15.6 Flashcards
What is the sole purpose of the gas turbine?
To absorb 60-70% of the energy
Two types of turbine on GTE?
Axial flow
Radial Flow
What is always the first main component of a turbine?
Stationary vanes
What is another name for these stationary vanes?
Turbine nozzle guide vanes
What are the rotors connected to?
Turbine disc
What turbines are always single stage?
Radial flow
Where are radial flow turbines normally used?
Small GTE such as the APU
Advantages of Radial flow turbines?
They are simple design and easy to manufacture
What are the the disadvantages of Radial turbines and what does this mean?
Only allow small airflows so are less efficient
Why in the radial flow turbine is only small amounts of airflow allowed making it inefficient?
High Aerodynamic losses and airflow must enter against centrtifugal fores
Where are axial flow turbines mainly used?
In a modern GTE
How many stages are axial flow turbines limited to?
As many as is necessary to operate compressor
What is an advantage of axial turbines?
They allow high amounts airflow to create high thrust
What does the turbine convert the gas energy to?
Torque
What is used to accelerate the gas flow from the combustion chamber?
Convergent duct
Where is the convergent duct?
Between nozzle guide vanes
What is an impulse turbine and how is it recognised?
A turbine which makes the rotation only by the impact of the gas flow on the rotor blades, recognisable by the special shape of the rotor blades
What are the 3 types of rotor blades?
Pure Impulse configuration
Pure Reaction configuration
Reaction-Impulse configuration
In a pure impulse type turbine where does the total pressure drop occur and why?
The fixed nozzle guide vanes because of their convergent shape
What does all the edgy convert to after hitting the rotor blade?
Kinetic energy
In a reaction configuration what are the fixed nozzle guide vanes designed to do?
Alter the gas flow direction without changing pressure
What (to do with the gas) causes the rotor to spin in a pure reaction type turbine?
Expansion and Acceleration of the gas
What type of duct is involved in Reaction impulse (reaction) turbine?
Convergent
What happens to the velocity and pressure in a reaction impulse (reaction) turbine?
The velocity increases and pressure drops. The direction is what changes
Where does the Impulsive reaction hit the aerofoil on the rotor blade?
On the root
Where does the Reaction aspect hit the aerofoil on the rotor blade?
The tip
What are rotor blades made out of?
Precision cast
Why are turbine blades constructed of a single crystal?
Strong and heat properties
What do heat barrier coatings and ceramic/airflow coatings ensure about the turbine?
It is cooler
What does a cool turbine ensure about the efficiency of the engine and why?
The exhaust temp is higher increasing efficiency
What is the benefit of shrouded turbines?
Vibration resistance
Allows lighter stage weights
Efficiency
Disadvantage of shrouded turbines?
Limits turbine speed so you require more blades
What do shrouded turbines form around the perimeter of the turbine rotor?
A band
What does the rotor element of a turbine section consist of?
Shaft and a wheel
What type of balance is required on a turbine wheel?
Dynamically balancing.
What does dynamic balancing consist of?
Blades that have a rotating disc
The rotational speeds cause what loads on a turbine?
Centrifugal loads
When does a disc become a turbine wheel?
When it has blades attatched
What do the blades transmit their energy to?
The rotor shaft
How is the disc attached to the shaft?
Bolted or welded
What does the disc rim absorb?
Heat from the gases and turbine blades
How can you ensure that the disc rim isn’t subject to high temperatures?
Bleed cooling air back onto the face of the disc
What allows attachment of turbine blades to the disc?
A series of grooves and notches that have the same contour as the blade
What compensates for thermal expansion in the attachment of the blade root to the notch?
A clearance
What causes the blade root to fit tightly in the disc rim?
Operation of engines and thermal expansion
What is the turbine shaft made from?
Alloy steel
What are the 2 methods of joining the shaft to the disc?
Bolting and welding (which had a butt or protrusion)
What design is common in the attachment of the turbine blades?
Fir tree
What must be capable of withstanding torque loads?
Shaft
What must the turbine shaft have a means of connection to?
Compressor rotor hub
How do you attach the Turbine shaft to Compressor Rotor Hub?
Using spline cuts on the forward end of the shaft
What does the spline fit into?
A coupling device
What if a coupling device is not used?
Then the turbine shaft may fit into a splined recess
Where is a splined coupling method used?
Centrifugal compressor engines
What method does axial use?
A splined recess
How are the bases retained in their respective grooves?
Peening, welding, lock tabs and riveting
Explain the peening method (used frequently)
It requires a small notch to be ground before blade installation
After blade is inserted the metal flows into the notch by a small punch mark
What does the blade root being made as a stop ensure?
The blade can be removed and inserted in one direction only.
When the blade root is being used as a stop, what is on the opposite side of removal and application?
A tang that is bent to secure the blade in the disc.
Where are the turbine inlet nozzle guide vanes located?
Aft of the combustion chamber and forward of the turbine wheel
Where does the inlet nozzle guide vane experience high temperature?
Aft of the combustion chamber but forwards of the turbine wheel
What does the inlet nozzle convert heat and pressure energy to?
Velocity so it can be transferred to mechanical energy
How do nozzles convert heat efficiently?
They deflect gasses at a certain angle
What is the second purpose of the turbine inlet nozzle?
Why?
To direct airflow to the direction of turbine rotation
The gas must enter the turbine whilst it is still spinning
What does the inlet nozzle assembly consist of?
An inner and an outer shroud
Why must the nozzle vanes be contrasted to allow for thermal expansion?
There can be severe warping or distortion otherwise
How can you compensate for thermal expansion?
Loose assembly of the shorts (The slots area slightly larger than the vanes)
What increases strength rigidity and support in nozzle guide vanes?
Inner and outer support rings
What do the support rings also facilitate?
Easy removal of nozzle vanes as a unit (without them, the vanes could fall out as you remove the shroud)
What allows for thermal expansion when the vanes are riveted or welded onto the inner and outer shroudes?
The inner and outer shrouds should be cut into segments.
Are cracks permitted?
How is detection mostly carried out?
No, they need to be replaced
Visually
What is a structural inspection technique of cracking in a turbine?
Dye penetrant
What can be done to pitting raised by FOD?
It can be polished by blending out or stoning
What two general equipment can be used to carry out an inspection more efficiently?
Light and magnifying glass
How do stress rupture cracks normally appear as?
Hairline cracks that occur on leading/trialing edge at right angles to edge length. (Not less than 1/16)
How does deformation caused by overheating appear as?
Wavy
On first stage turbine blades, what must you automatically suspect when any stress crack deformation has occurred?
Overheat condition
What materials experience creep?
Metals and plastics
What is Creep?
A slow deformation on a molecular level caused by prolonged exposure to heat.
What promotes creep?
High temperatures
What can creep cause concludingly?
Dimensional inconsistency
What may scoring identify?
The length of the blade is out of tolerance
Why must you number the blades before removal?
To ensure they’re put in the correct place that they were taken off of
What is another name for the rotor?
Turbine wheel
What proceeds first, the nozzle guide vane or the rotor/turbine wheel?
Nozzle guide vane
