15.6 Flashcards
What does the turbine transform
Part of the kinetic (velocity) energy of exhaust gases into mechanical energy to drive gas generator compressors and accessories
Purpose of gas generator turbine
Absorb 60-70% of total pressure energy from the exhaust gases
Where is the turbine located
Aft or downstream of the combustion chamber
Directly behind gas chamber outlet
Turbine main components
Stationary vanes - turbine nozzle guide vanes
Rotor blades on a turbine disk
Radial flow stage ?
Single stage used on APU
Radial flow disadvantage
Small airflows and less efficient due to high aerodynamic losses
Where are axial flow turbines used
Modern GTE
Advantage of axial flow turbine
Very high airflow required to create a high thrust
Turbine stage
Stator and rotor
(Stator first)
How many stages can an axial flow turbine have
Any number of
What does the turbine convert
Gas energy from combustion chamber into torque
After the combustion chamber what’s the first part the gas flows through
Stationary turbine nozzle guide vanes Rotor blades
How is the gas flow in the combustion chamber accelerated
Convergent shape duct between nozzle guide vanes Rotor blades
What is an impulse turbine
Turbine that only rotates due to impact of gas flow
3 types of turbine blade
Impulse
Reaction
Reaction - impulse
In an impulse turbine where does the total pressure drop occur
Fixed nozzle guide vanes because of their convergent shape
In a impulse configuration what is pressure energy converted to
All pressure energy converted into kinetic energy
What is a reaction type configuration designed to do
Alter gas flow direction without changing the pressure
What turbine combination do GTE use
Reaction - impulse
What forces are acting on a reaction turbine
Both impulse and reaction forces that acts in the plane of rotation to drive the turbine
What do impulse turbines require
High - velocity gas to obtain the maximum rate of momentum change
How does a reaction turbine cause its rate of momentum to change
By nozzling action of the rotor blade
What does a uneven workload cause if the blade
Gases to exit from the blade at different velocities and pressures
What type of cast are most blades
Precision
Why do we cast as single crystal
Gives blades better strength and heat properties
How do we keep turbine blades and inlet nozzles cooler
Using heat barrier coatings like ceramic coatings
Airflow cooling
What effect does keeping inlet nozzles cooler have
Increase efficiency of the engine
Advantage of shrouded turbine blades
Improves efficiency and vibration characteristics
Lighter stage weights
Disadvantage of shrouded turbine blades
Limits turbine speed and requires more blades
Why do we need additional turbine stages
Single turbine wheel cannot absorb enough power from exhaust gases to drive the components
What does the rotor element of a turbine section consist of
Shaft and wheel
Why do we control engine speed and temperature
To keep turbine within operations limits due to high centrifugal loads and temperatures on it
How is a turbine wheel balanced
Dynamically
What is the disc attached to
Main power-transmitting shaft of the engine
What is the purpose of grooves/notches in the rim of the disk
Allow attachment of the turbine blades and space for thermal expansion
What material is turbine shaft
Alloy steel that can absorb high torque loads
How do you cool the disk rim
Bleeding cooling air back onto the face of the disk
Most common method of connecting shaft to turbine disk
Bolting
How is the turbine shaft attached to compressor rotor hub
Spline cut on the forward end of the shaft
What does a spline it into
Coupling device between compressor and turbine shafts
What do we use if a coupling is not used
Splined end of the turbine shaft may fit into a splined recess in the compressor rotor hub
Where is the splined coupling arrangement used
Centrifugal compressor engines
Most common way to attach turbine blades
Fir tree design
Methods of retaining blades
Peening , welding , lock tabs and riveting
What is a notch filled with
Disc material
Why is the tang bent
Secure blade in the disc
What does using the Blade root as a stop do
Made on one end so the blade can only be inserted and removed in one direction
2 parts of turbine assembly
Stator - nozzle and inlet guide vanes
Turbine blades (rotor)
Stator 3 different names
Turbine inlet nozzle vanes
Turbine inlet guide vanes
Nozzle diaphragm
Where is the turbine inlet nozzle vane located
Aft of the combustion chambers and immediately forward of the turbine wheel
Where are turbine inlet nozzle assembled
Between outer and inner shrouds on rings
Where do vanes fit
Contoured slot in the shrouds
How can we fit vanes into inner and outer shrouds
Weld or river them into position
Why is the shroud ring cut into segments
To allow for thermal expansion
What’s a structural inspection technique
Dye penetrant
How can we get rid of putting in blades
Stoning and polishing
How do stress rupture cracks from
At aright angle to the edge length
How does deformation due to over temperature appear
Waviness
Length of visible cracks
1/16th inch upwards
What happens if cracks or deformation occurs on the leading edge of the first stage turbine blade
Over temperature condition must be suspected
What is creep
Slow structural deformation on a molecular level
How does creep occur
Prolog exposure to high stresses and high temperatures
What does scoring on the turbine case suggest
Length of blades out of tolerance
