15.6 Turbine Section Flashcards
What is the primary purpose of the gas generator turbine?
To absorb approximately 60 to 70% of the total pressure energy from the exhaust gases.
Where is the turbine section of a gas turbine engine located?
Aft, or downstream, of the combustion chamber.
What are the two types of turbines in gas turbine engines?
- Radial flow turbine
- Axial flow turbine
What is the first main component of a turbine?
A set of stationary vanes, known as the turbine nozzle guide vanes.
What is the main advantage of radial flow turbines?
Simple design and easy to manufacture.
What are the disadvantages of radial flow turbines compared to axial flow turbines?
- Allow only small airflows
- Less efficient due to high aerodynamic losses
What is the main application of axial flow turbines?
Used on modern gas turbine engines.
How does the turbine convert gas energy from the combustion chamber?
By converting it into torque.
What must the gas flow do before impacting the turbine rotor blades?
Pass through the stationary turbine nozzle guide vanes.
What type of turbine is driven only by the impact of gas flow on the rotor blades?
Impulse turbine.
How is pressure drop managed in the impulse configuration?
Occurs in the fixed nozzle guide vanes, increasing gas velocity and reducing pressure.
What is the role of the fixed nozzle guide vanes in the reaction configuration?
To alter the gas flow direction without changing the pressure.
What type of turbine configuration combines both impulse and reaction forces?
Reaction-impulse configuration.
What is a key characteristic of impulse turbines?
Require high-velocity gas to obtain maximum rate of momentum change.
What is a key characteristic of reaction turbines?
Do not require excessively high nozzle exit velocities.
What happens if the power load is not evenly distributed along the blade?
Causes gases to exit from the blade at different velocities and pressures.
What construction methods are used for turbine blades?
- Forged
- Cast
What is a benefit of casting turbine blades as a single crystal?
Provides better strength and heat properties.
What type of coating helps keep turbine blades cooler?
Heat barrier coatings, such as ceramic coating.
What is the disadvantage of shrouded turbine blades?
Limits turbine speed and requires more blades.
Why might more than one turbine stage be necessary?
A single turbine wheel cannot absorb enough power from the exhaust gases.
Fill in the blank: The turbine transforms a portion of the kinetic energy of the exhaust gases into _______.
[mechanical energy]
What are the main components of the turbine rotor element?
A shaft and a wheel
The wheel consists of blades attached to a rotating disc.
What causes the turbine wheel assembly to rotate at high speeds?
Exhaust gases leaving the turbine inlet nozzle vanes act on the blades
This results in high centrifugal loads and high temperatures.
What happens to the strength of turbine materials at high temperatures?
It lowers
Engine speed and temperature must be controlled to maintain operational limits.
How is the disc related to the turbine blades?
The disc acts as an anchoring component for the turbine blades
When blades are installed, the disc becomes the turbine wheel.
What is the effect of high temperatures on the disc rim?
It absorbs heat from hot gases and turbine blades
This leads to high thermal and rotational stresses.
What is one method of relieving thermal stresses on the turbine disc?
Bleeding cooling air back onto the face of the disc
This helps manage high temperatures.
What design feature allows for the attachment of turbine blades to the disc?
Grooves or notches in the rim of the disc
These notches accommodate thermal expansion of the disc.
What materials are typically used to fabricate the turbine shaft?
Alloy steel
It must absorb high torque loads.
What are the two common methods of connecting the turbine shaft to the disc?
Welding and bolting
Bolting is more common.
What is the purpose of a spline on the turbine shaft?
To attach the turbine shaft to the compressor rotor hub
It fits into a coupling device between the compressor and turbine shafts.
What is the most common method of attaching turbine blades?
Fir tree design
This design allows for effective retention in the disc.
Name some methods of retaining turbine blades in their grooves.
- Peening
- Welding
- Lock tabs
- Riveting
Peening is frequently used, involving a small notch in the blade root.
What does the peening method of blade retention involve?
Filling a notch in the blade root with disc metal
A punch mark is used to flow the metal into the notch.
How does the tang in the blade root secure the blade in the disc?
It is bent to prevent removal
This allows insertion and removal in one direction only.
