15.06 - Turbine Section Flashcards
(16 cards)
What % of total pressure energy from exhaust gasses is absorbed by the turbines.
60-70%
Radial flow turbines.
Single stage only. Inefficient.
Axial flow turbines.
Modern, allow very high airflow.
The 3 types of turbine blades
Impulse
Reaction
Reaction-impulse.
Impulse turbine blades.
Fixed nozzle guide vanes are convergent ducts, so pressure decreases.
Reaction type turbine blades.
Fixed nozzle guide vanes only alter direction, do not cause pressure drop.
Reaction-impulse the turbine blades.
Impulse type is at the roots.
Reaction type is at the tips.
Pressure drop for the reaction is at the tips.
Why are many turbine blades cast as single crystal
Gives better strength and heat properties.
What is an example of a heat barrier used for turbine blades.
Ceramic coating.
What are shrouded turbine blades characteristics.
Reduce vibrations. Improves efficiency.
Requires more blades.
Limits turbine speed.
Turbine blade to disc attachment.
Turbine wheel is dynamically balanced.
The disc is attached to the shaft.
High rotational speeds cause extremely high centrifugal forces.
The blades attach to the disc which then becomes known as the turbine wheel.
Shaft to disc welding or bolting. Bolting is more common.
A spline is cut on end of shaft, fits into coupling device of compressor.
For tree design most common for attaching blades.
How are turbine blades cooled
The disc rim is exposed to high heat so cooling bleed air goes through the face of the disc and the turbines.
Nozzle guide vanes.
A stage on a turbine is a nozzle guide vane (stator), and then a rotor.
The nozzle converts heat and pressure energy to velocity energy
Deflects gasses to a certain angle .
All have inner and outer shrouds., welded or riveted in place.
Turbine blade creep.
Slow structural deformation on a molecular level due to prolonged stresses.
Extreme heat promotes creep.
Turbine blade stress.
Inspections for cracks is important. Cracks are not permitted and need replacing. Dye penetrants are used.
Slight pitting can be blended out.
Stress rupture cracks are normally found on the leading or trailing edges.
- leading edge = over temp.
