Module 15.4 Compressors Flashcards
What are the two main components of a compressor?
- Stator
- Rotor
What are the two types of compressor?
- Axial flow
- Centrifugal flow
What is the purpose of the stator vanes?
- Guide and slow airflow to increase pressure
What are stator vanes in front of a compressor called?
Inlet Guide Vanes (IGV’s)
Stator vanes in the fan duct are usually named what?
Outlet guide vanes
How many stages are centrifugal compressors limited to due to efficiency concerns?
Two
What are the two main functional components of a centrifugal compressor?
Impeller and diffuser
In centrifugal compressors what can be said about the rise per stage?
High pressure rise - 8:1
What are the two types of impeller fan?
- Single entry
- Double entry
What can be said about double entry impellers?
Smaller diameter but require a higher rpm
Where are centrifugal compressors commonly used?
- Turboprop
- Turboshaft
What shape are the compressor blades?
Aerofoil shaped
What does a gradual decrease in diameter of the rotating drum maintain?
Steady axial velocity
Two spools is much more suitable for what type of engine?
Dual flow
In a 3 spool engine what does the fan also act as?
Low pressure compressor
What is the purpose of the stators?
- Act as guide vanes
- Act as diffusers
To achieve high compression in axial flow what is required?
Lots of stages
Why is it beneficial to have a centrifugal compressor in an APU?
- Low start up power
What is the compression ratio from one stage to the next in an axial compressor?
1.1 to 1.2 to 1.3 (1.3 mentioned in 15.5)
Does fan blade balancing require engine disassembly?
No
The centrifugal force depends on what?
- Rotation speed
- Mass of the object
- Distance between CoG and the rotational axis
What is the moment weight?
Product of the blade’s mass and the distance of the CoG from the rotational axis
Do blades have the same moment weight if they have the same mass?
No, not necessarily
What numbers will be displayed on the root?
- Part no.
- Serial no.
- Moment weight
How are the fan discs matched?
Computerised balancing
No.1 fan blade position is identified by what?
- Index mark on fan disc
- Offset hole
What are the two methods of fan blade change?
- Replace blade with one of moment weight extremely close to the opposite fan blade (pair)
- Replace the pair
What effect does increasing rotor speed have on the angle of attack?
Increases the angle of attack
If relative airflow speed increases what effect does this have on the AoA?
Decreases the AoA
What is the AoA between in the compressor?
Chord line of the vane and resultant rotor inlet velocity
A low AoA ensure what?
Airflow into the rotor is smooth
What is a compressor stall?
Airflow separation on the vanes
How do compressor stalls occur?
Rapid decrease of airflow causes stall on some blades or stages
Stall causes a blockage in airflow
Low pressure zones, airflow stops and reverses
What are some reasons for stall and surge?
- Dents or ice
- Damaged blades
- Damaged or broken combustor
- Damaged turbine components
- Speed of engine below designed operating speed
- Incorrect acceleration or deceleration
What are the 3 methods to prevent compressor stall or surge?
- Compressor bleed valves
- Dual or triple spool
- Variable compressor stator vanes
Where are bleed valves found?
Mid compressor section sometimes aft compressor section as well.
When are engine bleed valves open?
Low engine speeds
What is the most effective method of preventing compressor stall?
Variable stator vanes
If the pressure ratio is 1.3:1 and the air pressure to stage 1 is 10 psi. What is the input pressure into stage 2?
13 psi