What are nozzle guide vanes?
Nozzle guide vanes are components in a turbine assembly that prepare mass airflow to drive the turbine rotor.
What is the purpose of turbine inlet nozzle vanes?
The purpose of turbine inlet nozzle vanes is to convert heat and pressure energy to velocity energy and to deflect gases at a specific angle toward turbine rotation.
Where are the turbine inlet nozzle vanes located?
Turbine inlet nozzle vanes are located directly aft of the combustion chambers and immediately forward of the turbine wheel.
What happens if the turbine inlet temperature is not controlled?
If the turbine inlet temperature is not controlled, damage will occur to the turbine inlet vanes.
What does the turbine inlet nozzle assembly consist of?
The turbine inlet nozzle assembly consists of an inner shroud and an outer shroud with fixed nozzle vanes.
What is the construction method of turbine inlet nozzle vanes?
Turbine inlet nozzle vanes can be constructed to allow thermal expansion and may be fitted into shrouds, welded, or riveted into position.
What is the role of the inner and outer shrouds in turbine inlet nozzle construction?
The inner and outer shrouds provide support, allow for thermal expansion, and facilitate the removal of nozzle vanes.
True or False: The nozzle vanes must be tightly fit into the shrouds to prevent distortion.
False. The nozzle vanes must have a loose fit in the shrouds to allow for thermal expansion.
Fill in the blank: The turbine inlet nozzle vanes form several small nozzles discharging gas at _______.
extremely high speed
What can happen to metal components due to rapid temperature changes in turbine nozzles?
Severe distortion or warping can occur due to rapid temperature changes.
How do support rings contribute to the assembly of nozzle vanes?
Support rings provide increased strength and rigidity and facilitate the removal of the nozzle vanes as a unit.
What is the effect of cutting the inner or outer shroud ring into segments?
Cutting the shroud ring into segments allows for sufficient thermal expansion to prevent stress and warping of the vanes.
What is the relationship between the turbine inlet nozzle vanes and the turbine blades?
The turbine inlet nozzle vanes direct gas flow into the turbine blades, which convert velocity energy to mechanical energy.
List the three most commonly used names for the stator element of a turbine.
- Turbine inlet nozzle vanes
- Turbine inlet guide vanes
- Nozzle diaphragm
What must occur after the combustion chamber introduces heat energy into the mass airflow?
The heat energy must be delivered evenly to the turbine inlet nozzles.
What is the effect of creep on turbine blades?
Creep causes slow structural deformation on a molecular level due to prolonged exposure to high stresses and extreme temperatures.
Why is inspection for cracks in turbine sections important?
Cracks are not normally permitted, and they can lead to significant failures, requiring replacement of the disc and turbine rotor.
What methods are used for crack detection in turbine blades?
Visual inspection, dye penetrant methods, and structural inspection techniques.
What do stress rupture cracks usually look like?
Minute hairline cracks on or across the leading or trailing edge at a right angle to the edge length.
What signs indicate over-temperature conditions in turbine blades?
Stress cracks or deformation found on the leading edges of the first-stage turbine blade.
What is the consequence of finding cracks on the turbine disc?
It requires the replacement of the disc and turbine rotor.
How can slight pitting caused by Foreign Object Damage (FOD) be treated?
By blending it through stoning and polishing.
What are the visual inspection tools used for turbine blades?
Strong light and a magnifying glass.
What is the significance of the leading edge of a turbine blade?
It must be straight and of uniform thickness, except for areas repaired by blending.
What should be done if deformation is found on turbine blades?
Inspect and measure according to the manufacturer’s instructions and replace damaged blades as specified.
True or False: Creep increases with temperature.
True.
Fill in the blank: Creep results in visible _______ changes to the blades.
dimensional
What must be checked on the turbine disc during inspections?
Hardness and stretch.
What should be done with blades removed for inspection?
They must be reinstalled in the same slots/position and numbered before removal.
What do deformation signs caused by over-temperature look like?
Waviness along the leading edge.
What is the typical length range for visible stress cracks?
From one-sixteenth inch upward.
What materials experience creep besides metals?
Plastics.
