15.4 Compressors Flashcards
What is the compressors role?
Must increase pressure of the mass of air received from the air inlet duct and then discharge it to the combustion change in the quantity that is required
What is the secondary function of the compressors?
To supply sir to the systems that require bleed air to operate
What are the two main components of the compressor?
Compressor rotor, compressor stators
What do the rotor blades on the rotor do?
Suck in air and push it to the outlet side of the rotor, this action increases the energy of airflow
What do the compressor stator vanes do?
Guide and slow the airflow to cause an increase in pressure
What comes first rotor or stator in the compressor stage?
Rotor
What are the two types of compressors currently used in the gas turbine?
Centrifugal flow compressor, axial flow compressor
What does a centrifugal flow compressor consist of?
Impeller (rotor), diffuser (stator) and a compressor manifold
Why are centrifugal compressors limited to two stages?
Concerns about efficiency
Is the impeller single or double entry and what does it do?
Can be either and is to accelerate the air outwardly to the diffuser
How does double entry and single entry work?
Double has a smaller diameter but is usually operated at higher rotational speeds, single impeller permits convenient ducting directly to the impeller eye
What does a axial flow compressor consist of
An axial compressor consists of lots of aerofoil blades in lots or rows and made up of stators and rotors
Where are centrifugal compressors usually found?
Used in turboprop and turboshaft
What do compressors usually consist of?
Multiple stages
Why is a gradual decrease in diameter happening on the rotating drum?
To maintain a steady axial velocity, as the pressure of the air increases
What does the total number of stages depend on in a compressor?
Desired pressure increases
What are the advantages of centrifugal flow compressors?
High pressure rise per stage
Efficiency over a wide rotational speed range
The simplicity of manufacture and low cost
Low weight
Low starting power requirements
What are the disadvantages of centrifugal flow compressor?
Its large frontal area for a given airflow
Losses in turns between stages
What are the advantages of the axial flow compressor?
High peak efficiencies
Small frontal area for given airflow
Straight through flow, allowing high ram efficiency
Increased pressure rise by increasing the number of stages, with negligible losses
What are the disadvantages of axial flow compressors?
Good efficiencies over only a narrow rotational speed range
Difficulty of manufacture and high cost
Relatively high weight
High starting power requirements; (partially overcome by split compressors)
What are the only part of the engine that can be balanced without disassembly?
Fan blades
What are the three parameters the centrifugal force depends on?
Rotor speed, blade mass, distance of the centre of gravity of the fan blade from rotational axis
What units can moment weight be given in ?
- gram inches
- cm,g
- ounce inches
Where can the moment weight usually be found ?
On the blade’s root
How is vibration minimised from the fan blades ?
Individual moment weights are generally matched in pairs on the fan disk by computerised balancing
How can fan blade number 1’s position be identified on a fan disc ?
An index mark or by an offset hole
When replacing a fan blade, if the moment weight is too high, what should be done?
Balance weights can be used to equalise fan blade moment weight
What is moment weight?
The product of the blade mass and the distance of the centre of gravity from the rotational axis
What can be different about a fan blade even if they have the same mass?
Moment weight
What numbers can be found on the blade root?
Part number
Serial number
Moment weight
Why are both compressors sensitive to change in airflow?
Because the engine compressors have only one optimum operating condition for a given pressure ratio, rotational speed and airflow
When replacing a fan blade if you can’t find a fan blade the same weight what should you do?
You should find a fan blade most similar to the weight or replace the pair fine to roughly the same weight
If rotor speed increases what happens to angle of attack?
Angle of attack increases
If rotor speed decreases what happens to angle of attack?
Angle of attack decreases
If airspeed increases what happens to angle of attack?
Angle of attack decreases
If airspeed decreases what happens to angle attack?
Angle of attack increases
Why do you want a small angle of attack?
So airflow into the rotor is smooth
What happens when airflow separation in the engine?
Compressor stall
What does a compressor stall do?
Changes the proper airflow in the compressor causes airflow to slow down stop or even reverse its direction
What is a compressor surge?
A very severe form of compressor stall
How is a compressor surge generated?
Rapid decreased of airflow causes stall on some blades or stages
Stall causes a blockage in the airflow which leads to a stronger stall
Causes low pressure zones in which the airflow comes to a stop and reverses its direction
Where can compressors surge occur?
Surge can happen on all engine compressors. It can occur in the forward or the aft compressor stages
What happens on forward stage surge?
Surge in the forward stages usually affects only a part of the compressor blades. This is due to the bigger dimensions of the blades. The effect on the compressor operation is not very dangerous
What happens on an aft stage surge?
Surge in the aft stages develops very rapidly over a large part of the compressor. This behaviour is supported by the high pressure in the aft stages and by the short compressor blades
Why do compressor stalls and surges happen?
Compressor stall and surge is caused by problems in engine operation or by damaged engine components
What primary engine components can cause compressor stall if they are damaged?
1)Dents or ice
2)Damaged rotor blades or stator vanes
3)Dirty compressor blades or stator vanes can cause stall
4)Damaged or broken combustor components can cause a blockage in the airflow and decrease the velocity in the compressor.
5)Damaged turbine components will decrease the airflow and dents in the jet nozzle or broken objects in the exhaust system can also cause a blockage to the airflow.
What are the two operational reasons for compressor stall and surge?
The compressor can stall if the speed of the engine is too far below the design speed
It can also stall at incorrect acceleration or deceleration
What are the three different design methods to prevent compressor stalls and surges?
Compressor bleed valves
Use of dual or triple spool rotors instead of single spool rotors
variable compressor stator valves
When are engine bleed valves open?
They are open at low engine speeds so that the excessive airflow from the forward stages can escape overboard
What is the most effective method to prevent stall?
Variable stator vanes
What’s the main advantage of variable stator vanes?
At an optimum AOA the following rotor blades can be reached for every engine speed
What is another name for the low- pressure compressor ?
N1 compressor
What is another name for the high pressure compressor ?
N2 compressor
How is each compressor rotor driven in a double or triple spool rotor ?
By a separate turbine at the exhaust section of the engine
What is the pressure ratio of each stage or an axial flow compressor ?
1.3:1
Why must many stages be used in an axial compressor?
One stage only raises a small amount, so many are needed for the correct pressure
