Turbine Section
What is the function of the turbine inlet guide vanes?
- They are considered to be the stationary part of the turbine assembly. - After the combustion chamber has added heat energy to the mass airflow and delivered it evenly to the guide vanes, the vanes must prepare the mass air flow to drive the turbine rotor. - The vanes are contoured and set at such an angle that they form a number of small nozzles discharging gas at extremely high speed to drive the turbine blades. - This temperature must be controlled or damage will occur to the blades. - The vanes also deflect the gases in the general direction of turbine rotation, enabling the gases to enter the blade passageway while the turbine is still rotating.
How is the turbine shaft attached to the compressor rotor hub?
1- By a spline cut on the forward end of the shaft. The spline fits into a coupling device between the compressor and turbine shaft. 2- A coupling is not used and the splined end of the shaft is fitted into a splined recess in the compressor rotor hub. The splined coupling is used exclusively on centrifugal compressor engines, while axial compressor engines may use either of the methods.
State the two methods used to allow thermal expansion of the turbine inlet vanes.
1- Loose-fitting vanes - Each vane is fitted into a contoured slot in the shrouds, which conforms to the airfoil shape of the vane. The slots are slightly larger than the vanes to provide a loose fit. The shrouds are encase by support rings which provide strength and rigidity, as well as, facilitating the removal of the vanes. 2- Welded vanes - Each vane is welded or riveted into position in the the shrouds. To allow thermal expansion, either the inner or outer shroud is cut into segments.
What are the two methods used to relieve the thermal and rotational stresses acting on the disk rim of the turbine wheel?
1- To bleed cooling air back onto the face of the disk. 2- A series of grooves, conforming to the blade root design, are broached in the rim of the disk. These grooves allow attachment of the blades to the disk,as well as, thermal expansion of the disk.
What are the constituents of a turbine stage?
A turbine stage consists of a row of stationary vanes, followed by a row of rotating blades.
What material is used to fabricate the turbine shaft?
Alloy steel because it is capable of absorbing high torque loads.
... in the turbine section are not normally allowed and slight pitting may be blended by ... and ...
Cracks Stoning Polishing
... of turbine blades caused by over-temperature may appear as waviness and/or areas of varying thickness along the leading edge.
Deformation
Where is the turbine section located?
Directly behind the combustion chamber outlet.
Define the term creep.
It is a slow structural deformation on a molecular level caused by prolonged exposure to high stresses.
Signs of ... on the turbine housing may indicate that the length of turbine blades is out of tolerance.
Scoring
What are the basic elements of the rotor part of the turbine section?
Shaft Turbine wheel - The turbine wheel consists of blades attached to a rotating disk. The disk is attached to the main power-transmitting shaft of the engine.
In multiple spool engines (eg. turbofan), each spool has its own set of turbine ...
Stages.
... on turbine blades may appear as minute hairline cracks on or across the leading or trailing edge at a right angle to the edge length.
Stress rupture cracks.
What is the one characteristic peculiar to all turbine inlet vanes?
The vanes must be constructed to allow thermal expansion. Otherwise, there would be severe distortion of the metal components because of rapid temperature changes.
Why do some engines incorporate a multistage turbine and multiple compressor rotors?
To accommodate the heavy rotational loads.
Why do some turbine wheels have shrouded turbine blades (band on the outer perimeter of the wheel)?
To improve vibration characteristics.
What is the function of the turbine section?
To transform a portion of the kinetic (velocity) energy of the exhaust gases into mechanical energy to drive the gas generator compessor and accessories.
Whereis the highest temperature that comes in contact with metal components in a gas turbine engine?
Turbine inlet guide vanes.
What are the two basic elements of the turbine assembly?
Turbine inlet guide vanes. Turbine blades.
Most turbofan engines have ... spools.
Two - a low psi spool and a high psi spool.