Module 15.4 Compressors Flashcards
What is the primary function of the compressor?
To supply a sufficient quantity of air to the combustion chamber.
What must the compressor do to the air received from the inlet duct?
Increase the pressure of the mass of air, and then discharge it to the combustion chamber.
What is the secondary function of the compressor?
To supply air to the systems that require bleed air for operation.
Locations of the bleed ports depend on what?
The pressure or temperature required by that system.
The compressor has what two main components?
Compressor stator
Compressor rotor
What does each rotor disk have on it?
Rotor blades
What do the rotor blades do to the air as they rotate around the axis?
They suck in air and push it to the outlet side of the rotor. This increases the energy of the airflow.
What is the compressor stator?
A fixed component. The stator vanes guide and slow the airflow to cause an increase in pressure.
The combination of a rotor and stator is called what?
A compressor stage.
What part is always the first part of the compressor stage?
The rotor
What stage is a compressor with two rotor-stator pairs
A two stage compressor.
The space between each rotor and each stator is called what?
Annulus space
What are the set of stator vanes in front of a compressor stage called?
Inlet Guide Vanes (IGVs).
What do the Inlet Guide Vanes (IGVs) do?
Improve the airflow into the first compressor stage.
What are the stator vanes in the fan duct called?
Outlet Guide Vanes (OGVs).
What do Outlet Guide Vanes (OGVs) do?
Improve the (bypass) airflow into the fan nozzle.
What are the two types of compressors?
Centrifugal flow compressor
Axial flow compressor
The gap between each stator and each rotor forms what?
A divergent duct. This is done to decrease airflow velocity and increase pressure.
Rotor blades rotating around the axis does what to the velocity of the airflow?
Increases it
What is happening to velocity as air goes from rotor to stator etc?
Velocity is constantly increasing and decreasing.
From the first to final stage, velocity has stayed the same but the pressure has increased.
What does the centrifugal-flow compressor consist of?
Impeller (rotor), diffuser (stator), and compressor manifold.
What is the pressure rise per stage with centrifugal compressors?
A high-pressure rise per. 8:1
How many stages are centrifugal compressors limited to?
Two stages due to efficiency concerns (pressure loss).
What is the diffuser and manifold assembly called?
The diffuser
Where is the impeller located?
Mounted on the shaft and inside the diffuser.
How does the impeller work?
It accelerates air outwardly to the diffuser.
What two types of impeller can be used?
Single entry or double entry.
What is one stage of the centrifugal compressor?
An impeller and diffuser together
Why does a single entry impeller allow for over a double entry type?
More convenient ducting directly to the impeller eye (inducer vanes) as opposed to the more complicated ducting needed to reach the rear side of the double-entry type.
Axial flow compressors are normally found on what type of engine?
Turbofan engines
Centrifugal flow compressors are normally found on what type of engine?
Turboprop and turboshaft engines.
Define an axial flow compressor.
One that consists of one or more rotating components which have aerofoil-shaped blades attached.
A rotor-stator pair in a centrifugal compressor is referred to as what?
A stage
What happens to the rotating drum and case from front to back in an axial compressor?
Diameter gradually decreases.
Why does the diameter of the drum and case decrease in an axial compressor?
To maintain a steady axial velocity (no stagnated air).
What does a single-spool compressor consist of?
A single drum and multiple stators.
Two spool are more suitable for what?
Dual flow engines.
In a two spool, what is the front part of the compressor designed to do?
Initially compress a larger mass of air than the mass injected into the High Pressure (HP) compressor.
In a triple spool engine, what is the fan driven by?
A dedicated segment of the turbine, making its shaft the Low Pressure (LP).
What does the fan act as in a triple spool engine?
A low-pressure compression stage (slightly pressurising the air that goes into the core).
Where is the smaller proportion of the flow, known as the core flow or hot air passed through?
The Intermediate Pressure (IP) and HP compressor, the combustion chamber, and the turbine.
From one stage to the next in an axial compressor, the axial compression ratio is…
Small
What is the typical compression ratio in an axial compressor?
Between 1:1 and 1:2.
What is done to achieve a high compression rate in axial flow compressors?
A large number of stages.
What does a spool consist of?
A turbine, shaft and compressor
How many compressors in a twin spool?
How many in a triple spool?
Two in a twin spool and three in a triple spool.
What two ways can rotors (blades) be balanced?
