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Gas Turbines > Turbines > Flashcards

Turbines Flashcards

1
Q

What is the purpose of a Turbine?

A

To convert most of the kinetic/heat energy from the airflow into mechanical work.

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2
Q

Why is the turbines the most stressed component?

A

Due to high temperatures and rotational forces

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3
Q

What are the major components that make up a Turbine?

A
  • Case
  • Nozzle guide vanes
  • Turbine disk / wheel
  • Turbine blades
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4
Q

What is the Nozzle Guide Vanes and what is it’s purpose?

A

A ring of stationary blades before the turbine rotors
- Directs hot gasses at optimum angle
- Increases velocity

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5
Q

When is the gas stream velocity the highest in the engine?

A

After passing the Nozzle Guide vanes?

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6
Q

How are Nozzle Guide Vanes constructed and what materials are used?

A

Made hollow to allow cooling
- Made of nirmonic steel or ceramics

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7
Q

Why do most Nozzle Guide Vanes operate in the Choked Nozzle condition?

A

To allow a relatively constant flow to the turbines

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8
Q

What are Turbine Wheels made of?

A

Machine forged heavy nickel alloy

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9
Q

What is the preferred attachment for turbine blades and why is it preferred?

A

Fir Tree Root
- Preferred due to high heat transfer and strength

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10
Q

How do Fir Tree Root blades work?

A

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.

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11
Q

What are examples of older blade attachments?

A
  • De Laval Bult Root
  • BMW Hollow Blade
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12
Q

What is a Blisk?

A

A turbine wheel machined from a single slab of steel where the blades and wheel are a single unit.

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13
Q

What are Turbine blades constructed from?

A

Nickel alloys super alloys (Inconel/Nirmonics)

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14
Q

What are the requirements for a Turbine Blade?

A
  • High thermal strength
  • Corrosion resistance
  • Low co-efficient of expansion
  • Good creep resistance
  • High fatigue resistance
  • Low manufacturing costs
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15
Q

When is a Turbine Tip Shroud used?

A
  • Found on older turbines with slower rotational speeds because of high tip loading.
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16
Q

How do Tip Shrouds aid efficiency?

A
  • 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
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17
Q

What is a Knife-Seal Edge?

A

Reduces air losses by cutting into abradable shroud ring and forming a seal

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18
Q

What is an indication of a worn Knife Edge Seal?

A

Rise in EGT

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19
Q

What are the 3 types of Turbines Blades and how do they work??

A

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

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20
Q

What is the primary factor that limits the power output of an engine?

A

Maximum temp the turbine inlets can handle
- The hotter the turbine inlet temp, the more efficient an engine is.

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21
Q

How can the Turbine Inlet Temp be increased without adding weight to blades?

A

By providing the Inlet Nozzle vanes and Turbine blades with cooling.

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22
Q

What is the most critical temperature in the engine to be monitored?

A

Turbine inlet Temp
- Controlled by the FCU and taken from the 1st stage of nozzle vanes

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23
Q

What air is used to provide the 1st stage of blades and nozzle guide vanes with cooling?

A

Compressor bleed air
- If cold is used it can cause thermal shock

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24
Q

What are the two methods to cool turbine blades?

A

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.

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25
Q

How much energy from the airstream is extracted in Turbojets, Turbofans and Turboprop/turboshafts?

A

Turbojets:
- 65%
Turbofans:
- 75%
Turboprops / Turboshafts:
- 90%

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26
Q

What is mechanical and gas coupling in a turbine?

A

Mechanical coupling: Turbine stages bolted together
Gas coupling: When turbine is free of the compressor drive (FREE POWER TURBINE)

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27
Q

Why must the Engine Speed and TIT be closely controlled in a turbine?

A

Due to high thermal and centrifugal loads.

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28
Q

What do excessive temperatures do on Blades?

A
  • Blade creep
  • Deformation
  • Fatigue
29
Q

What are operations that produce high EGT?

A
  • Starting
  • Acceleration
  • Max thrust at high altitude.
30
Q

What does the generation of impulse-reaction loads do to the bade?

A

Produces bending and twisting loads that increase with engine RPM

31
Q

Why must the turbine blade tips not exceed the sound barrier?

