15.4 COMPRESSORS👌🏼 Flashcards
What are the 2 components of a compressor
- rotor
- stator
What do stator vanes do
- guide and slow airflow to cause pressure increase
Combination of a rotor and stator is named what
- compressor stage
Are rotors or stators first in a compressor
Rotors
What are the set of stator vanes in front of a compressor stage named
Inlet guide vanes
Explain inlet guide vanes
Straighten airflow into first compressor stage
What are the 2 types of compressors
- centrifugal flow compressor
- axial flow compressor
What do the rotors do
Increase energy of airflow
How many stages do centrifugal compressors have and why
- 2
- They have a high pressure rise per stage
Explain the double entry type impeller
- smaller diameter but operated at higher rotational speed to assure sufficient airflow
What compressors are usually found on turbofan engines
Axial
What compressors are commonly used in turboprop and turboshaft engines
- centrifugal compressors
Explain axial compressors
- stators and rotors
- rotating components are aero foil shaped
- multiple stages (small increase per stage)
Why is there a gradual decrease in diameter of the rotating drum and casing
- As it moves from low to high pressure
- necessary to maintain steady axial velocity, as pressure and density of air increase
In triple-spool engines what is the fan referred to as
LP compressor
Centrifugal compressors advantages
- HP rise per stage
- efficiency over wide rotational speed range
- simple
- low weight
- low starting power requirements
Centrifugal compressors disadvantages
- large frontal area
- losses in turns between stages
Axial flow compressor advantages
- small frontal area
- straight through flow
- increasing pressure rise by increasing stages
Axial flow compressor disadvantages
- difficult to manufacture, expensive
- heavy
- high starting power requirements
3 parameters that create centrifugal force on a engine
- rotor speed
- blade mass
- distance of CG of fan blade from rotational axis
Explain moment weight
- distance of CG from rotational axis
Where is the moment weight usually seen on the blade
Blade root
What information is on the blade root
- part number
- serial number
- moment weight
How are the blades moment weight calculated
By computerised balancing
How can fan blade no1 be identified
- index mark on fan disc
- offset hole
How are the blades numbered
- often numbered in the direction of rotation
What is the best method to replace a fan blade
- replace with a new blade that has a moment weight as close as possible
Explain balance weights
- use to balance the difference in the moment weight
Where are the balance weights installed
- fan disc
- spinner cone
What are the 2 methods of a fan blade change
- replace single blade with one that has similar moment weight
- replace a pair of blades with similar moment weights
What must be ensured to allow smooth airflow into the rotor
- AOA to the rotor blades is small
- Blade is aerofoil shaped
Explain stator vanes
- decelerate airflow and guide it to the next rotor stage of compressor
When will the AOA increase on the rotor
- when inlet air velocity decreases
Airflow separates from the compressor aerofoils and causes turbulent airflow is known as what
Compressor stall
Explain compressor stall
- airflow slows down, stop or even reverse direction
Explain compressor surge
Severe form of compressor stall
Explain surge in the forward stages
- affects a part of the compressor blades
- due to bigger blade dimensions
- not very dangerous
Explain surge in the aft stages
- develops rapidly over a large part of the compressor
- due to high pressure in aft stages and the short compressor blades
What are the 2 operational reasons for compressor stall and surge
- engine speed below design speed
- stall at incorrect acceleration or deceleration
3 methods to avoid compressor stall and surge
- compressor bleed valves
- use dual or triple spool rotors instead of single spool rotors
- use variable compressor stator vanes
What is the modern method to avoid stall and surge
- variable compressor stator vanes
Explain bleed valves
- found in mid-section/aft-section of engine compressor
- open at low engine speeds
What is the N1 compressor known as
Low pressure compressor
What is N2 compressor known as
- high pressure compressor
Explain variable inlet guide vanes/variable stator vanes
- most effective to prevent stall
- moveable along vertical axis
- found in FWD stages of HP compressor
Each stage of an axial flow compressor has a pressure ratio of?
1.3:1
What compensates for the lost thrust as aircraft speed increases
Ram recovery (total pressure recovery)
