15.4 Compressors Flashcards
What is the primary function of the compressor section in a gas turbine engine?
To supply a sufficient quantity of air to satisfy the requirements of the combustion chamber.
What is a secondary function of the compressor in a gas turbine engine?
To supply air to systems that require bleed air for operation.
What are the two methods to compress air in engines?
- Decrease the volume in a cylinder by a piston.
- Make use of Bernoulli’s Principle.
What are the two main components of a gas turbine engine compressor?
- Compressor rotor
- Compressor stator
What is the role of the rotor blades in a gas turbine engine compressor?
To suck in air and push it to the outlet side of the rotor, increasing the energy of the airflow.
What are stator vanes and what is their function in a compressor?
Fixed blades attached to the stator that guide and slow the airflow to cause an increase in pressure.
What is the combination of a rotor and stator called?
A compressor stage.
What are inlet guide vanes (IGVs) used for?
To improve the airflow into the first compressor stage.
What are outlet guide vanes (OGVs) and their purpose?
Stator vanes in the fan duct that improve the airflow into the fan nozzle.
What are the two types of compressors currently used in gas turbine engines?
- Centrifugal flow compressor
- Axial flow compressor
Fill in the blank: The compressor rotor has _______ mounted on it.
[rotor blades]
True or False: The compressor stator is a rotating component.
False
What additional components support the compressor rotor bearings?
- Compressor inlet case
- Compressor outlet case
What are the main components of a centrifugal-flow compressor?
Impeller, diffuser, compressor manifold
The impeller is responsible for accelerating air, while the diffuser directs air to the manifold.
What is the typical pressure rise per stage for centrifugal compressors?
Around 8:1
This indicates the efficiency of centrifugal compressors in generating pressure.
What are the two main types of impellers in centrifugal compressors?
Single entry, double entry
The main differences are in size and ducting arrangement.
How does the double-entry impeller differ from the single-entry impeller?
Smaller diameter, higher rotational speed
The double-entry impeller allows for more compact ducting but requires more complex designs.
What is the function of the plenum chamber in double-entry compressor engines?
To provide positive airflow into the compressor
This is necessary for optimal compressor performance.
What is the role of the diffuser in a centrifugal compressor?
Directs airflow from the impeller to the manifold
Designed to retain energy imparted by the impeller while increasing pressure.
What are the outlet ducts of a compressor manifold known as?
Air outlet ducts, outlet elbows, combustion chamber inlet ducts
These ducts change the airflow direction from radial to axial.
True or False: Axial flow compressors compress air while changing its direction.
False
Axial flow compressors maintain the original direction of airflow to minimize energy loss.
What defines a stage in a compressor?
A rotor-stator pair
Each stage contributes to the pressure increase in the air compression process.
What are Inlet Guide Vanes (IGVs) used for in compressors?
To guide airflow towards the first stage
They improve the efficiency of the compressor by directing flow properly.
What is a single-spool compressor?
A compressor with a single drum and multiple stators
Total stages depend on the desired pressure increase.
What is the main advantage of a multi-spool compressor?
Higher compression ratio with optimal speed control
This setup is more efficient for dual-flow engines.
What is the primary function of the fan in a triple-spool engine?
Acts as a low-pressure compression stage and propeller
The fan helps manage airflow into the core and bypass flow.
What is the typical compression ratio between stages in axial flow compressors?
Between 1:1 and 1:2
This small ratio minimizes flow separation and stall.
What are the advantages of centrifugal flow compressors?
- High pressure rise per stage
- Efficiency over a wide rotational speed range
- Simplicity of manufacture
- Low weight
- Low starting power requirements
These advantages make centrifugal compressors suitable for specific applications.
What are the disadvantages of axial flow compressors?
- Good efficiencies over a narrow rotational speed range
- Difficulty of manufacture
- High cost
- Relatively high weight
- High starting power requirements
These factors limit their use in certain engine designs.
What is fan balancing?
Fan balancing is the process of ensuring that fan blades create identical centrifugal forces to run free of vibration.
What components require balancing in turbine engines?
All turbine engine rotating assemblies require balancing by the manufacturer.
Why do high-bypass fans require particular attention to balance?
High-bypass fans require attention due to their large diameter and high rotational forces.
What can cause a fan to become unbalanced?
Even minor damage can compromise the integrity of the rotating assembly and its bearings.
What three parameters affect centrifugal forces in fan blades?
- Rotor speed
- Blade mass
- Distance of the centre of gravity of the fan blade from the rotational axis
What is moment weight?
Moment weight is the product of the blade mass and the distance of the centre of gravity from the rotational axis.
How does moment weight change?
Moment weight changes with the blade mass and the position of the centre of gravity of the fan blade.
What units can moment weight be given in?
- gram inches
- cm.g
- ounce inches
Why can’t fan blades be manufactured to have the same moment weight?
Fan blades have different moment weights due to manufacturing tolerances.
What is the range of moment weights for the fan blades on a CFM56-3 engine?
The moment weights vary between 29,001 gram inches and 33,951 gram inches.
What information is typically found on the blade root?
- Part number
- Serial number
- Moment weight
What does the computer do in computerized fan blade distribution?
The computer works out the best position for each blade on a fan disc.
How are fan blades typically numbered?
Fan blades are often numbered in the direction of rotation, but can also be numbered in the opposite direction.
What is the preferred method for changing a single fan blade?
