M15.06 Turbines Flashcards
What is the main function of a turbine?
. Provide power necessary to drive the engine compressor and accessory gear box
What are the two different types of turbines in gas turbine engines?
. Radial flow turbine
. Axial flow turbine
What is the first main component of a turbine?
. Set of stationary vanes
What are the stationary vanes called??
. Turbine nozzle guide vanes
What stage turbines are radial flows?
. Single stage turbines
Where are radial flow turbines used?
APU
as only allow small airflows but are small and easy
Where are axial flow turbines used?
On modern gas turbine engines
How many stages are there on an axial flow turbine?
Many stages
What is the basic operation of a turbine?
Turbine converts the gas energy from the combustion chamber into torque
The gas flow from the combustion to the turbine why?
Because of the convergent shaped ducts between the nozzle guide vanes (first) to the turbine blades
The impact of the gas flow on the turbine blades causes what?
Causes the turbine to rotate, which makes the torque to drive the turbine shaft
How does an impulse turbine work?
Makes the rotation of the turbine only by the impact of gas flow into the rotor blades
(Very old)
Impluse reaction turbines?….?…. Find out how tf they work
An impluse reactuin turbine has another name, what is it??
Reaction turbine
The loads carried by a turbine component determine what??
How long they remain in service
What loads must turbine materials withstand??
. Extremely high temperatures
. Very Hugh centrifugal forces at high rotational speeds
What are the two factors that reduce service life of a turbine??
. Material fatigue by power cycles (which is reduced by correct engine operation)
. corrosion by sulphuric acid (use the correct jet fuel)
At take off each turbine must withstand what?
Hugh centrifugal forces of several tonnes
What is turbine creep??
When centrifugal loads are applied over a long time especially at high temps - causing material deformation
When would creep start developing?
When the engine is operated at max power
How would you get a large increase in the rate of creep??
. High energise speed and high temperatures
How can the pilot prevent creep?
Keep max power for a short amount of time only
What method is used to reduce creep?
Decrease the temperature/ cooling
Two reasons why turbines are cooled?
. Increases service life
. Better turbine efficiency
How are HP turbine nozzle guide vanes and rotor blades cooled??
. With air from the engine HO compressor
Which turbine cooling method is the easiest?
. Convection cooling
How does convention cooling work?
. Cooling air from compressor enters through holes at the bottom of blades and flow through the channels, cooling them
What cooling method is better for turbine nozzle vanes and rotor blades?
. Impingement cooling
How does film cooling work?
. Cooling air is blown into hot gas flow via small drill holes in the turbine aerofoil,which forms a thin air film in the outer walls
What method of cooling is the most effective?
Film cooling
What is the main disadvantage of film cooling?
. Very expensive
The first stage of nozzle vanes and rotor blades are cooled by??
. Convection
. Impingement
. Film cooling
. Because very high gas temperatures from the combustion chamber
The second stage nozzle guide vanes are cooled by??
. Convection
. Impingement
. Because gas temperature is cooler at this stage
The second stage rotor blades are cooled by??
. Convection only
. Because a lot cooler gas temperatures
Modern gas turbines have an active clearance control system, why??
. To keep the tip clearance as small as possible in all operating conditions
What is the tip clearance??
. The gal between the tip of the rotor blades and the casing of the compressor of turbine
Why do too clearances change?
. Be Jane if the different expansions if engine materials at different engine operations
Why do you want a small turbine gap clearance?
. Because a large gal clearance decreases the efficieny of the gurbine stages as a larger proportion of the gas would pass through the gap of the turbine casing, which would not help drive the turbine.
Why does the turbine casing expand faster than turbine rotors blades?
. Case is thinner than the rotor
. Case is in contact with high temperatures
When the rotor speeds get faster, it increases the centrifugal force, which does what to tip clearances?
. Reduce tip clearance because rotor blades expand more
When would tip clearance decrease?
Engine shut down, turbine rotor and casing decrease
What is used to cool the turbine casings??
. Cold fan air
. Sometimes compressor bleed air
Why do we have active clearance control systems?