Statically and dynamically balanced.
What is the moment if the rotors are dynamically balanced?
Moment is 0
If a rotor blade is dynamically imbalanced, what is needed?
A balance weight on the opposite blade.
All turbine engine rotating assemblies require balancing by who?
Manufacturer
Which set of blades can be balanced without disassembly?
Fan blades
When can the rotor of the engine run free of vibration?
If the fan blades that are opposite each other create identical centrifugal forces.
The centrifugal force of fan blades depends on what three factors?
The rotor speed
The blade mass
The distance of the centre of gravity of the fan blade from the rotational axis
Can fan blades have a different moment weight even if they have the same mass?
Why?
Yes, because the centre of gravity of each fan blade is different.
What three measurements are there for moment weight?
gram inches
cm.g
ounce inches
What three numbers does a fan blade root have?
Part number
Serial number
The moment weight
How are blades with individual moment weights generally matched on the fan disc?
Using computerised balancing. The computer works out the best position for each blade on a fan disc.
How is fan blade no.1 usually identified?
Index mark on the fan disc or an offset hole.
How are the blades often numbered?
In the direction of rotation.
What is the best method of changing a fan blade?
Replace the blade with a new blade that has a moment weight as close as possible.
When would no further balancing be required after a fan blade change?
If the difference in moment weight between the old and new blade is small enough (as specified by the manufacturer).
If the difference in moment weight between the old and new blade is larger than specified by the manufacturer, what should be done?
Balance weights can be used.
What are balance weights used for when changing a blade?
To balance the difference in the moment weight of the old and new fan blades.
Where are balance weights installed?
On the fan disc or in the spinner cone.
What other method of changing fan blades are there?
Change a pair of blades (opposite blade) with similar moment weight blades, even if only one blade is damaged.
Define the compression ratio of an internal combustion engine.
The ratio of the volume of its combustion chamber from its largest to its smallest capacity.
Is compression ratio proportional to RPM?
No, but it does vary with RPM.
What is the pressure ratio at each stage of an axial compressor?
1.3:1
What does a 1.3:1 pressure ratio mean?
If the air pressure enters the first stage with 10 psi, then the air pressure is increased to 13 psi on the output side of that compressor stage.
This pressure now becomes the input pressure to the next stage.
What is the pressure ratio like at each stage of the compressor in axial flow compressors?
Very small
What three methods are there to prevent a compressor stall?
Bleed valves
Dual or triple-spool rotors instead of single spool
Variable compressor stator valves
How do bleed valves prevent a compressor stall?
They are open at low engine speeds (RPM) so that the excessive airflow from the forward stages can escape overboard.
Where would bleed valves be placed in multi-stage axial compressors?
At several positions
Where is the bleed air bled off to?
C-duct (bypass duct)
Where would a 3rd stage bleed valve be?
3rd stage of the compressor.
On a dual spool engine, what is the fwd and aft compressor called?
Fwd - Low-pressure compressor or N1 compressor.
Aft - High-pressure compressor or N2 compressor
What is the most effective method to prevent compressor stall?
Variable stator vanes and variable inlet guide vanes
Where are variable stator vanes and variable inlet guide vanes located?
In the forward stages of the high-pressure compressor.
What is the advantage of variable stator vanes?
An optimum angle of attack on the following rotor blades can be reached for every engine speed.
What does the angle of attack of the rotor depend on?
Air inlet velocity and rotor speed.
What does the correct relationship between the inlet velocity and the rotor speed allow?
A smooth air flow.
What does the angle of attack of the rotor blades need to be for smooth airflow
Small
When does the angle of attack of the rotor blade change?
When the inlet air velocity decreases (rotor speed/rpm is now too fast for inlet velocity).
What is a compressor stall?
When airflow separates from the compressor aerofoils and causes a turbulent vortex.
What does a compressor stall do to the air?
Causes the airflow to slow down, stop or reverse its direction.
What is a compressor surge?
A very severe form of compressor stall. Surge can happen on all engine compressors.
Where can compressor surges occur?
In the forward or the aft compressor stages.
What does a surge in the fwd compressor stages affect?
Only a part of the compressor blades due to the bigger dimensions of the blades.
How does a surge affect the aft compressor stages?
It develops very rapidly over a large part of the compressor.
Where in the compressor can bleed valves found?
The mid-section and sometimes the aft section.
