15.4 Compressor Flashcards
What is the primary function of the compressor?
To supply a sufficient quantity of air to satisfy the requirements of the combustion chamber.
What is the secondary function of the compressor?
To supply air to the systems that require bleed air for operation.
What are the two main components of a compressor?
- Rotors.
- Stators.
What’s the purpose of the Rotors?
To suck in air and push it to the outlet side of the rotor. This action increases the energy of the airflow.
What’s the purpose of the Stators/Stator Vanes?
The stator vanes guide and slow the airflow to cause an increase in pressure.
What is classed as a single stage of the compressor?
One pair of stators and rotors.
What component is always the first part of a compressor stage ?
The Rotor.
What is the name of stator vanes found just in front of the compressor inlet?
Inlet guide vanes (IGVs).
What’s the purpose of IGVs?
Improve the airflow into the first compressor stage.
Are IGVs fixed, or moveable?
Can be EITHER fixed or moveable!
What are Oulet guide vanes (OGVs) and where are they located?
Located in the fan duct, and improve airflow into the fan nozzle.
What is the purpose of the compressor inlet and outlet cases?
- To give support for the compressor rotor bearings.
- Improve and straighten the airflow into and out of the compressor.
What are the two types of compressor used in GTEs?
- Centrifugal flow.
- Axial Flow.
What are the main components of a Centrifugal flow compressor? (3)
- Impeller (rotor)
- Diffuser (stator)
- Compresssor manifold.
What is the pressure rise ratio per stage in a Centrifugal compressor? (It’s classed as high).
8:1.
How many stages are Centrifugal compressors limited to, and why?
2 stages, due to efficiency concerns.
What material is the impeller usually made from?
Forged aluminium alloy, heat treated, machined, and smoothed for minimum flow restriction and turbulence.
What is the function of the impeller?
To pick up and accelerate the air outwardly to the diffuser.
What are the two types of impeller?
- Single entry.
- Double entry.
What are the features of a single entry impeller?
- More efficient.
- Simpler and more convenient ducting direct to diffuser.
- But a larger overall diameter of the engine is required.
What are the features of a double entry impeller?
- Smaller diameter.
- Higher speed.
- More complex ducting
What’s a unique feature of the double entry impeller?
It has a plenum chamber included in the ducting.
What’s the purpose of the plenum chamber in the double entry impeller?
The air must enter the engine at almost right angles to the engine axis. The plenum chamber creates this effect.
How is positive flow into the compressor inlet achieved? (Regarding pressure).
Postive pressure surrounding the compressor.
How do ‘Blow-in doors’ within the plenum chamber assist with positive pressure?
The doors open automatically whenever engine compartment pressure drops below atmospheric pressure.
How are the Blow-in doors held shut when the engine is not operating?
Via a spring.
What also assists in holding the blow-in doors shut? (During T/O and flight).
Ram air.
What is the design and function of the diffuser within a centrifugal compressor?
An annular chamber with vanes that create divergent passages into the compressor manifold, from the impeller.
What is the purpose of the Compressor Manifold?
Diverts flow of air from the diffuser into the combustion chamber, with one outlet duct per chamber for even distribution.
What important action do the manifold outlet ducts peform?
Turn the radial airflow into axial airflow for the combustion chamber.
What assists the outlet ducts in turning the airflow from radial to axial?
Cascade (turning) vanes. Present a smooth turning face.
In what engine-type are centrifugal compressors most common?
Turboprop and turboshaft engines.
What are the main design features of an axial flow compressor?
Multi-stage, consisting of rotors that rotate around fixed stator vanes, that are fixed to the compressor case.
Why do axial flow compressors gradually diverge (reduce in diameter)?
To maintain a constant axial flow velocity, as pressure and density increase.
What is a single spool engine?
A single compressor drum with multiple stators, driven by a seperate turbine.
What is a twin spool engine?
- A LP compressor (fan) driven by an LP turbine (N1).
- And a HP compressor driven by a HP turbine (N2).
What is a triple/multispool engine?
- A LP compressor (fan) driven by an LP turbine (N1).
- A HP compressor driven by a HP turbine (N2).
- And an IP compressor driven by an IP turbine (N3).
What is the ‘driving mechanism’ for compression?
The diffusion of air between the stators and rotors.
What is the approximate compression ratio between stages of an axial flow compressor?
Between 1:1 and 1:2. Hence, many stages are required.
Why do axial flow compressors use many stages of a small pressure increase?
Reduces the pressure losses, and increases performance and efficiency.
Advantages of Centrifugal flow compressors?
- High pressure rise per stage.
- Efficiency over a wide rotational speed range.
- Simplicity of manufacture and low cost.
- Low weight.
- Low starting power requirements.
Disadvantages of Centrifugal flow compressors?
- Its large frontal area for a given airflow.
- Losses in turns between stages.
Advantages of axial flow compressors?
- 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.
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 is a requirement of all fan blades?
They must be balanced.
What force of the fan blades needs to be balanced?
The centrifugal force.
What 3 parameters does the fan blade’s centrigual force 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 the ‘moment weight’ of a fan blade?
The product of the blade mass and the distance of the centre of gravity from the rotational axis.
What are the 3 units for measuring moment weight?
- gram inches.
- ounce inches.
- cm.g
Where can the moment weight value of a blade be found?
Stamped on the root of the blade, alongside the P/N and S/N.
What can cause a change in the moment weight value?
Bending. The old value will be crossed-out, and a new value stamped on.
How are blade positions around the engine, numbered?
