Module 15.6 Turbine Section Flashcards
What does the turbine tranform?
Portion of the kinetic (velocity) energy of the exhaust gases into mechanical energy to drive the gas generator compressor and accessories.
What percentage of total pressure energy from the exhaust gases does the gas generator turbine absorb?
60 to 70%
The turbine stages can be used to drive what?
Drive a low-pressure compressor (fan), propeller, and shaft.
Where is the turbine section located?
Directly behind the combustion chamber outlet.
Where are the two types of turbines?
Radial flow turbine
Axial flow turbine
What is the first main component of a turbine?
A set of stationary vanes.
What are the stationary vanes called?
Turbine nozzle guide vanes.
What component comes after the Turbine nozzle guide vanes.
Rotor turbine blades.
How many stages does a turbine have?
One or more.
Radial flow turbine are what stage turbine?
Single-stage
What engines are radial flow turbines used for?
Small gas turbine engines such as an APU.
What are axial flow turbines mainly used in?
Modern gas turbine engines.
How many stages can axial flow turbines have?
Any number of turbine stages.
Is the advantage of axial flow turbines?
They allow very high airflow.
Does the turbine turn the gas energy into thrust or torque?
Torque
How is the flow from the combustion chamber accelerated?
Convergent shape of the ducts between the nozzles guide vanes.
What is an impulse turbine configuration?
Rotation is made by the fast velocity impact of the gas flow on the rotor blades.
What happens to the pressure and velocity of air at each nozzle guide vane of the impulse configuration?
Pressure decreases and velocity increases due to the converging blade passages.
What is a reaction turbine configuration?
Gas (air) flow direction is changed without changing pressure.
The converging rotor blade passages experience a reaction force resulting from the expansion and acceleration of the gas.
What turbine configuration do gas turbine engines use?
Reaction-impulse
What is a reaction-impulse turbine configuration?
Where the blade is impulse type at the roots and reaction type at the tips
What does the reaction-impulse configuration do to blade exit pressure?
Keeps it relatively constant.
What happens to the pressure and velocity of air at each nozzle guide vane of the reaction configuration?
Pressure and velocity stays the same. The NGV is parallel.
What are turbine blades cast from?
A single crystal.
What are the two types of heat barrier coatings?
Ceramic coating
Airflow cooling
What do heat barrier coatings do?
Help keep the turbine blades and inlet nozzles cooler.
Where are most turbine blades open?
Outer perimeter (tip)
What is a shrouded turbine blade?
Blades that form a band around the outer perimeter of the turbine wheel.
Name the two types of turbine blade tips.
Open
Shrouded
What does the shrouded blade tip improve?
Efficiency and vibration characteristics.
What does the rotor element consist of?
A shaft and a wheel.
What is a turbine wheel?
A unit consisting of blades attached to a rotating disc.
When does a turbine disc turn into a turbine wheel?
When the turbine blades are installed onto the disc.
What is the disc attached to?
The main shaft of the engine.
How is the disc attached to the shaft?
Bolted or welded
Where are the grooves/notches installed on the disc?
In the rim of the disc
Why are there grooves on the disc?
Allows attachment of the turbine blades to the disc
When does the blade fit tightly into the disc groove?
During engine operation. Expansion of the disc decreases the clearance
The turbine shaft must have some means of attachment to what?
The compressor rotor hub.
What is the most common attachment design of a turbine blade?
Fir tree design.
What method is most used to retain the blade in its groove?
Peening
What is peening?
A small notch is grounded in the edge of the blade fir tree root before the blade installation. After the blade is inserted into the disc, the notch is filled by the disc metal
What three names are given to the turbine stator element?
Inlet nozzle vanes
Inlet guide vanes
Nozzle diaphragm
Where are turbine inlet nozzle vanes located?
Directly aft of the combustion chambers and immediately forward of the turbine wheel
What do the inlet/nozzle guide vanes do?
Converts the heat and pressure energy to velocity energy that can then be converted to mechanical energy through the turbine blades.
What is the secondary purpose of inlet/nozzle guide vanes?
To deflect the gases to a specific angle in the direction of turbine rotation.
How is the inlet nozzle constructed?
An inner shroud and an outer shroud between which the nozzle vanes are fixed.
Where are vanes for the inlet assembled?
Between the outer and inner shrouds or rings.
The nozzle vanes must be constructed to allow what?
Thermal expansion
Where do the inlet vanes slot into?
The shrouds
What is the fit of the inlet vane slots?
Loose fit
What can cause pitting?
Foreign Object Damage (FOD).
What should be done if there’s a crack on the disc
Replace the disc and turbine rotor.
How can pitting be corrected?
Blended by stoning and polishing.
Why are turbine blades susceptible to damage?
Because of the extreme heat they’re under.
What is used to inspect turbine blades?
Strong light and a magnifying glass
How may deformation caused by temperature appear on a turbine blade?
As waviness along the leading edge.
When must an over-temperature condition of the blade be suspected?
When any stress cracks or deformation is found on the leading edges of the first-stage turbine blade.
What is creep?
A slow structural deformation on a molecular level
What is a creep caused by?
By prolonged exposure to high stresses.
As temperature increases, what happens to creep?
It increases aswell.
What may indicate the length of the turbine blades is out of tolerance?
Signs of scoring on the turbine.
How is the turbuine shaft attached to the common rotor hub?
Splines
