COMPRESSORS Flashcards
What is a low pressure compressor module also called?
A booster module.
What are the two ways to compress air in the engines?
- Decrease volume in cylinder by a piston
- Bernoulli’s principle to get compressed air (used in compressor of gas turbine engines)
What are the main components of a compressor?
Rotor
Stator
What is a rotor In a compressor?
Blade which rotates around compressor and sucks air to the outlet side of the compressor.
(Increases energy of the airflow)
What is a stator in a compressor?
Fixed component, blades called stator vains which guide and slow airflow to increase pressure.
What are inlet guide vains?
Improve the airflow into the first compressor stage.
Either fixed or moveable
What are stator vains In the fan duct usually called?
Outlet guide vains.
Improve airflow into the fan nozzle
What is the purpose of the inlet/outlet case on the compressor?
Provide support to the compressor rotor bearings.
Also improve and straighten airflow into and out of the compressor.
What are the two types of compressor?
- axial flow compressor
- centrifugal flow compressor
Where are centrifugal flow compressors usually found?
Small gas turbine engines such as APUs
What does a centrifugal flow compressor consist of?
- impeller
- diffuser
- compressor manifold
Describe the operation of an centrifugal flow compressor.
- impeller rotate, air sucked to centre
- rotation pushes air outward increasing air velocity and energy
- divergent ducts on impeller some velocity increase becomes static pressure
- airflow through diffuser velocity decrease pressure increased
What ratio can centrifugal compressors give?
5:1
How many compressor stages are deemed as most efficient in centrifugal compressors?
2
What is a double entry centrifugal compressor?
Two impellers mounted face to face.
- air can enter from both sides decreasing frontal area.(saves weight)
- no higher pressure but higher airflow.
What are the channels between rotors?
Diffusers.
- increases velocity but also slows it to create higher static pressure
What ratio are axial flow compressors usually?
1:3:1
Air enters with 10 psi, air pressure is 13 psi on outer side.
Why is compressor cross section gradually decreased?
Air pressure increases and density increases which means the air needs less volume
At optimum compressor speeds, what type of compressor is more efficient?
Axial flow compressor.
What type of compressor will cause less drag?
Axial flow. Smaller frontal areas.
Centrifugal flow compressors gain higher or lower pressure per stage?
Gain high pressure per stage.
In terms of weight saving, which type of compressor is lighter?
Centrifugal flow
Simpler to manufacture and lower cost
What type of compressor will need less power for engine start?
Centrifugal flow.
What conditions create the ‘design point’?
Compressors which have one optimum operating conditions for pressure ratio, rotation speed and air flow
What is required for smooth airflow into the rotor?
The correct relationship between the inlet velocity and rotor speed.
(Direction of airflow = resultant rotor inlet velocity)
In terms of rotors, what will determine the outlet direction of the airflow?
The shape of the rotor.
Speed of airflow at inlet is same as rotor outlet
What is the purpose of stator vains in compressors?
Decelerate and direct airflow into the next rotor stage.
When will the angle of attack change on stator vains?
If the rotor speed and inlet velocity changes
What is compressor stall?
Angle of attack increases and inlet air velocity decreases. Airflow separates from compressor aerofoil and causes a turbulent vortex.
What can compressor stall cause?
- Airflow to slow down, stop or even reverse.
- engine runs roughly and decrease rotor speed
- if stall is strong, can affect all stages (compressor surge)
What is compressor surge?
A very severe form of compressor stall.
How is compressor surge generated?
- Rapid decrease of airflow
- stall causes blockage of airflow stronger stall other stages
Compressor surge at the front stages of the compressor will cause what?
Usually only affects part of the compressor blades due to the bigger diameter of the blades.
(Not dangerous at this stage)
What will surge in the aft part of the compressor cause?
Airflow decreases rapidly and causes strong reverse flow.
Reverse flow could be from the combustion chamber to the inlet
What is the attitude of the air during compressor stall?
Airflow will collapse and build up in short intervals.
- causes loud banging and vibration
All primary engine components can cause compressor stall or surge. What issues can arise to creat this?
- dents or ice
- damaged rotor blades
- dirty rotors
- damaged combustion components
- damaged turbine
What issues will fast deceleration cause?
Compressor stall or surge as the HP compressor slows faster.
LP compressor withdraws air which cannot pass through the HP compressor.
Why will too fast acceleration cause compressor stall or surge?
Too much fuel in combustion chamber causing excess back pressure for a while.
What systems are In place to prevent compressor stall or surge?
- compressor bleed valves
- double or triple spool rotors
Variable compressor stator vains
What is the purpose of compressor bleed valves?
(Mid or aft section of compressor)
- Open at low engine speeds so excess air can go overboard.
- prevents blockage in aft compressor stage
What is a major disadvantage of compressor bleed valves?
Air which is pushed overboard compressed which causes a major loss of efficiency
What is the most effective method of stall prevention?
Variable stator vains
What is the function of the fan blades?
Provide initial compression and pass air through primary and secondary gas paths
What does the LPC/LPR concern?
