15.4 Flashcards
What is the main role for a compressor section of a GTE?
To supply sufficient quantity of air to satisfy the combustion chamber
What is the secondary function of the compressor?
To supply air to the systems that require bleed air
What is the location of the bleed ports dependant on?
The pressure and temperature of the required systems
Two main components of a compressor?
Stator and Rotor
What do the rotors do and why?
They contain rotor blades that suck in air and push it to the outlet side of the rotor to increase the energy of the airflow
What is the name given to the blades on a stator?
Stator vanes
What do stator vanes do?
Guide and slow the airflow which increases static pressure
What is the combination of a rotor and a stator called?
Compressor stage
What is always the first part of the compressor stage?
Rotor
What is an inlet guide vane?
A set of stator vanes infront of compressor stage
What do the IGV’s do?
Improve airflow into the first compressor stage
Are IGV’s fixed or moveable?
Both
What are the stator vanes in the fan duct called?
Outlet guide vanes or OGV’S
What do OGV’S do?
Improve airflow into the fan nozzle
Two types of compressors used in gas turbine engines?
Centrifugal and Axial
What does the centrifugal flow compressor consist of?
Impeller (Rotor)
Diffuser (Stator)
Compressor manifold
What is the ratio of centrifugal compressors and what does this say about it?
8:1 so a High-pressure rise per stage
How many stages are centrifugal compressors limited to and why?
2 due to concerns in efficiency
What is the impeller made out of?
Aluminium alloy
How many entries can the impeller have?
One or two
Because an impeller has a smaller diameter, what does this mean for the rotational speed?
It must be higher to ensure sufficient airflow
What does the single entry impeller ensure and what comes after the eye?
Direct ducting through the eye to inducer vanes (easier than the double entry which is more complicated)
How do axial compressors compress air and why is this good?
They compress the air in its direction of flow which reduces energy lost due to turns
Where are axial flow compressors found?
turbofan engines
What does the axial flow compressor consist of?
One or more rotating aerofoil blades
Where are centrifugal flow compressors used?
Turboprop and turboshaft
Why do you need multiple stages in a compressor?
Because the energy output (pressure increase) from a single stage is small
Where are the stationary vanes (stators) mounted in an axial flow compressor?
To the compressor casing
What does a stage refer to?
A rotor-stator pair
What duct is included in the axial compressor?
Convergent
Why is there a gradual decrease in the diameter of the rotating drum and casing? (Convergent)
To maintain a steady axial velocity as pressure and density increase
What are multi-spool compressors driven by?
An independent turbine
What happens to the remainder of the flow in axial compressors?
It is bypassed and mixes in the exhaust
What does the fan act as?
A Low pressure compression stage
What is core flow/hot air?
The smaller proportion of flow
Where is core flow passed through?
The intermediate pressure and high pressure compressor
What is the compression ratio of axial flow?
Between 1:1 and 1:2
Why is the rate of pressure increase per stage purposelly low?
Because you don’t want air to move to quick so that it losses efficiency and stalls
How do you achieve a high compression rate?
Increasing stages
How many stages does the fan consist of?
One
Advantages of cenrtigiugal compressors?
High pressure rise per stage
Efficiency over wide rotational speed range
Simple
Low cost
Low starting power
Disadvantages of centrifugal compressors?
Large frontal area
Losses in turns between stages
Advantages of axial flow compressors?
High peak efficiency
Smaller frontal area
Straight through flow allowing high RAM efficiency
Increase pressure rise by increasing stages with negligible losses
Axial flow compressors disadvantages?
Only efficient at slight tolerances (narrow window)
High cost
Difficult to manufacture
Relatively high weight
High starting power (partially overcome by split compressors)
What is beneficial about balancing fan blades?
They can be balanced without dissassembly of the engine
What needs to have identical centrifugal forces on an engine to prevent vibrating rotors?
The fan blades
What three parameters do centrifugal forces depend on?
Rotor speed
Blade mass
Distance of CoG from fan blade to rotational axis
What is the moment weight?
The product of the blade mass and the distance of the CofG from the rotational axis
What is the moment weight measured by?
A special blade scale
Why do fan blades have a different moment weight even if they have the same mass?
The CofG is different
What can the moment weight be given in?
Gram inches
cm.g
Ounce inches
Where can the moment weight, serial and part numbers be found?
The root
What matches the blades on the fan disc using moment weight
Computerised balancing
What is the best method to change a single fan blade?
To replace the blade with a new blade with a moment weight as close to the old one as possible
What can be used if the difference in moment weight is larger than specified by the manufacturer?
Balance weights can be used
Where are the balance weights installed?
On the fan disc or in the spinner cone
What is another method for replacing a single fan blade?
To change a pair with similar moment weights even if one blade is damaged
What are axial and centrifugal compressors sensitive to?
Changes in airflow passing through compressor
What is the resultant rotor inlet velocity?
The correct relationship between the inlet velocity and the rotor speed which ensures smooth airflow
The airflow to the rotor is smooth as long as what?
The AOA to the rotor blades is small
What do stator vanes do? Also to speed?
Guide the airflow to the next rotor stage and decelerates
The airflow in the axial compressor is smooth as long as what?
The AOA to the rotor blades and stator vanes is small
What happens to the angle of attack when the inlet air velocity decreases?
Angle of attack increases
What happens to the angle of attack when the inlet air velocity increases?
Angle of attack decreases
What happens when the AOA increases? What is this known as?
Turbulent vortex caused because of airflow separation (Compressor stall)
What does a compressor stall cause air to do?
Airflow to slow down, stop or reverse direction
What is a compressor surge?
A severe form of compressor stall where airflow reverses, stops or slows
How are pressure surges formed?
Rapid decrease of airflow
Blockage in airflow
Low pressure zones formed which reverses airflow
Where do surges occur?
On the forward or aft compressor stages
What do surges in the forward stages usually affect?
Only a part of the compressor blade
When do surges develop very rapidly over a large part of compressor?
In the aft stage
What are the two main reasons for a compressor stall and surge?
Engine is too far below design speed
Incorrect acceleration or deceleration
What are the three design methods to prevent compressor stall and surge?
Bleed valves
Dual or triple spool rotors instead of single spool
Variable compressor stator vanes
Where are bleed valves found?
Mid section or aft of compressor
When are bleed valves open?
During low speeds so excessive airflow can escape overboard
On a dual spool engine, the forward part is named as what?
N1 or Low pressure compressor
On a dual spool engine, the aft part is named as what?
N2 or the High pressure compressor
Is each compressor rotor joined or does it use a separate turbine?
Separate turbine
What is the most effective method to prevent stall?
Variable stator vanes
What axis do variable stator vanes move along?
The vertical axis
Where can variable stator vanes/variable inet guide vanes be found?
Forward stage of High pressure compressor
What is an advantage of variable stator vanes?
Optimum angle of attack on rotor blades can be reached for every engine speed
Disadvantage of variable stator vane?
Needs complicated control mechanism.
What does the compression ratio represent?
Ratio of volume of combustion chamber from largest and smallest capacity
What remains constant from the front to the rear compressor?
Axial velocity of gas
What is compression ratio not proportional to?
RPM but does vary with it
Each stage of an axial flow compressor has a ratio of what?
1.3:1
Because the pressure increase at each stage is small, what needs to be in place to raise the pressure for efficient combustion?
Multiple stages
