Turbines Flashcards
What is the purpose of a Turbine?
To convert most of the kinetic/heat energy from the airflow into mechanical work.
Why is the turbines the most stressed component?
Due to high temperatures and rotational forces
What are the major components that make up a Turbine?
- Case
- Nozzle guide vanes
- Turbine disk / wheel
- Turbine blades
What is the Nozzle Guide Vanes and what is it’s purpose?
A ring of stationary blades before the turbine rotors
- Directs hot gasses at optimum angle
- Increases velocity
When is the gas stream velocity the highest in the engine?
After passing the Nozzle Guide vanes?
How are Nozzle Guide Vanes constructed and what materials are used?
Made hollow to allow cooling
- Made of nirmonic steel or ceramics
Why do most Nozzle Guide Vanes operate in the Choked Nozzle condition?
To allow a relatively constant flow to the turbines
What are Turbine Wheels made of?
Machine forged heavy nickel alloy
What is the preferred attachment for turbine blades and why is it preferred?
Fir Tree Root
- Preferred due to high heat transfer and strength
How do Fir Tree Root blades work?
Uses finely machines serrations across force loads are equally shared.
- When engine is stationary, blades are loose.
- When engine is running, centrifugal loading and heat expansion lock into place.
What are examples of older blade attachments?
- De Laval Bult Root
- BMW Hollow Blade
What is a Blisk?
A turbine wheel machined from a single slab of steel where the blades and wheel are a single unit.
What are Turbine blades constructed from?
Nickel alloys super alloys (Inconel/Nirmonics)
What are the requirements for a Turbine Blade?
- High thermal strength
- Corrosion resistance
- Low co-efficient of expansion
- Good creep resistance
- High fatigue resistance
- Low manufacturing costs
When is a Turbine Tip Shroud used?
- Found on older turbines with slower rotational speeds because of high tip loading.
How do Tip Shrouds aid efficiency?
- Allows for longer, thinner blade (weight saving)
- Prevents air loss at tip
- Prevent air distortion under heavy gas loads
- Dampens vibrations
- Provides mounting base for knife edge seals
What is a Knife-Seal Edge?
Reduces air losses by cutting into abradable shroud ring and forming a seal
What is an indication of a worn Knife Edge Seal?
Rise in EGT
What are the 3 types of Turbines Blades and how do they work??
Impulse:
- Stator forms a convergent duct and blades form a straight duct
- Rotor is turned by impulse as gas impinges on blades (Water Wheel)
Reaction:
- Produces rotation by aerodynamic action as air is accelerated between blades.
Impulse Reaction:
- Base is impulse, tip is reaction
What is the primary factor that limits the power output of an engine?
Maximum temp the turbine inlets can handle
- The hotter the turbine inlet temp, the more efficient an engine is.
How can the Turbine Inlet Temp be increased without adding weight to blades?
By providing the Inlet Nozzle vanes and Turbine blades with cooling.
What is the most critical temperature in the engine to be monitored?
Turbine inlet Temp
- Controlled by the FCU and taken from the 1st stage of nozzle vanes
What air is used to provide the 1st stage of blades and nozzle guide vanes with cooling?
Compressor bleed air
- If cold is used it can cause thermal shock
What are the two methods to cool turbine blades?
Convection cooling:
- Air flows through holes in the hollow blades and heat is directly carried away
Surface Film Cooling:
- Air flows from small exit ports on the leading/trailing edge and forms a cooling ‘film’ over the surface.
How much energy from the airstream is extracted in Turbojets, Turbofans and Turboprop/turboshafts?
Turbojets:
- 65%
Turbofans:
- 75%
Turboprops / Turboshafts:
- 90%
What is mechanical and gas coupling in a turbine?
Mechanical coupling: Turbine stages bolted together
Gas coupling: When turbine is free of the compressor drive (FREE POWER TURBINE)
Why must the Engine Speed and TIT be closely controlled in a turbine?
Due to high thermal and centrifugal loads.
What do excessive temperatures do on Blades?
- Blade creep
- Deformation
- Fatigue
What are operations that produce high EGT?
- Starting
- Acceleration
- Max thrust at high altitude.
What does the generation of impulse-reaction loads do to the bade?
Produces bending and twisting loads that increase with engine RPM
Why must the turbine blade tips not exceed the sound barrier?
Blades cannot extract energy if they are operating in the sonic range
- Brayton cycle breaks down.
What is blade creep?
The gradual but permanent elongation of the blades over time due to centrifugal loading and operations at high temperatures.
What are the 3 types of Creep?
Primary: Early creep that occurs at a higher rate when the blade is new and running in.
Secondary: The predominant working life of the blade where creep is significantly reduced.
What are Stress Rupture Cracks?
