15.6 Turbine Section Flashcards
what is the purpose of the turbines
to turn kinetic energy of the exhaust gasses into mechanical energy. also turns the fans, compressor and shafts
what percentage of total pressure is absorbed by the turbine ?
60-70%
what are the two different types of turbines used on gas turbine engines ?
- radial flow
- axial flow
what are the vanes called at the entrance of the turbine
Nozzle guide vanes
where are radial flow turbines used?
small gas turbine engines e.g APU
what are some disadvantages of a radial flow turbine?
- only allows small airflows
- less efficient due to high aerodynamic losses
where are axial flow turbines commonly used?
- modern gas turbine engines
what is the benefit of using axial flow turbines ?
- allows very high airflow required to create high thrust for modern engines
what does the turbine convert gas energy into ?
mechanical energy in the form of torque
what happens to airflow velocity as it goes through the nozzle guide vanes ?
velocity increases
what are the three clasifications of turbine blades ?
- impulse configuration
- reaction configuration
- reaction-impulse configuration
what happens to total pressure and velocity in an impulse configuration?
total pressure decreases rapidly and velocity increases
what happens to total pressure and velocity of airflow in a reaction configuration?
doesn’t change the pressure and changes direction of airflow
why might a reaction impulse configuration be used ?
mixture of both impulse and reaction most efficient way of absobing energy in the turbine
what does reaction impulse configuration require to obtain the maximum rate of momentum change?
high velocity gas
What are most turbine blades made of?
precision cast or made of composite alloys
what is the advantage of casting a single crystal?
better strength and heat properties
what are some ways turbine blades can be protected from heat?
- ceramic coating
- single crystal casting
- airflow cooling
what does the turbine assembly consist of ?
- stator ( nozzle and inlet guide vanes )
- turbine blades ( rotot )
- both connected via the turbine shaft
where are inlet nozzle vanes located?
- directed aft of the combustion chambers and immediately forward of the turbine wheel
why are inlet nozzle vanes contoured ?
to discharge gas at extremely high speed by converting heat energy into velocity
other than increasing speed, what is the other job of the inlet nozzle vanes?
to deflect airflow to a specific angle
how many shrouds are used in an inlet nozzle vane assembly ?
- inner and outer shrouds
inlet nozzle vanes are exposed to extreme heat, what should be taken into consideration when designing the assembly?
- thermal expansion
how is thermal expansion accounted for with inlet nozzle vanes ?
thermal expansion is absorbed by the loose assembly of the shrouds
what does the support ring facilitate ?
-keeps the vanes in place, otherwise they would fall out
- increases support and rigidity
how else could thermal expansion be accounted for in the shrouds?
vanes are welded/ riveted and the shrouds will have seperations in them to allow sufficient space for thermal expansion
what sort of cracks or damage are premitted on a blade?
none
how is crack detection carried out?
usually visual, however other methods such as dye penetrants can be used
how can slight pitting due to FOD be corrected ?
blended by stoning or polishing
what tools are used to inspect turbine blades for stress rupture cracks and deformation?
strong light and a magnifying glass
how do stress rupture cracks appear on a turbine blade ?
minute hairline cracks
how may deformation due to heat damage appear?
waviness along the leading edge
what else must be suspected if a stress crack or deformation is found ?
an over temperature condition
what is creep?
a slow structural deformation caused by prolonged exposure to high stress
what promotes creep?
extreme temperatures in the turbine area
what must be inspected once creep is identified ?
turbine disc for hardness and stretch
Why are there operational limits for turbines
Due to the centrifugal force and temperature
What is the difference between a turbine disc and a turbine wheel
A turbine wheel has blades installed onto the disc
How do you relieve temperature stresses on the disc
Been cooling air back onto the face of the disc
How are blades installed onto discs
In grooves or notches
What are the two types of disc retention
Bulb and fir tree
What are the two ways of holding discs in place
Bolting and welding
How are blades held in place in centrifugal compressors
With splined couplings
