M15.6 Flashcards
What are the 2 types of turbines?
Radial flow
Axial flow
What is the main purpose of the turbine?
To provide power to drive the engine compressor and gear box
What is the turbine made up of?
Rotors
Stators
Radial turbines are always what?
Single stage turbines
Where are radial flow turbines used?
On small GTE’s such as APU
What are the disadvantages of a radial flow turbine?
Small efficacy
Small airflow
What is the advantage of radial flow turbine?
Simple design
The axial flow turbine is mainly used on what?
Modern GTE’s
What is the advantage of axial flow turbines?
Allow for very high airflow
What is the operation of a turbine?
To convert gas into torque
What’s the purpose of the convergent duct?
To accelerate the air and create more pressure energy
What is a impulse turbine?
Makes rotation only by impact of gas flow on rotor blades
Where are impulse turbines usually found?
On very old GTE’s
Where is the highest heat to metal contact in the engine?
1st stage of hp compressor
What are the 2 types of turbine blades?
Impulse
Impulse - reaction
The force at the outlets acts in what direction?
Opposite to discharge gas flow
What is done by the pilot to try and reduce heat in the turbine?
High throttle is kept to a min
Why must the correct jet fuel be used?
Prevent corrosion from sulphuric acid
What are the 3 factors that contribute to creep?
Time
Temp
Centrifugal forces
When does creep occur?
When centrifugal forces are applied for a long time in high temps
What is the only way to reduce creep?
To keep material as cool as possible
What are the 2 reasons to cool turbines for?
Increase service life
Better turbine efficiency
What are the nozzle guide vanes and rotor blades cooled from?
Air from hp compressor
What is convention cooling?
Passing cool airflow through the hollow turbine materials while air is also passing inner wall of turbine aero foil
What is the easiest method of cooling?
Convection cooling
What is the best cooling method for nozzle guide vanes no rotor blades?
Impingement cooling
What is the most effective method of cooling?
Film cooling
What is the principle of film cooling?
Prevents direct contact of hot gas flow with turbine materials
What method is used to cool second stage hide vanes?
Convection
Impingement
What method is used to cool second stage rotor blades?
Convection
What is tip clearance?
The gap between the tip of the rotor blade and the casing
Why does a large tip clearance decrease the efficacy of the turbine stage?
Because a large amount of gas passes through the gap between the rotor blades and turbine casing
What are the 2 types of turbine clearance control systems?
Passive clearance control
Active clearance control
How is vibration reduced in LP turbine blades?
With shrouds
What causes stress rupture cracks in the turbine section?
Metal fatigue and corrosion
When does the active clearance control system operate on engines fitted with hydro mechanical fuel control systems?
During climb and cruise
What does the passive clearance control system use for cooling?
Fan air
What controls clearance control valves?
And what are they actuated by?
Engine control unit
Actuated by the fuel pressure from the hydro mechanical unit
Are the LP turbine blades cooled?
No
FADEC?
Full Authority Digital Engine Control
What works out the tip clearance?
ECU
What loads does the turbine frame carry?
Bearing loads of aft end of the LP turbine
The case of the HP turbine is bolted to what?
Rear flange of combustion case
How many pieces make up the HP turbine case?
1
Why is the HP turbine case made up of a abradable ceramic surface?
To give thermal resistance
How many nozzle guide vanes are in a nozzle guide vane segment?
2
What are the gaps between the nozzle guide vane segments sealed with?
Sealing metallic strips
What does the interstate seal do?
Prevents airflow between the rotor and stator gap
Where is the HP turbine case bolted to?
The rear flange of HP turbine
The LP turbine case is made of how many pieces on modern engines?
1
Where are the inner air seals located?
On the inner surface of the nozzle guide vane segments
How many vanes make up the nozzle guide vane segments?
6 or 7
The HP turbine rotor is attached to what?
The rear shaft of the compressor
What is the function of a disc on the turbine?
To hold the rotor blades and transfer torque
Why is the cross sectional area of the disc very large?
So the disc withstands high centrifugal forces
On HP turbines what must the interstate seal do?
Help transfer torque from stage 2 disc to stage 1 disc
What does the interstate seal do?
Act as a space between the 2 discs
What’s the purpose of the blade retainers?
Hold turbine blades in place
What does the seal lip do on the blade retainers?
Act as air seal
Why is the fur tree root fitting so efficient?
Has many contact surfaces
What is the fitting like on fur tree roots when the engines not running?
Loose
What is the most common method for attaching turbine rotor blades?
Fir tree root
What advantage does the axial flow turbine have over radial flow turbines?
They can be built with as many stages as required and allow for a very high flow rate
What is the primary air sources for a passive clearance control system?
HPC air for the HPT and fan air for LPT
What are the 2 types of rotor assembly?
Open ended blades
Shrouded blades
What type of blade is used in HP turbines?
Open ended blade
What’s the disadvantage of shrouded blades?
More weight
What’s the purpose of the knife edge seal on shrouded blades?
Helps reduce air loses at the tip of blade
Where are shrouded blades usually found?
LP turbines
How are turbine discs attached to the shaft?
Either splined or bolted
How are the HP turbine blades cooled?
Internally with air from HP compressor
What type of structure is the turbine frame?
Major
What does the turbine frame do?
Supports 1 or 2 turbine rotors and provides attachment points to rear engine mount
How are the turbine struts shaped?
Aerodynamically to cause a little drag as possible
What are the 2 types of strut?
Radial strut
Tangential strut
The radial strut is what?
Thick and hollow
What does the tangential strut do to minimise stress from thermal expansion?
Expands
On engine shut down what is the contraction sequence of the turbine and casing?
Rotor shrinks
Then
Casing
On what stage of a turbine would open turbine blades normally be found?
On the HP turbine stage