Crack propagation from the leading edge to trailing edge
- Right angles to the edges
What causes Stress Rupture Cracks and what part of the blade is more suseptable?
Caused by 1 or more significant over-temp events
- Leading edge and trailing edge are most susceptible due to thinner cross section.
What is blade erosion and what causes it?
Wearing away of the metal on the leading edges from impurities in the gas stream
- The rate of erosion is determined by the type of debris and operating temp of turbine
What is Nozzle Bowing?
Outwards bowing of the Nozzle Guide Vanes due to pressure of the heated gas stream
What causes Nozzle Bowing?
Extended operations at max TIT and the loss of convergent duct between the nozzles
What are symptoms of Nozzle Bowing?
- Loss of HP compressor RPM
- Rise in EGT due to turbine extracting less energy from the airstream
What is Blade Untwist and what does it do to the blade?
- Loss of twist (Usually on outer 3rd of blade) caused by gas loads and extended operations at max TIT.
- Reduced efficiency from blade
- Extends length of the blade causing rubbing.
What is Blade Deformation?
Localized distortion of the blade
- Similar to Nozzle Bowing
- Reduces efficiency
What is Blade Rubbing and what type of wear is caused by it?
Turbine blades rubbing on the case due to elongation by blade creep or blade untwist
What is Blade Tip Curling?
When thin unshrouded tips curl due to rubbing.
What is an example of large FOD failure?
Bird strikes or large stones
Why is FOD ingestion an issue on turbines?
Causes an imbalance of the finely balanced rotor assemblies and leads to vibration and failure.
What is an example of a smaller FOD failure?
Engine components fail upstream
What is Turbine Metalization?
A coating of molten metal on the blades and nozzle which was sprayed on by other failed hot-end parts overheating upstream
What is Turbine Burning?
Discoloration or burning of the surface
- Found anywhere in hot section
In extreme cases of Turbine Burning, what is a characteristic sign of it on the blades?
Blistering of the blades to give an orange peel look
What is Sulfurdation of the blades?
Sulfur in the fuel leads to a chemical burn which leaves a green color.
What is Blade Pitting?
Holes or cavities on the surface of an over-stretched area
- Often made worse by corrosive products (Sulfur)
How does aerodynamic blockage using cascades and blocker doors work? And what activated this thrust reverser?
Redirects only fan air on unducted turbofans by activating the translating cowl and blocker doors, redirecting air to cascade vanes.
- Activated by T/R levers on idle and weight on wheels.
What are the 3 main sources of noise in an engine? And what is the most major of the 3?
- Fan
- Compressor
- Exhaust
Exhaust noise is the most major due to high velocity jet stream moving through a quiet atmosphere.
Describe the mixing of exhaust gases in the core atmosphere close to the exhaust
Little mixing with the atmosphere causes a high frequency noise due to fine grain turbulence
- Caused by violent mixing of exhaust gases with the atmosphere and shearing action.
Describe the mixing of exhaust gases in the core atmosphere far away from the exhaust
Jetstream mixes with the atmosphere and a coarser, low frequency noise is generated due to large eddies.
What type of frequency travels further and is more noticeable on the ground?
Low frequency
What does the noise levels produced by the engines vary with?
Vary with the amount of work done by the engine
Out of a large turbofan and a small turbojet, which one is quieter?
Large turbofan
Why is a large turbofan quieter than a turbojet?
They have bigger turbines which reduce the velocity of gases.
What are the two ways to reduce engine noise?
- Revising noise patter
- Changing frequency
What does the Corrugated Perimeter and Multi-tube types of noise suppressors do to reduce noise?
Break the main jet into smaller jet streams to reduce noise by making Eddie stream smaller.
What 2 effects does reducing the eddie streams have on noise?
- Higher frequency above the audible range
- Higher frequency noise is absorbed quickly by atmosphere
What is an Acoustic Lining and where is it placed?
Placed between the engine and the cowling. Made of a porous skin with honeycomb backing that creates separation between skin and engine.
- Converts acoustic energy into heat
What must be ensured when choosing an acoustic liner for reducing noise?
The acoustic properties of the liner and skin are the same.
What causes the noise generated by the compressor and turbine?
Comes from the interaction of pressure fields and turbulent wakes from rotating blades and stationary blades.
What are the two groups of frequencies produced by compressors and turbines?
Discrete tone (single frequency)
- Produced from the regular passage of blade wakes through stages
Broadband (multiple frequency)
- Caused by turbulent air as it passes the blades.
What is a hush kit and where is it used?
A silencer placed on the exhaust to reduce the lower frequency waves from travelling.
- Used on low bypass engines
What is a suppressor nozzle?
Increases the mixing rate of the hot stream and cold air
- Increases the frequency which is quickly attenuated.
