15.6 Turbine Flashcards
What is the primary purpose of the turbine?
Transform a portion of the exhaust gas kinetic energy, into mechnical energy to drive the compressor and its accessories. (about 60-70% of total pressure energy from exhaust).
What is the amount of energy absorption at the turbine, determined by?
Determined by the load the turbine is driving. (Compressor size, type, number of accessories).
Where is the turbine usually located within the GTE?
Aft (downstream) of the combustion chamber.
What are the two main components of the turbine?
- Turbine Nozzle Guide Vanes (stator).
- Rotor section, blades attached to a rotating turbine disc).
How many stages can a turbine have?
One or more, defined by number of pairs of rotors and stators, just like compressors.
What are the two types of turbine?
- Radial flow.
- Axial flow.
How many stages does a radial flow turbine have?
Always 1.
Where are radial flow turbines normally used?
Only in small GTEs, such as an APU.
What is the advantage of a radial flow turbine?
Simple design and easy manufacture.
What are the disadvantages of radial flow turbines?
- Only allows small airflow.
- Less efficient.
- High aerodyamic losses as air must pass through turbine opposing centrifugal forces.
Where are axial flow turbines used?
On modern GTEs and high bypass ratio engines.
Why are axial flow turbines the ideal choice for modern GTEs?
- They create very high airflow, necessary for high thrust.
- They allow for any number of stages necessary to operate the compressor and all the accessories.
How do the Nozzle Guide Vanes accelerate and direct the gas flow from the combustion chamber?
- They have a convergent shape to accelerate the gas flow.
- The shape and angle helps direct the airflow in the direction of rotation of the rotor blades.
What causes the turbine blades to rotate?
The impact of the gas flow.
What are the 3 types of turbine blade used?
- Impulse configuration.
- Reaction configuration.
- Reaction - Impulse configuration.
What is the key point of the impulse-type turbine blade?
The blades experience an impulse force caused by the impact of the gas on the blades.
With the impulse-type blades, where does the total pressure drop of each stage occur?
Occurs in the fixed nozzle guide vanes, which increases velocity as pressure decreases.
What type of energy is the pressure energy converted to, in an impulse-type blade configuration?
Kinetic energy.
How does the impulse force react?
It’s resolved into 2 components.
In a Reaction configuration, what do the fixed nozzle guide vanes do?
They alter the gas flow direction, without changing the pressure.
How do the converging blade passages experience a reaction force?
By the expansion and acceleration of the gas.
Which of the blade configuration types is normally used in GTEs?
Impulse-Reaction configuration.
Pure impulse/reaction configurations alone, are not normally used.
What is experienced at the root of the blades, in a reaction-impulse configuration?
Impulse.
What is experienced at the tip of the blades, in a reaction-impulse configuration?
Reaction.
What is the benefit of the reaction-impulse configuration?
Blade exit pressure can be held relatively constant.
How are turbine blades normally constructed?
Either forged or cast, but are normally precision cast.
Turbine blades cast as a ‘single crystal’ gives what benefits?
Better blade strength and heat properties.
What coating helps keeps the turbine blades and inlet nozzles resistant from heat?
Ceramic coating.
What is the MAIN benefit of having heat resistant turbine blades and inlet nozzles?
Allows for higher exhaust temperatures, which increases engine efficiency.
What other method cools the turbine blades and inlet nozzles?
By airflow cooling.
Where are most turbine sections ‘open’?
At the outer perimeter of the blades.
What is a ‘shrouded turbine’?
A band is formed around the outer perimter of the turbine wheel.
What are the advantages of a shrouded turbine?
- Improves efficiency.
- Improves vibration characteristics.
- Permits lighter stage weights.
What are the disadvantages of a shrouded turbine?
- Limited Turbine speed.
- Requires more blades in total.
Why are additional stages normally required?
As a single turbine wheel cannot absorb enough power to drive all the components alone.
What are the main components of the rotor element of the turbine?
- The turbine shaft.
- The tubine wheel.
What does the turbine wheel consist of?
