15.6 TURBINE SECTION Flashcards
The gas generator turbine is to absorb approximately —-% of total pressure energy from exhaust gases
60-70%
Where is the turbine section of a gas turbine engine located
- Aft
- directly behind the combustion chamber outlet
What are the 2 different types of turbines on gas turbine engines
- radial flow turbine
- axial flow turbine
The 2 types of turbine have the same main components which are?
- stationary vanes (turbine nozzle guide vanes)
- set of moving rotor blades
Explain radial flow turbines
- always single-stage turbines
- used on small gas turbine engines (e.g. APU)
What are the disadvantages of radial flow turbines
- allow small airflow and less efficient (high aerodynamic losses)
Advantages of radial flow turbines
- simple design
- easy to manufacture
Explain axial flow turbines
- can be built with any number of turbine stages as necessary
Advantage of axial flow turbines
- allow very high airflow that is required to create high thrust
On modern gas turbine engines, which flow turbine is used
Axial
What does the turbine convert
Gas energy from combustion chamber into torque to drive turbine shaft
Explain impulse turbine
Turbine that makes rotation only by the impact of the gas flow on rotor blades
What are the 3 classification of turbine blades
- Impulse configuration
- Reaction configuration
- Reaction-impulse configuration
Explain impulse configuration
- pressure drop across each stage occurs in the fixed nozzle guide vanes (due to convergent shape)
- increase gas velocity, reducing pressure
Explain reaction configuration
- the fixed nozzle guide vanes are designed to alter the gas flow direction without changing the pressure
Explain reaction-impulse configuration
- used on gas turbine engines
Explain shrouded turbine
- limit vibration
- improves efficiency
Disadvantage of shrouded turbine
- limits turbine speed
- requires more blades
How is the turbine wheel balanced
Dynamically
What 2 things must be controlled to keep turbine operation within operational limits
- engine temperature and speed
What does high temperature and high centrifugal loads cause
Lower material strength
When will the turbine become a ‘turbine wheel’
- when turbine blades are installed onto disc
How does the disc rim absorb heat
- from the turbine blades by conduction
Why is there clearance between blade root and notch
Permit for expansion and contraction of blade
What are the 2 methods of connecting a shaft
- weld shaft to the disc
- bolting
What are the 2 common ways to attach turbine blades
- bulb
- Firtree
How are the blades retained in their respective grooves
- peening (most common)
- welding
- lock tabs
- riveting
What will occur if a crack was found on the disc
- replacement of disc and turbine rotor
Explain stress rupture cracks
- appear as minute hairline cracks on or across leading or trailing edge at right angle to the edge length
Explain creep
- slow structural deformation
- caused by prolonged exposure to high stresses
If you remove turbine blades should they be reinstalled in the same slots
Yes
Thermal expansion of the turbine nozzles is absorbed by
Loose assembly
How is the turbine shaft attached to the compressor rotor hub
Spline
- fits into coupling device between turbine and shaft
What gives blades better heat and strength properties
Single crystal cast
