15.6 Flashcards
The turbine drives what?
The compressor and gear box
Two types of turbines?
Radial flow turbine
Axial flow turbine
What is the first main component of turbine?
A set if stationary vanes
-called the turbine nozzle guide vanes
What type of turbine is only used in the APU?
Radial flow turbine
What type of turbine is mainly used on modern engines?
Axial flow turbines
Explain operation of turbine
Turbine converts gas energy from combustion chamber to torque
Gas flow comes from combustion chamber and passes through nozzle guide vanes
Gas flow from combustion chamber is accelerated due to convergent ducts
Deflects towards the direction if rotation of turbine blades and causes it to rotate
A turbine which makes the rotation only by gas flow impact on rotor is known as what?
Impulse turbine
Impulse reaction turbine is often referred to as what?
Reaction turbine
What determines the service life of turbine?
The loads it takes
Turbine materials must be able to withstand what?
Extremely high heat and centrifugal forces due to high RPM
What are two factors that reduce service life of an engine?
- Material fatigue caused by power cycles (reduced by correct engine operation)
- Corrosion caused by sulphuric acid (sulphur is from fuel and high gas temperatures)
Why do blades elongate?
High centrifugal loads (Increased by heat)
What is creep?
The material deformation when centrifugal loads are applied over long period of time
(This deformation doesn’t return back)
What is a opposing factor that makes it hard to stop blade deformation?
High turbine inlet temperatures are required however, to stop deformation low temperatures are needed.
Why are turbine cooled?
Increase service life
Increase efficiency
What is convection cooling?
Easiest method of turbine cooling
Air from compressor is pushed onto blades to cool then pushed into the airstream
What is the cooling method for the turbine nozzle guide?
Impingement cooling
Why is film cooling the most preferred cooling method?
Reaches maximum cooling effect with minimum cooling air
First stage nozzle guide vanes are cooled by what?
Convection
Impingement
Film cooling
The first stage rotor blades are cooled by what?
Convection
Impingement
Film cooling
The second stage nozzle are cooled by what?
Convection
Impingement
The second stage rotor is cooled by what?
Convection only
Turbine clearance control does what?
Keeps tip clearance as small as possible in all operating conditions
What are the two reasons that turbine casing expands faster than the turbine rotor?
Turbine casing is thinner
Turbine casing is in contact with higher temperature areas
Is the expansion of a material by centrifugal force bigger than expansion by heat?
Yes
How are turbine cases cooled in a passive control system?
Cooled by a continuous airflow that is not regulated
Sources of cooling for high pressure turbines in passive clearance system?
Uses air from last stages of high pressure compressor as the cooling source
Sources of cooling for low pressure turbine in passive clearance control?
Fan air
How does active clearance control differ from passive clearance control?
Use the same air but active clearance control, controls the quantity of air
Tip clearance control saves what?
Lots of fuel
On engines with hydro-mechanical control units, the active clearance control system only operates when?
During climb and rise
A hydro-mechanical engine control unit uses two signals to activate what?
Clearance control system
The turbine section of a typical twin spool engine consists of what?
A high pressure turbine (HPT)
A low pressure turbine (LPT)
A turbine frame
A typical stator assembly has what components?
Stator case
Nozzle guide vanes
Sealing and wall segments
Clearance control air manifolds
High pressure turbine cases are usually in how many pieces?
One piece
How are nozzle guide vanes held in place?
Nozzle guide vane support
Each segment usually has how many nozzle guide vanes?
2
The interstage seal prevents what?
Gas flow through the gap between stator and rotor
Modern turbo fan engines have what around high pressure turbine case?
Cooling air tubes
The cooling air tubes around the high pressure turbine case are used for what?
Active clearance control
Why are there borescope ports in the turbine case?
For inspecting of rotor blades
Low pressure turbine case is bolted to what?
To the rear flange of high pressure turbine
LP turbine usually made of how many pieces?
One peice
Some engines have split stator cases
The inner seals are located where on nozzle guide vanes?
Inner surface
In low pressure turbines the segments are made up of how many vanes?
6 or 7
The high pressure turbine rotor has what main components?
Turbine discs
Rotor blades
Rotating interstage seal
Forward and rear blade retainers
What do the turbine discs do?
They hold the rotor blades and transfer torque from blades to compressor rear shaft
How big is the cross sectional area of the turbine discs and why?
Very large
Discs must withstand high centrifugal loads
The interstage seal acts as what between the discs?
A spacer
The blade retainers are designed to hold what in place and why?
Hold the turbine blades in place
So they do not move from their seats due to gas loads
What type of retainers are in LP turbines?
Strip type blade retainers
-they fit into dovetail slots and bent up at each end
High pressure turbine uses what type of connection to attach blade to disc and why?
Fir tree root
As they have lots of contact points
What are the two types of blades used in modern gas turbine engines?
Open end blades-high rotational speed turbines
Shrouded blades- found in some high pressure turbines
Shrouded blades form what around the tips of rotor blades?
What does this help with?
Forms a band
Helps reduce vibrations
What type of seal is used around outer surface of shroud seal?
Knife edge seals
What is an advantage of a knife edge seal?
Help reduce air loss at tip of blades
What is a disadvantage of knife edge seals?
Adds more weight
The added weight of a shrouded blades can be offset by what?
Thinner blades?
Turbine discs are either directly splined or bolted to what?
Drive shaft
High pressure blades are internally cooled by what?
By air from HP compressor
The turbine frame supports what?
Turbine rotors
Two functions of turbine frame?
Supports turbine rotors
Provide attachment points for rear engine mount
A typical turbine frame is built of what?
Frame hub with bearing support
Outer frame casing
Set of struts which connects the hub with the outer casing
Two arrangements of struts on gas turbine engine?
Radial struts
Tangential struts
Radial struts are designed how and why?
Designed short as possible
Minimises expansion of struts
Tangential struts minimise stress by thermal expansion how?
If they thermally expand, they cause the hub to rotate slightly
