15.6 Turbine Section Flashcards
What does the turbine section do?
Transforms kinetic energy to mechanical energy to drive the compressor and accessories.
What is the purpose of a gas generator turbine?
To absorb 60 to 70% of total pressure energy.
What are the 2 types of turbines used?
Radial flow and axial flow.
What can be said about the stages of a radial flow turbine?
Always only single stage.
Where is a radial flow turbine used?
On APUs.
What are the negatives of a radial flow turbine?
Small airflow.
Less efficient.
Where are axial flow turbines used?
On modern engines.
What is the advantage of the axial flow turbine?
Allows very high airflow to create high thrust.
What does a stage consist of in the turbine?
Nozzle guide vane (stator), then a rotor.
What effect do nozzle guide vanes in the turbine have on velocity and pressure?
V increases.
P decreases.
What effect do rotors in the turbine in the turbine heave on velocity and pressure?
V decreases.
P increase.
How do turbine blades operate.
Converts gas energy (convergent shaped) from the combustion chamber into torque.
Impact of gas flow on the blades rotates the turbine.
What is key about how an impulse turbine operates?
The turbine only rotates from gas flow impact on the rotor blades.
What are the 3 configurations of turbine rotor blades?
Impulse.
Reaction.
Reaction - impulse.
What is key about the fixed nozzle guide vanes in an impulse configuration and why ?
Due to their convergent shape, It is where the total pressure drop occurs at each stage.
Describe the total pressure drop of an impulse turbine blade configuration?
It is too big.
What does an impulse turbine blade configuration do to energy?
Converts all static pressure energy to kinetic energy.
What do fixed nozzle guide vanes do in a pure reaction turbine blade configuration?
They alter gas flow direction without changing pressure.
Describe the pressure drop of a pure reaction turbine blade configuration?
It is too little.
What turbine blade configuration is normally used?
Reaction- impulse.
Known as a reaction turbine.
Where does impulse force act in a reaction turbine blade configuration.
At the roots.
Where does reactions force act on a reaction turbine blade configuration?
At the tip.
How are turbine blades constructed?
They are precision cast as a single crystal.
Why are the turbine blades single crystal?
Provides better strength and heat properties.
What is done to turbine blades to keep them and the inlet nozzles cooler?
A ceramic coating is added.
What does a ceramic coating on turbine blades allow for ?
A higher exhaust temperature, increasing the efficiency of the engine.
Where are shrouded turbine blades used?
On low pressure turbines.
What is the advantage of shrouded turbine blades?
Increased efficiency and vibration characteristics resulting in lighter stage weight.
What is the purpose of the holes in turbine blades?
Purely for cooling.
Define a turbine wheel.
A dynamically balanced unit of blades attached to a rotating disc.
What is the turbine disc attached to ?
To the main transmitting shaft.
What is the most common attachment for turbine blades?
Fir tree.
What relieves stresses from hot gasses on the disc rim?
Bleeding cool air.
What can be found on the rim of the turbine disc ?
Grooves for blade attachment.
The turbine shaft must be capable of absorbing high torque loads. How is this achieved?
Welding the shaft to the disc.
Bolting.
What is the purpose of a spine cut on the fwd end of the shaft?
Attachment for the shaft to the compressor rotor hub.
Where is a splined coupling arrangement used?
On centrifugal compressor engines.
What’s the most common method to retain turbine blades in grooves?
Peening.
What is peening?
The blade is installed then the notch is filled by the disc metal.
What is another method of blade retention?
Using the blade root as a stop and the other end has a tang bent to secure the blade in the disc.
Where are nozzle guide vanes located in a turbine?
Aft of the combustion chamber.
What is the primary purpose of the nozzle guide vanes?
To convert heat and pressure to velocity energy.
What is the secondary purpose of the nozzle guide vanes?
Deflect gasses to a specific angle.
How are nozzle guide vanes assembled?
Between inner and outer shrouds/rings.
Each vane fits into a contoured slot in the shrouds.
How are turbine blades inspected for stress and creep?
Mainly visually.
Or dye penetrant.
What can cause stress rupture cracks?
Overtemprature conditions.
What is creep on turbine blades?
Slow structural deformation on a molecular level due to prolonged exposure to high stresses.
What makes creep on turbine blades worse?
High temperatures.
How big are turbine cracks types?
1/16 of an inch or bigger.