To keep the tip clearance as small as possible
Where is passive clearance control method used??
. HP and LP turbines
What is used to cool turbine cases using passive clearance control system??
Cooled by continuous airflow that is not regulated
Where does the HO turbine cooling air for a passive clearance system come from??
. Uses air from the last stages of HP compressors as cooling source
What does the LP turbines uses for passive clearance cooling source?
Fan air
Explain how active clearance control systems work?
Use the same cooling air as passive clearance control systems but can also control the quantity of air
What shape are LPT cooling tubes??
Circular shaped tubes
(Which point directly to turbine case)
What shape are the HPT cooling tubes??
. Rectangular shaped
(Bleed holes are at the edges)
The clearance control valves are always separate for?
. Valves for HPT
. Valves for LPT
How are the clearance control valves controlled?
. By engine control unit and actuated by fuel pressure from hydro mechanical unit
What does the tip clearance control save??
Saves a lot of fuel usage
On engines with hydro mechanical control units, the active clearance control operates when??
. Only during climb and cruise
The hydro mechanical engine control unit uses two signals to active the clearance control system, what are they??
. Core engine speed
. Aircraft Altitude from pressure port
Cruise power operation is usually in the range of?
80 - 95% N2
With the two signals the hydro mechanical engine unit can?
Open and close the clearance control valve
On engines with FADEC system, tip clearance can be controlled??
For all operating conditions
A typical twin spool engine has?
. HP turbine (HPT)
. LP turbine (LPT)
. A turbine frame
Where the the HPT rotor fitted?
To the HP compressor rear shaft
What is the LPT surrounded by?
The LP turbine case
What is the typical turbine stator assembly?
. Stator case
. Nozzle guide vanes
. Sealing and wall segments
. Clearance control air manifolds
What is the purpose of the HPT case?
. Supports the nozzle guide vanes and wall segments opposite the rotor blades
How many pieces are HPT cases normally made from??
1
What is special about the wall segments in a turbine cases?
Have an abradable case ceramic surface which give optimal thermal resistance
What are the nozzle guide vanes held in place by?
Nozzle guide vanes support
The nozzle guide vanes are made of?
Many vane segments
What prevents gas flow through stator and rotors gap??
Interstate seal
On the HP turbine case there are cooling tubes for?
Active clearance control
There are ports in the turbine case for??
For borescope, inspecting the rotor blades
The stator case of the LP turbine is bolted to the?
Rear flange of the HP turbine
LPT are usally made from how many pieces?
1
The HP root has what 4components?
. Turbine discs
. Rotor blades
. Rotating interstage seal
. FWD and AFT blade retainers
What is the purpose of the turbine disc?
Hold the rotor blades and transfer the torque
What is the purpose of the interstage seal on the rotor??
. Acts as a spacer in between the two discs
What is the function of the blade retainers?
. Holds the turbine blades in one place so that it doesn’t move from their seats due to gas loads
Pg 81
The Rory or blades on HP turbine have a special shape?
Special shape root - known as fir tree root
What are the two types of turbine blades used??
. Open end blade
. Shrouded blade
Where would open end blades be used?
On turbines with High rotational speed - HP turbines
Where would shrouded blades be used??
. Mainly LP turbines
Sometimes HP
What do shrouded blades help with??
. Reduce vibrations
. Also knife edge seat on outer surface
What is a disadvantage of a shrouded blade?
. Adds more weight to each rotor blade
(Therefore mainly LP with lower rotational speeds)
HP blades are cooled internally by air from the HP compressor, how?
Fed into gap from the front or rear of turbine disc
Where is the turbine frame?
The aft major structural compartment of the engine
What are the two functions of the turbine frame??
. Supports the turbine rotors
. Provides attachment points for rear engine mount
What are the two different frame struts??
. Radial strut
. Tangential strut
How does a radial strut work??
. As short as possible
. Minimises expansion so keeps the stress on frame hub low
How does a tangential strut work??
. Minimises stress caused by thermal expansion because if expand, the hub will rotate slightly
