15.06 Flashcards
What two types of turbines is there
Axial And radial flow
What does the turbine state provide power to drive?
Accessory gearbox & compressors
What are the two main components of a turbine ?
Stationary guide vanes
Turbine nozzle guide vanes
How many stages does a radial flow turbine always have?
One!
Where are radial flow turbines normally used?
They’re only used on small gas turbine engines like an APU
What are the pros and cons of a radial flow turbine
Advantage: they’re a simple design so easy to manufacture
Disadvantage: only allow small airflow
Less efficient
Where are axial flow turbines used?
Modern by bypass engines
What are the advantages of a axial flow turbine?
They can be built up with any number of stages, sufficient enough to drive gearbox and large fan
Secondly they allow very high air flow required to create thrust on bypass engines
What does the turbine convert gas energy into?
Torque
The gas flow from the combustion chambered is accelerated by what?
The convergent duct between the nozzle guide vanes
A turbine which makes the rotation only by the impact of gas flow in the rotor blade is called what?
An impulse turbine, it’s recognisable by the special shape of the rotor blade and they’re generally found on very old gas turbine engines.
Turbines must operate in very extreme environments, what two things must turbine materials be able to withstand?
Extremely high temps and very high centrifugal forces
Two factors reduce turbine life what are they?
- Material fatigue caused by many cycles
2. Corrosion causes by sulphuric acid due to high sulphur levels in fuel and high temp
What is material deformation known as?
Creep
What factors can increase the rate of creep?
High engine speeds and high temperatures and the amount of time
What are the two main reasons why turbines are cooled in modern engines?
- To increase service life done by internal cooling of nozzle guide vanes and rotor blades
- Better turbine efficiency, done by case cooling
What is convection cooling?
It’s the easiest method and it’s when cooling air flow passes through hollow turbine nozzle vanes and rotor blades
How does impingement cooling work?
Better cooling method, the airflow first flows through an insert fixed in the hollow turbine aerofoil which has many small holes which serves as a jet nozzle which has better heat transfer
What is film cooling
There’s further cooling improvement, cooling air is blown into the hot gas flow via small drill holes in turbine aerofoil. The gas stream deflects airflow and makes a film
Advantage and disadvantages of film cooling?
- It’s the most effective method because it uses minimal cooling air
- This cooling method is very expensive because small drill holes are difficult to make
The fist stage of nozzle guide vanes are cooled by what methods?
Convection
Impingement
Film cooling
What are the first stage rotor blades cooled by?
Convection
Impingement
Film cooling
What are the second stage nozzle guide vanes normally cooled by?
Convection
Impingement
What is the second stage rotor blades normally cooled by?
Convection only! Due to lower temps at the back
What does active tip control do?
This system keeps the tip clearance as small as possible during operation
How does large tip clearance affect a turbine stage?
It decreases the efficiency because large amounts of airflow pass over the blade
If the tip clearance increases by 0.25mm or 0.01 inch how much will the specific fuel consumption increase by ?
1% or by 30000kg of extra fuel for a single engine
When does the turbine casing expand faster than the turbine rotor?
At engine start
What two types of bleed methods are used to cool the turbine casing?
Fan air or on some engines compressor bleed air
What is passive clearance control ?
Is when the turbine cases are cooled by continuous airflow which is not regulated
The air is picked up form the fan discharge duct and sprayed onto lp turbine case
What is active clearance control ?
Active clearance control uses the same cooling air as passive however active clearance control can control the quantity of air
What are the turbine clearance control components ?
1 one or more cooling air supply duct
- A manifold which distributes air
- Spray rings
How does tip clearance control benefit the engine?
Saves a lot of fuel
For an engine with a hydro mechanical control unit when does the active clearance control work ?
During climb and cruise
On a hydro mechanical engine control unit what two signals are used to activate tip clearance control?
80%-95% N2 And aircraft altitude
For an engine with a fadec system when can the tip clearance control be used ?
During all operation condition
The turbine section of a typical twin spool engine consists of ?
- A high pressure turbine
- A low pressure turbine
- A turbine frame
What components make up a typical turbine stator assembly ?
- The stator case
- the nozzle guide vanes
- Sealing and wall segments
- Clearance control air manifold
What does the turbine case segment have to provide optimum thermal resistance
An abradable ceramic surface
What does the interstate seal prevent?
Gas flow between through the gap between the stator and the rotor
Are there borescope ports in the turbine case
Yes
What is the stator case of a low pressure turbine bolted to ?
The rear flange of the high pressure turbine
What components does the hpt rotor have ?
Turbine discs
Rotor blades
A rotating interstage seal
Forward and rear blade retainers
Why is the cross sectional area of the turbine discs very large?
Because the disc must withstand high centrifugal loads.
What are the blade retainers designed for ?
Designed so that the turbine blades are held in place so they do not move from their seats due to gas loads
Rotor blades from the high pressure turbine rotor have a special shape know as ?
Fir tree root
What are they two types of turbine blades used in a modern gas turbine engine ?
- Open end blades
2. Shrouded blades
What is the disadvantage of the shrouded blade ?
It adds more weight to each rotor blade
What air is used to cool the hpt blades ?
Internally cooled air from the high pressure compressor
What is the turbine frame built up of?
- A frame hub with the beating support
- Am outer frame casing
- A set of struts which connect the hub with the outer casing
