15.04 Compressors Flashcards
What is a combination of a rotor and a stator known as?
A compressor stage
What is always the first part of a compressor stage?
The rotor
What is the purpose of the stator?
It guides and slows the airflow - increasing pressure
What are the set of stator vanes in front of the compressor stage called?
Inlet guide vanes (IGVs)
What is the purpose of the IGVs?
They improve the airflow into the compressor
What is the purpose of the compressor casings?
- they support the bearings
- they improve and straighten the airflow
What are the two types of compressors?
- axial flow compressors
- centrifugal flow compressors
What pressure ratio can centrifugal flow compressors give?
A pressure ratio of 5:1
On a centrifugal flow compressor, what has the same function as a rotor and stator on an axial flow compressor?
The impeller and diffuser
How many stages of a centrifugal flow compressor are the most efficient?
2 stages - due to a pressure reduction when the airflow changes direction in more than 2 stages
What is the double entry type impeller?
Two impellers mounted face to face. Also known as the double-face impeller
What is the advantage of a double-face impeller?
It saves weight and gives the engine a smaller frontal area
Higher airflow at given diameter, not higher pressures
In an axial flow compressor, what direction is the airflow?
Along the horizontal axis of the compressor
What is the purpose of the channels (diffusers) inbetween the rotor blades?
They slow the airflow increasing the static pressure
What pressure ratio does each stage of an axial flow compressor have?
1.3:1
What happens as the air pressure increases in the compressor stages?
The density increases and the volume decreases
What is the purpose of the compressor cross-section gradually decreasing?
It keeps the velocity of the airflow constant
What are the advantages to axial flow compressors?
- at optimum compressor speed, they are more efficient than centrifugal flow compressors
- smaller frontal area which causes less drag
- higher pressures due to more compressor stages
What are the advantages to centrifugal flow compressors?
- smaller and cheaper to manufacture
- lighter in weight
- need less power to start
- high pressure rise per stage
- good efficiency over a wide range of speeds
Axial and centrifugal flow compressors have one thing in common. What is this?
They are very sensitive to changes in the airflow passing through the compressor due to the compressors only having one optimum operating condition for a given rotational speed, airflow and pressure ratio.
What is the name given to the optimum operating conditions of a compressor?
The design point
The airflow into the compressor is smooth as long as…
The angle of attack of the rotor blades is small
What is the purpose of the stator vanes?
They decelerate the airflow and guide it to the next rotor stage of the compressor
What is a compressor stall?
The airflow separation from the compressor blades causing a turbulent vortex
If a compressor stall affects all compressor stages, what is this known as?
A compressor surge
What occurs in a compressor surge?
Low pressure zones occur and the airflow comes to a stop and reverses in direction. The airflow collapses and builds up again in short intervals along with heavy vibrations and loud banging noises.
A surge in the forward part of the compressor isn’t dangerous, why is this?
The stall only occurs on part of the blades as they are larger
Why is a surge in the aft part of the compressor dangerous?
The airflow decreases rapidly and causes a strong reverse flow which can be from the combustion chamber to the inlet.
What are reasons for a compressor surge?
- dents or ice on inlet
- damaged rotor or stator vanes
- dirty compressor blades/combustion components
- damaged combustion components
- damaged turbine components
What are the two operational reasons a compressor can stall or surge?
- speed of the engine is too low below the design speed
- incorrect acceleration or deceleration
The rotor speed of the HP compressor decreases faster than….
The rotor speed of the LP compressor. This means the LP compressor can supply too much air which cannot pass through the HP compressor
What happens if the engine is accelerated too quickly?
Excess fuel in the combustion chamber increases the back pressure and can stall the compressor
What are the 3 methods used to prevent compressor stall and surge?
- VSVs
- Compressor bleed valves
- Double/Triple spool engine’s
Where are compressor bleed valves usually found?
In the mid and sometimes the aft parts of the compressor
When are the compressor bleed valves open?
They open at low speeds so that excessive airflow can escape
What is the disadvantage of using compressor bleed valves?
They reduce efficiency
What is the advantage of double and triple spool engines?
It reduces the weight of the shafts and therefore the rotors can be speed-matched more easily.
This means they have excellent acceleration capabilities
What are is the most effective way of preventing compressor stall?
Variable Stator Vanes
What is the disadvantage of VSVs?
They have a complicated control mechanism
How are major compressor modules usually built up of?
They usually are built up from sub-modules
What is the spinner cone also known as?
The nose cone or compressor inlet cone
What is the low pressure compressor rotor also known as?
The booster rotor
What two designs are spinner cones usually?
- sharp tip
- curved tip
What ice protection measures can the spinner cone have?
- heated nose cone which removes ice
- soft tip that causes imbalance and removes ice
What are the spirals or white markings on the spinner cone used for?
To scare birds away
Where is the spinner cone always attached?
To the front face of the fan disc
Where are balanced weights attached?