Attached and secures the LPC rotor to the LPT shaft.
What is the fan stator case?
Provides flow path for fan discharge air and connects engine with necelle.
What is the intermediate fan case?
Major structural component. Engine mounting location.
What is the rotor drum for in a low compressor module?
Attached to the fan disc which has dovetail slots for rotor blades.
What does a knife edge which can cut in creating a small gap between the rotor assembly and the shroud improve?
Efficiency of the compressor.
What are the 2 construction methods for a compressor stator case?
Closed stator case (all stator vanes fitted to shrouds between rotor stages)
Split stator case. (Stator vanes installed in halves and bolted together afterwards)
What does the fan case assembly house?
The fan rotor and low pressure compressor.
What do flanges on the outer surface of the case help?
Stiffen the case and prevent deformation.
What are the lining elements for on a fan case?
Found on the inner surface, providing a smooth surface for the airflow in the fan duct.
What are the very small holes for on the surface of the lining element in the fan case?
Reduce noise created by the fan
What is the shrouded area on the fan case for?
Made of soft material which prevents damage to the fan blades if they come in contact
(Also named abraidable shroud)
What is the ring element made up of which is called the fan blade containment?
Honeycomb construction covered by layers of Kevlar.
What are the outlet guide vanes in the fan case assembly made of?
Aluminium or composite materials.
What is the module between the low pressure compressor and high pressure compressor?
Fan frame.
What do the variable bleed valves do for the low pressure compressor?
Protect it against surge. Bleed off excessive air from the low pressure compressor into fan discharge duct.
What do the fan frame struts do?
Carry loads from the engine bearings to the engine mount.
Always hollow to save weight
What does the centrifugal force of fan blades depend on?
- Rotor speed
- Blade mass
- Distance of centre of gravity of fan blade from rotational axis
What does the moment weight of the fan blade change with?
Blade mass and centre of gravity of fan blade.
Blade mass x distance
Can fan blades me manufactured to have the same moment weight?
No.
Where can the moment weight be found?
On the root of the blade.
Part number, serial number, moment weight
How is fan blade no. 1 identified?
Index mark on the fan disc or an offset hole.
What are used to balance the difference in moment weight of old and new fan blades?
Balance weights.
Installed on fan disc or in the spinner cone [balance screws]
What is the compressor outlet case often named?
The diffuser case, because the gas path is a divergent duct.
Where is the rear bearing of the high pressure compressor usually?
In the combustion case between the compressor and turbine.
How many stages do modern high pressure compressors have?
9-14
What does a labyrinth seal do in the stator vanes on a high pressure compressor?
Prevents reverse airflow through the gap between the rotor and the stator.
What does a CDP seal (behind the last high pressure compressor stage) do?
Prevents compressor discharge airflow from escaping into the bearing area below the combustion chamber.
How many different ways are there to fit rotor blades to a HP compressor rotor?
3.
What movement of blades is prevented by using blade retainers?
Axial.
What is attachment method of using bolts for connecting blades to the HP rotor usually for?
Used for long blades at the first stage of compressor.
What does the front compressor case for a high pressure compressor support structural loads of?
Bending and torsion.
Why do the inner ends of vane segments in stator vanes have an inner platform?
Minimise vibration caused by airflow.
What do variable stator vanes have at each end?
A trunnion in each end.
What are VSV’s?
Variable stator vanes.
Installed together with an actuation lever
What is the HP compressor a source of?
Pressurised air for the pneumatic system.
Compressor stator has openings to supply bleed air.
Where is the bleed air supplied through on a HP compressor?
Hollow stator vanes or hollow holes around the case.
What do bleed manifolds do?
Collect bleed air from pick up holes and supply it to consumers.
What is the spinner cone also known as?
Inlet cone or nose cone.
What sections is a fan rotor divided into?
- spinner come
- fan disk
- fan blades
- low pressure compressor rotor (booster rotor)
What two designs are there for spinner cones?
Sharp tip
Curved tip
What are the white spiral markings on spinner cones used for?
Scare birds away.
Where can balance weights be fitted to the spinner cone?
- Attached to the mounting bolts
- installed on into threaded inserts at the outer surface of the spinner
What type of fan blade fitting is found on most engines?
A dovetailed fitting
A loose fit is necessary when fitting rotor blades. Why are rotor blades fit loose?
All sudden forces or torque loads do not exceed the maximum allowable stress limits of the fan blades
What loads are on rotor blades?
Centrifugal
Air loads
What system do long rotor blades employ to keep the loading on them safe?
Shroud which forms a ring
What are the shrouds on rotor blades also known as depending on the manufacturers?
Mid-span shroud
Part-span shroud
What method protects rotor blade shrouds from rubbing against each other and breaking?
A special coating
Shrouds transfer blade flutter to friction
What type of fan blade do most aircraft use?
Wide chord fan blade
Why would wide chord fan blades be used over conventional blades?
They are more efficient
Wide chord fan blades are able to withstand FOD damage better than conventional. Why is this?
High flexibility, able to absorb impact energy