Replace the blade with a new blade that has a moment weight as close as possible to the old one.
What should be done if the difference in moment weight between old and new blades is larger than specified?
Balance weights can be used to balance the difference in moment weight.
Where can balance weights be installed?
Balance weights can be installed on the fan disc or in the spinner cone.
What is the second method for changing a single fan blade?
Change a pair of blades with similar moment weights, replacing the damaged blade and the blade opposite it.
What is the primary operating condition for a given pressure ratio, rotational speed, and airflow in compressors?
Design point
At the design point, the airflow in the compressor is perfectly matched.
What is the resultant rotor inlet velocity in an axial compressor?
The combination of air inlet velocity and rotor speed
A smooth airflow into the rotor requires the correct relationship between these two velocities.
What happens to airflow at the stator vanes in an axial compressor?
The stator vanes decelerate the airflow and guide it to the next rotor stage
This sequence continues through all the compressor stages.
What is compressor stall?
A condition where airflow separates from compressor aerofoils, causing turbulence
Stall can slow down, stop, or reverse airflow in the compressor.
What can cause an increase in the angle of attack on rotor blades in an axial compressor?
A decrease in inlet air velocity or changes in rotor speed
What is compressor surge?
A severe form of compressor stall characterized by rapid airflow decrease and reverse flow
Surge can happen on all engine compressors, affecting operation significantly.
How does compressor surge develop?
It develops from a blockage in airflow caused by stall in subsequent stages
This leads to low-pressure zones where airflow can reverse.
What are the effects of compressor surge on engine operation?
Decreased thrust, fluctuating engine speed, and increased exhaust gas temperature
What are some causes of compressor stall and surge?
Damaged engine components, incorrect operational conditions, and disrupted airflow
Examples include dents or ice on the engine inlet, damaged rotor blades, and dirty components.
What operational condition can lead to compressor stall?
Engine speed too far below design speed or incorrect acceleration/deceleration
What happens to airflow in the aft stages of a large axial compressor when operating below design speed?
Air piles up, slows down, and can reverse direction
Fill in the blank: Compressor stall can occur at some ______ only, making it weak and almost unnoticeable.
aerofoils
True or False: Surge in the forward stages of a compressor is usually dangerous.
False
Surge in the forward stages usually affects only a part of the compressor blades.
What can fast deceleration in dual and triple-spool engines cause?
The rotor speed of the high-pressure compressor decreases faster than that of the low-pressure compressor
What is the consequence of fast acceleration in a compressor?
Too much fuel in the combustion chamber increases back pressure and can cause stall
What do HP Bleed Valves do?
They help prevent compressor stall and surge by allowing excessive airflow to escape.
Found in the mid-section and sometimes in the aft section of the engine compressor.
What is the function of Variable Inlet Guide Vanes?
They adjust the airflow entering the compressor to maintain optimum efficiency and prevent stall.
They are located in the forward stages of the high-pressure compressor.
What are Variable Stator Vanes used for?
They are used to optimize the angle of attack on rotor blades, preventing stall and ensuring efficiency.
Like inlet guide vanes, they are also moveable along the vertical axis.
What does the control mechanism for moveable vanes do?
It controls the rotation of the variable inlet guide vanes and variable stator vanes to maintain optimal engine performance.
This mechanism is complex.
List the methods to prevent compressor stall and surge.
- Compressor bleed valves
- Dual or triple-spool rotors
- Variable compressor stator vanes
True or False: All three methods to prevent stall can work independently.
True
What is a disadvantage of using bleed valves?
They cause a loss in efficiency due to airflow being blown overboard.
This is especially significant in large multi-stage axial compressors.
What are the components of a dual spool engine?
- Low-pressure compressor (N1)
- High-pressure compressor (N2)
What is an advantage of using dual or triple-spool rotors?
They provide excellent acceleration capability due to smaller rotor mass.
Triple spool engines include an N3 compressor for even better performance.
Fill in the blank: The most effective method to prevent stall is ______.
Variable stator vanes
Where are bleed valves located in an engine?
In the mid-section and sometimes in the aft section of the engine compressor.
What does the compression ratio of an internal combustion engine represent?
The ratio of the volume of its combustion chamber from its largest to its smallest capacity.
What is a fundamental specification for many combustion engines?
Compression ratio.
In a turbine engine, what conditions affect the optimum effect of compressor components?
Mass flow, pressure ratio, and rpm.
What remains constant from the front to the rear of a compressor?
Axial velocity of the gas.
Is the compression ratio proportional to RPM?
No.
What is the typical pressure ratio for each stage of an axial flow compressor?
About 1.3:1.
If the air pressure enters the first stage of a compressor at 10 psi, what is the output pressure after the first stage?
13 psi.
How do you calculate the output pressure of the second stage of a compressor?
Multiply the output pressure of the first stage by the pressure ratio of 1.3.
What is the output pressure of the second stage if the first stage’s output is 13 psi?
16.9 psi.
Why are many stages needed in a compressor?
To raise the pressure to high values necessary for efficient combustion.
What happens to the density of air as the pressure increases in a compressor?
The density of air increases.
What adjustment is made to the compressor cross-section as air pressure increases?
The cross-section is gradually decreased.
Fill in the blank: The compression ratio varies with _______ but is not proportional to it.
RPM
True or False: The pressure increase at each stage of a compressor is significant.
False.