To the mounting bolts of the spinner cone or in threaded inserts at outer surface of the spinner cone
What is the fan rotor disc usually made from?
Titanium alloy
What type of fan blade fitting is used on most engines?
The dovetailed fitting
Why is a loose fit required for the fan blades?
So that sudden forces or torque loads do not exceed the maximum allowable stress limits of the fan blades
What do long fan blades have to keep the fan blade deflection within safe limits?
Shrouds - dampening blade fluttering into friction
What is the coating for on the fan blade shrouds?
Is ensures the blades can easily slide on each other
What are the advantages of modern wide chord fan blades?
- more efficient than conventional blades (main advantage)
- can be run at lower speeds (reducing noise)
- flexible (absorbing impact damage better than older blades)
What type of seal is located inbetween the dovetail slots of the fan blades?
A labyrinth seal
What are the shroud segments filled with inside the stator assembly?
They are filled with an abradable material for the knife edges to cut in. This improves the efficiency of the compressor
What are the two construction methods for the compressor stator case?
- split stator case (VSVs fitted to shrouds and matched during assembly)
- closed stator case (VSVs installed in two stator halves)
What are flanges for on the fan case assembly?
They help stiffen the case and prevent deformation
What are the lining elements of the fan case also known as?
Acoustic panels
What are the holes for on the fan case acoustic lining?
They reduce noise created by the fan
What is important to remember about the outer surface of the acoustic lining?
It is very thin to save weight and can be easily damaged by FOD, tools or incorrect maintenance
There is a shrouded area on the fan case instead of acoustic panels. What is this for?
It is known as the abradable shroud - the tips of the blades can cut into this for maximum efficiency and smallest tip clearance
What is located opposite the abradable shroud on the outer wall of the fan case?
Fan blade containment - usually made of honeycomb construction and covered by layers of Kevlar
What are the outlet guide vanes (located in the fan duct) made from?
Aluminium or composite materials
What is the fan frame?
It is the next main component behind the fan module between the LPC and the HPC
What is the centre hub of the fan frame used for?
- It supports the forward bearings of the engine rotors
- It houses the internal gearbox on the forward end of the N2 rotor shaft
What would you find on the aft side of the fan frame?
The forward engine mount
What is located on the outer surface of the fan frame?
Oil tanks, ignition exciters and computers
What are the fan frame struts for?
They carry the loads from the engine bearings to the engine mount. They are always hollow to save weight.
What do the centrifugal forces for balanced rotors depend on?
- rotor speed
- blade mass
- the distance of the CofG from the rotational axis
What is the moment weight?
The product of the fan blade mass and the distance of the CofG from the rotational axis
Why do fan blades have a different moment weight even if they have the same mass?
The CofG of each blade is different due to manufacturing tolerances
Where can the moment weight of each fan blade be found?
At the root of each blade. The information found:
- Part Number
- Serial Number
- Moment weight
If the moment weight of a fan blade changes because of blending, what happens to the moment weight information?
It is crossed out and the new moment weight is marked on the root next to it
How can fan blade No.1 be identified?
An index mark on the fan disc or by an offset hole
The rest of the blades are usually numbered in direction of rotation
If a fan blade needs to be replaced, what two methods are proposed by the engine manufacturers?
- replacement with a blade with a moment weight as close as possible to the old one (preferred)
- balancing weights can be used
Where are balancing weights installed?
On the fan disc or the spinner cone
What is the compressor outlet case (the case between the last compressor stage and the combustion section) also known as?
The diffuser case - because the gas path through the case is a divergent duct
Where is the rear bearing of the high pressure compressor located?
In the outlet case of the HP compressor (which is in between the compressor and the combustion chamber)
Modern high pressure compressors contain how many stages?
Between 9 and 14 stages
What is the purpose of the labyrinth seal?
It prevents reverse airflow through the gap between the rotor and the stator.
Where is a CDP seal found?
It’s located behind the last compressor stage
What is the purpose of the CDP seal?
It prevents compressor discharge airflow from escaping into the bearing area
What do the retainers that hold the fan blades also do?
They prevent axial movement
On small compressor blades the discs have circumferential dovetail slots. What do each of these slots also have?
A loading slot and two locking slots
How are the compressor blades locked in place?
By a locking lug when all the blades from the first stage have been inserted
What type of blade fitting is usually used for the longer blades at the first compressor stages?
Bolts inserted through the disc and into the blade root
How are stator vanes usually installed?
Circumferential dovetail slots
What do the variable stator vanes have installed at each end?
Trunnions
How are all VSVs moved simultaneously?
Via the actuator ring
How is bleed air supplied to the cooling or pneumatic system?
Through hollow stator vanes or through holes in the outer platform of the stator vanes. Bleed pickup holes are located around the whole compressor case
Where does the bleed air from the pickup holes go to?
The bleed manifold supplying consumers