A dynamically balanced unit of turbine blades fitted to a rotating disc.
How does the turbine disc abosorb heat?
Via conduction.
How is the turbine disc cooled?
Bleeding cool air back onto the face of the disc.
What is the turbine disc attached to?
The power-transmitting shaft of the engine.
What is the turbine shaft normally made of?
Alloy steel.
In what two methods is the turbine disc attached to the shaft?
- Welded.
- Bolted through a shaft hub.
What is the most common method for attaching the turbine disc to the shaft?
Bolted through a shaft hub.
What causes the turbine shaft to rotate?
The blades extract power from the exhaust gas, and the kinetic energy rotates the disc, which is welded or bolted to the engine drive shaft.
What else is the turbine shaft attached to?
The compressor shaft.
How is the turbine shaft fitted to the compressor shaft?
Via splined ends.
What must be controlled in order to keep the turbine within safe operating limits?
Engine speed and temperature.
What stresses does the turbine section experience? (2)
- Rotational (torque) stress.
- Thermal stress.
How do the turbine blades fit onto the turbine disc, with thermal expansion considered?
They fit loosely onto the turbine disc, so that they become tight when the blades expand under thermal expansion when the engine is operating.
What are the two usual types of blade root design?
- Bulb type.
- Fir-tree.
What are the two main design methods of blade retention on the turbine disc?
- Peening.
- Blade root stop.
What is the peening method for turbine blade retention?
A small notch is ground in the edge of the fir-tree root before the blades are installed. Upon installation, the disc metal fills the notch and helps lock the blade in place.
What is the ‘blade root stop’ method for turbine blade retention?
A stop on one end of the turbine blade root allows the blades to be installed or removed in only one direction. The other end is secured via a tang that can be bent over and locked in place.
What are the 3 common names for the stator element of the turbine section?
- Turbine inlet nozzle vanes.
- Turbine inlet guide vanes.
- Nozzle diaphragm.
Where are the turbine inlet nozzles located?
Directly aft of the combustion chamber, immediately forward of the turbine wheel.
What is true about the temperature of the turbine inlet nozzle vanes?
It’s usually the hottest area that comes into metal contact with the engine.
Why are the turbine inlet nozzle vanes contoured, and at a specifc angle?
To convert a portion of the heat and pressure energy into velocity energy, which is then converted into mechanical energy.
What’s a secondary purpose of the turbine inlet nozzles?
To deflect the gases in the direction of turbine rotation.
How are the turbine inlet nozzles contructed to allow for thermal expansion?
They are assembled loosley into the inner and outer vane shrouds.
What are support rings?
They allow for the removal of the turbine inlet nozzle vanes as a one complete unit.
What other method can the turbine inlet nozzle vanes be secured to the shrouds by?
Riveted or welded.
If the turbine inlet nozzle vanes are riveted or welded to the shrouds, how is thermal expansion compensated for?
The supporting rings are installed as segmented pieces.
What type of damage is not permitted on blades?
Cracks.
What can assist in the visual inspection of turbine blades?
Dye penetrant
What must be done if a crack is found on a the turbine disc?
The disc and the turbine rotor must be replaced.
What can be done if slight-pitting is found on the turbine blades?
They can be blended within limits.
How do ‘stress rupture cracks’ usually appear?
As hairline cracks on the edges of the turbine blades, at 90 degree angles to the edge.
What cause deformation of the turbine blades?
Overtemp conditions.
How can deformation appear on turbine blades?
Waviness along the leading edges of the turbine blades.
What must the condition of the turbine blade leading edges be?
Straight and uniform across the entire edge, except for areas that have had blended repairs.
What is blade ‘creep’?
Slow structural deformation on a molecular level, by prolonged exposure to high stress.
What promotes blade creep?
Extreme temperatures.
How is blade creep identified?
Dimensional inconsistencies of the blades.
What can ‘scores’ on the turbine case be an indication of?
The length of the turbine blades are out of tolerance. (Maybe due to creep or overtemp).
