose of the diffuser?
To decrease the air’s velocity and to increase its pressure.
Where does the air pressure increase as it passes through a centrifugal compressor?
Air pressure increases progressively through the impeller and increases rapidly through the diffuser.
What is configuration /description of a centrifugal compressor?
Short in length, spoke like design, easy to manufacture, with high compression per stage.
How does velocity and pressure vary through a centrifugal compressor?
Velocity and pressure both increase through the impeller, however through the diffuser the velocity decreases and the pressure increases.
What is the configuration/ description of an axial flow compressor?
Multiple stages of rotating aerofoil blades and fixed vanes (stators), with a larger mass flow capacity with high efficiency.
What are the advantages of an axial flow compressor over a centrifugal compressor of the same frontal diameter? 2
Higher compressor pressure ratios and greater mass airflow.
What are the advantages of an axial flow compressor over a centrifugal compressor? 4
- A lower frontal area for a given mass airflow, which allows for a smaller diameter engine.
- Achieves higher compression ratios more efficiently.
- A lower pressure rise per stage.
- A much lower airflow exit velocity compared to its inlet velocity.
What is the definition of the compressor pressure ratio?
The ratio of the compressor outlet pressure to the compressor inlet pressure.
What is the typical total compressor pressure ratio of a single stage centrifugal compressor?
Up to about 7.5 :1
What is the typical total compressor pressure ratio of a double stage centrifugal compressor?
Up to about 15:1
What is the typical total compressor pressure ratio of a modern axial flow compressor?
20 : 1, but up to 30 : 1 in some cases.
How does the temperature, pressure, and velocity of the air change as it passes through each stage of an axial flow compressor?
Temperature and pressure progressively increase and the axial velocity increases across the rotors and decreases across the stators, but overall velocity decreases, especially when considering the effect of the diffuser.
What is the function of the inlet guide vanes?
The inlet guide vanes are designed to direct the airflow onto the first stage rotors at an appropriate angle of attack.
What is the function of variable inlet guide vanes?
To change angle depending on flow characteristics, directing the airflow onto the first stage rotors at the optimal angle of attack, thereby reducing the chance of a compressor stall during operation off design (at low) rpm.
What is the function of the variable stators in an axial flow compressor?
The variable stators are designed to direct the airflow onto the subsequent stage of rotors at the optimum angle of attack, thereby increasing the compressor pressure ratio.
What is the primary function of each row of stator blades?
To convert the kinetic energy of the airflow into pressure energy.
What is the secondary function of the stator blades?
To stop swirl in the air thereby maintaining axial flow as it passes through the compressor.
What are the function of the rotors and stators in an axial flow compressor?
The rotors accelerate the airflow and the stators decelerate the flow to increase the air’s pressure.
What shape does an axial flow compressor form towards the combustion chamber?
The annulus area of an axial flow compressor reduces towards the high pressure end, forming a convergent duct.
Why does the air annulus area decrease towards the high pressure end of an axial flow compressor?
To allow for the reduction in the volume of the air as its pressure progressively increases and to maintain the axial velocity of the air constant as density increases.
Why is there only a small pressure increase across each stage of an axial flow compressor?
Because the aerofoil blades are, by necessity, quite small and consequently only produce a small amount of lift at each stage.
What effect does the diffuser have on the air leaving the compressor?
A diffuser reduces the velocity and increases the pressure of the airflow.