15.4 Compressors Flashcards
What is the primary function of a compressor?
to supply a sufficient quantity of air to satisfy the requirements of the combustion chamber.
What is the secondary function of a compressor?
to supply air to the systems that require bleed air for operation.
What is the compressors two main components?
Compressor rotor
Compressor stator
How does the rotor increase the energy of airflow?
By the action of the rotor blades sucking in air and push it to the outlet side of the rotor as it rotates in the compressor
What are the blades on a stator called?
Stator vanes
What type of component is a stator?
Fixed component
What do stator vanes do?
increase the pressure of air by slowing the airflow
What is the combo of a rotor and stator called?
A compressor stage
What are the stator vanes in front of a compressor stage called?
IGVs
Are IGVs fixed or moveable?
fixed or movable.
What are the stator vanes in a fan duct called and what do they do?
Outlet Guide Vanes
improve the airflow into the fan nozzle.
What are the two types of compressors used in GTEs?
The centrifugal flow compressor
The axial flow compressor
What does a centrifugal flow compressor consist of?
Impeller (rotor)
Diffuser (stator)
Compressor manifold
What is the energy rise per stage in a centrifugal compressor?
8:1
So high
How many stages can a centrifugal compressor be?
Limited to 2 for efficency reasons
What is the impeller made from? Designed for what?
Forged aluminium alloy
Minimum flow restriction
What is the impellers function?
pick up and accelerate the air outwardly to the diffuser.
What are the two types of impeller?
Single entry
Double entry
What is the advantage of a single entry impeller?
Permits ducting directly to the impeller eye (inducer vanes)
Double entry ducting is more complicated
How does axial flow compressor reduce energy loss?
Compresses the air in its original direction of flow, so no energy used on turning it.
Where are axial compressors used?
Where are centrifugal compressors used?
Axial: Turbofan
Centrifugal: turboshaft or turboprop
Whats the structure of an axial flow compressor?
Stationary vanes (stator) mounted to the compressor casing between the rotating blades (rotor)
Axial flow compressor: Why is there a gradual decrease in the diameter of the rotating drum and the casing?
to maintain a steady axial velocity, as the pressure and density of the air increase.
decrease in diameter: increases the velocity
What does a single spool consist of?
A single drum and multiple stators
How are the two or three drums of rotors driven in a multi- spool compressor?
Each driven by a separate turbine segment at the optimum respective rotational speed
What does the fan act as in triple- spool engine?
As a LP compression stage
What do stators also do in axial compressors?
Correct the angle of flow between rotors?
What is the compression ratio in axial compressor?
small, between 1:1 1:2 and 1:3
(1:3 mentioned in 15.5)
Why is the compression ratio kept small in axial compressors?
Amount of diffusion and angle of deflection per stage is kept small to minimise flow separation and stall
Adding more stages makes designing more difficult
Why is it better to have fans at the front of an engine?
Safer to have such a large component in the cold part of the engine
What are centrifugal compressors advantages:
- high pressure rise per stage
- efficient over a wide rotational speed range
- simplicity of manufacture
- low weight
- low starting power
Centrifugal compressor disadvantages:
- Large frontal area for a given airflow
- Losses in the turns between stages
What gives an axial compressor high ram efficiency?
It’s straight through flow
Axial compressors efficiency:
High peak efficiencies, but only over a narrow rotational speed range
Axial compressor disadvantages:
Difficulty of manufacture and high cost
Relatively high weight
High starting power requirements; (partially overcome by split compressors)
Does the engine have to be disassembled for fan blade balancing?
no
Why do high bypass fans require more attention on their balance during service?
because of their large diameter and high rotational forces.
What is needed for no vibrations on the rotor of an engine?
fan blades that are opposite each other must have identical centrifugal forces.
What does the centrifugal force of a blade depend on?
The rotor speed
The blade mass
The distance of the centre of gravity of the fan blade from the rotational axis
What is a blades moment weight?
product of the blade mass and the distance of the centre of gravity from the rotational axis
Why do blades with the same mass have different moment weights?
the centre of gravity of each fan blade is different due to manufacturing tolerances
What is moment weight given in?
gram inches
cm.g
ounce inches
What is find on the blade root?
- part number
- serial number
- moment weight
How are blades matched?
Computerized balancing
How is fan blade no.1 identified?
an index mark on the fan disc or by an offset hole
What are the two methods for a fan blade change:
Preferred method: replace the blade with a new blade that has a moment weight as close as possible to the old one.
Other: change the pair of blades
