Module 15.6 Turbine section Flashcards
What is the sole purpose of the gas generator turbine?
Absorb 60-70% of the total pressure energy
The turbine turns KE into what?
Mechanical energy
What are the two types of turbine?
- Radial flow
- Axial flow
How many stages are radial turbines?
One
What is an advantage of axial turbines?
Very high airflow
What is one compressor stage?
Rotor then stator
What is one turbine stage?
Stator then rotor
Which turbine makes rotation only by the impact of the gas flow on the rotor blades?
Impulse turbine
What are the 3 types of turbine blades?
- Impulse configuration (called pure impulse)
- Reaction configuration (called pure reaction)
- Impulse-reaction configuration (commonly called reaction type)
In the impulse configuration, what is the total pressure drop across?
Nozzle Guide Vane (NGV)
Kinetic energy increases
What shape are the fixed NVG’s?
Convergent
In the reaction type configuration, what are the fixed NVG’s designed to do?
Alter gas flow direction without changing the pressure
What forces act on the blade of a reaction turbine?
Impulse and reaction
Where do the impulse forces act on the reaction turbine?
At the blade root
Where do the reaction forces act on the reaction turbine?
At the blade tip
In the impulse configuration, what increases across the NGV’s?
The gas velocity
Better strength and heat properties are obtained by casting blades as?
Single crystal
What is used to cool nozzles and turbine blades?
- Ceramic coating
- Airflow cooling
What type of turbine blades improve efficiency and vibration and permit lighter stage weights?
Shrouded turbine blades
What is the disadvantage to shrouded turbine blades?
Limits the turbine speed
Why is the shrouded turbine rpm limited?
As there is a higher moment weight
Where should shrouded turbine blades be used?
Low pressure turbine as it spins slower
Where can’t shrouded turbine blades be used?
High pressure turbine
What are the turbine blades installed on to?
Turbine disc
What is the turbine disc attached to?
Main power transmitting shaft
What is it called when the blades are installed on the turbine disc?
Turbine wheel
What balance type is a turbine wheel?
Dynamically
What is one method of relieving stresses on the disc rim?
Bleed cooling air on the face of the disc
What are the two methods of connecting the shaft to the disc?
- Welding
- Bolting
What allows the shaft to spin the compressor?
Splines cut into forward end of the shaft
What do the turbine shaft splines fit into?
- Coupling device
- Recess in compressor rotor hub
What compressor shaft connection do centrifugal compressors use?
Coupling device
What are the two types of blade to disc attachments?
- Bulb
- Firtree
What are the 4 methods of retaining the fan blades into the disc?
- Peening
- Welding
- Lock tabs
- Riveting
What is the purpose of the NGV?
- Straighten and guide gas flow
- Decrease static pressure
- Increase velocity
What comes after the first NGV’s in the blade section?
Rotor, High pressure turbine
What are the NGV’s fitted between?
The outer and inner shrouds/rings
What fit do the slots give the vane?
Loose fit
What are the 3 methods of fitting vanes into the shrouds?
- Riveting
- Welding
- Contoured slot
What does the fit of the vanes allow for?
Thermal expansion
How is inspection for cracks conducted?
Mainly visual, but some structural techniques like dye penetrant may be used
What damage may be blended by stoning or polishing?
Pitting
Cracks cause replacement
What is blade creep?
Elongation of the blade by the centrifugal forces and temperature
When stress cracks or deformation are found on the LE of 1st stage turbine blades, what is suspected?
Over temperature condition
How do stress cracks appear visually?
Hairline cracks at right angles to edge length