Normally in the direction of rotation of the engine. But it can also be the opposite.
How are fan blade positions first assigned and balanced?
Using computerised balancing, where a computer uses weight values to determine best location around the spinner.
(Think a bit like wheel-balancing weights on a car wheel)
What is the BEST method for replacing a fan blade?
Replace the blade with a new blade that has the closest possible moment weight to the old blade.
What is the other method for replacing a fan blade?
Replace both the damaged blade, and the blade opposite to this blade.
Both blades should be replaced with blades of a moment weight as close as possible to the blades removed.
What can be added to assist in fan balancing?
- Balance weights, placed in the spinner or fan-disc.
- Balance screws.
What is the defintion of ‘Design Point’ of the compressor?
Optimum operating condition for the compressor where the airflow, pressure ratio, and speed are perfectly matched.
What is the ‘Resultant Rotor Inlet Velocity’?
The direction of the airflow, as a result of the relationship between inlet velocity and rotor speed.
What is important for smooth airflow into a rotor?
The Angle of Attack (AoA) of the airflow to the rotor blades must be small.
What determines the direction of the airflow into the following rotor stage?
The stator outlet velocity and the rotor speed.
Optimum compressor condition is only achievable at a single….?
Speed.
What affects the AoA onto the rotor blades?
- Chnages in rotor speed.
- Changes in inlet velocity.
What causes an increase in the AoA onto the rotor blades?
A decrease in inlet velocity.
How does an increase in airflow AoA on the rotor blades, intiate a stall in the compressor?
The airflow seperates from the aerofoils in the compressor, causing turbulent flow.
What can happen to the airflow when it stalls in a compressor stage?
It can slow down, stop, and even reverse direction.
What is a compressor surge?
A very severe form of compressor stall.
How is a surge generated, in 3 steps?
1) A rapid decrease of airflow causes stall on some blades or stages.
2) The stall causes a blockage in the airflow which leads to a stronger stall in the subsequent stages.
3) This causes low-pressure zones in which the airflow comes to a stop and reverses its direction.
Where in the compressor can surges occur?
Forward or Aft stages.
What types of compressors can experience a surge?
ALL types.
What are the characteristics of a surge in the Forward section of a compressor?
- The blades are larger, and thus only part of the blades are affected.
- The effects are not as dangerous as an Aft section stall.
What are the characteristics of a surge in the Aft section of a compressor?
- Shorter compressor blades and higher pressures results in rapid stall development.
- Rapid decrease in airflow that can reverse.
What is the most extreme effect of a compressor surge in the Aft section?
Reverse flow from the combustion chamber, back to the engine inlet.
What are the observed effects of compressor surge?
- Heavy vibrations.
- Loud banging.
What are the effects of compressor surge on the aircraft’s performance?
- Thrust decreases.
- Engine speed fluctuates.
- EGT increases.
What are the causes of compressor surge and stall, in relation to the compressor components?
- Dents or ice on the engine inlet cause turbulent or disrupted airflow.
- Damaged rotor blades or stator vanes.
- Dirty compressor blades or stator vanes.
- Damaged or broken combustor components can cause a blockage in the airflow and decrease the velocity in the compressor.
- Damaged turbine components .
- Dents in the jet nozzle.
- Broken objects in the exhaust system.
What are the causes of compressor surge and stall, in relation to how the aircraft is operated?
- Engine speed is too far below design intended speed.
- Incorrect acceleration or deceleration.
How do very low speeds on an axial flow compressor, potentially cause a stall?
The forward stages of the compressor are more efficient than the aft stages, so the air backs-up in the aft section and reverses.
How can fast deceleration cause a compressor stall?
The rotor speed of the HP compressor (aft) decreases at a faster rate than the LP compressor (fwd).
(Same effect as very slow speeds, where air backs-up in the aft section).
How can fast acceleration cause a compressor stall?
Results in too much fuel in the combustion chamber, causing an increase in back-pressure.
What are the 3 methods for preventing compressor stall and surge?
- Compressor bleed valves.
- Dual/triple spool.
- Variable compressor stator vanes.
Which of the 3 methods is most effective at preventing stalls and surges?
The variable compressor stator vanes.
Where are bleed valves found in the compressor?
Mainly the mid-section, but sometimes the aft section.
How do bleed valves help prevent compressor stall?
They open at low speeds to allow excessive airflow from the forward stages to escape, preventing blockages in the aft section.
What’s the disadvantage of compressor bleed valves?
Large efficiency losses, as some of the air is compressed and then bled off.
How does adding dual/triple spools help prevent compressor stall?
The compressors are split and driven by their own turbines.
What’s another advantage of dual and triple spool engines, unrelated to stall prevention?
Excellent acceleration capabillities, due to smaller mass of rotors.
How do variable compressor IGVs/stator vanes help prevent a stall?
Optimal AoA on the following rotor blades can be achieved at every engine speed. (Maxium efficiency).
Where are variable stator vanes usually located?
The forward stages of the HP compressor.
What is the only axis that variable stator vanes are moveable along?
ONLY the vertical axis.
What’s the disadvantage of variable stator vanes?
Very complex control mechanism.
What is the approximate compressor ratio value for every stage in an axial flow compressor?
1.3:1
(Air enters stage at 10psi, exits stage at 13psi.
What happens to pressure and density as air flows through the compressor stages?
Pressure and density both increase.
How is a constant velocity maintained through the compressor, whilst pressure and density are increasing?
By gradually reducing the diameter of the compressor per stage.
