How does a turbine differ from a compressor?
The expanding gas is moving in a direction of decreasing pressure.
As pressure increases in the compressor, the direction of gas flow wants to take the easiest route out which is back on itself unlike the turbine stage where it’s energy is so high, it wants to propel itself out its natural gas flow direction.
What does a turbine stage consist of?
Nozzle Guide Vanes followed by a row of rotating turbine blades
What are the turbine blades attached to?
A turbine disk which is then attached to a drive shaft
As the gas flow moves rearwards from the HP turbine to the LP turbine in a multi-spool, explain what is happening to the NGVs
The NGV stages gradually get bigger to account for the correct amount of energy to be extracted due to the loss of energy in the gas stream
What are the number of turbine stages dependent on?
- Amount of power the turbine has to produce
- Number of compressors driven
- Largest turbine diameter the engine design allows
What are the sub assemblies of a single stage turbine?
- NGVs
- Turbine blades
- Turbine disk
Describe the basic construction and operation of NGVs
Convergent duct to allow an increase in kinetic energy in the gas stream by converting thermal and pressure energy
State the function of a shroud on a NGV
To ensure rigidity, create a smooth passage for the gas flow and prevent tip leakage
What is the purpose of a turbine blade?
To extract energy from the gas flow to drive the compressor, accessories and/or high speed output shaft
What shape is a turbine blade?
Aerofoil shaped
What are the 3 types of turbine blades?
Impulse
Reaction
Impulse-Reaction
Where are pure impulse blades found?
Used in the air starter and APUs
Why are reaction blades not used?
They are inefficient
Because the impulse-reaction blades are 50-50, what shape does it take?
A twisted aerofoil shape from root to tip
What is the twisted profile known as on a turbine blade?
Stagger angle
What does a stagger angle ensure?
Ensures that the gas flow does equal work at all positions along the length of the blade
Ensures it enters the exhaust system with a uniform axial velocity
How are turbine blades attached to the turbine disk?
Fir tree root method
What must the turbine disk be capable of withstanding?
High centrifugal loads
High thermal loads
How does the turbine disk cope with the high loads?
Wider at the root and tapers towards the rim to carry the bade root fixings and air seal
What type of turbine blades are used for smaller GTE to reduce the centrifugal loading imposed on the disk?
A BLISK
What is required in design to account for a temporary increase in length during operation?
A clearance gap between the turbine blade tip and the casing
What is permanent increase in length of the turbine blade known as?
Blade creep
What is creep defined as?
The continuous extension of a component under loads, even though the load is less than that which would cause fracture
What is the degree of creep dependent on?
Operating temperature
Higher the temp, higher the creep