A

Blades cannot extract energy if they are operating in the sonic range
- Brayton cycle breaks down.

32
Q

What is blade creep?

A

The gradual but permanent elongation of the blades over time due to centrifugal loading and operations at high temperatures.

33
Q

What are the 3 types of Creep?

A

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.

34
Q

What are Stress Rupture Cracks?

A

Crack propagation from the leading edge to trailing edge
- Right angles to the edges

35
Q

What causes Stress Rupture Cracks and what part of the blade is more suseptable?

A

Caused by 1 or more significant over-temp events
- Leading edge and trailing edge are most susceptible due to thinner cross section.

36
Q

What is blade erosion and what causes it?

A

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

37
Q

What is Nozzle Bowing?

A

Outwards bowing of the Nozzle Guide Vanes due to pressure of the heated gas stream

38
Q

What causes Nozzle Bowing?

A

Extended operations at max TIT and the loss of convergent duct between the nozzles

39
Q

What are symptoms of Nozzle Bowing?

A
  • Loss of HP compressor RPM
  • Rise in EGT due to turbine extracting less energy from the airstream
40
Q

What is Blade Untwist and what does it do to the blade?

A
  • 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.
41
Q

What is Blade Deformation?

A

Localized distortion of the blade
- Similar to Nozzle Bowing
- Reduces efficiency

42
Q

What is Blade Rubbing and what type of wear is caused by it?

A

Turbine blades rubbing on the case due to elongation by blade creep or blade untwist

43
Q

What is Blade Tip Curling?

A

When thin unshrouded tips curl due to rubbing.

44
Q

What is an example of large FOD failure?

A

Bird strikes or large stones

45
Q

Why is FOD ingestion an issue on turbines?

A

Causes an imbalance of the finely balanced rotor assemblies and leads to vibration and failure.

46
Q

What is an example of a smaller FOD failure?

A

Engine components fail upstream

47
Q

What is Turbine Metalization?

A

A coating of molten metal on the blades and nozzle which was sprayed on by other failed hot-end parts overheating upstream

47
Q

What is Turbine Burning?

A

Discoloration or burning of the surface
- Found anywhere in hot section

48
Q

In extreme cases of Turbine Burning, what is a characteristic sign of it on the blades?

A

Blistering of the blades to give an orange peel look

49
Q

What is Sulfurdation of the blades?

A

Sulfur in the fuel leads to a chemical burn which leaves a green color.

50
Q

What is Blade Pitting?

A

Holes or cavities on the surface of an over-stretched area
- Often made worse by corrosive products (Sulfur)

51
Q

How does aerodynamic blockage using cascades and blocker doors work? And what activated this thrust reverser?

A

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.

52
Q

What are the 3 main sources of noise in an engine? And what is the most major of the 3?

A
  • Fan
  • Compressor
  • Exhaust
    Exhaust noise is the most major due to high velocity jet stream moving through a quiet atmosphere.
53
Q

Describe the mixing of exhaust gases in the core atmosphere close to the exhaust

A

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.

54
Q

Describe the mixing of exhaust gases in the core atmosphere far away from the exhaust

A

Jetstream mixes with the atmosphere and a coarser, low frequency noise is generated due to large eddies.

55
Q

What type of frequency travels further and is more noticeable on the ground?

A

Low frequency

56
Q

What does the noise levels produced by the engines vary with?

A

Vary with the amount of work done by the engine

57
Q

Out of a large turbofan and a small turbojet, which one is quieter?

A

Large turbofan

58
Q

Why is a large turbofan quieter than a turbojet?

A

They have bigger turbines which reduce the velocity of gases.

59
Q

What are the two ways to reduce engine noise?

A
  • Revising noise patter
  • Changing frequency
60
Q

What does the Corrugated Perimeter and Multi-tube types of noise suppressors do to reduce noise?

A

Break the main jet into smaller jet streams to reduce noise by making Eddie stream smaller.

61
Q

What 2 effects does reducing the eddie streams have on noise?

A
  • Higher frequency above the audible range
  • Higher frequency noise is absorbed quickly by atmosphere
62
Q

What is an Acoustic Lining and where is it placed?

A

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

63
Q

What must be ensured when choosing an acoustic liner for reducing noise?

A

The acoustic properties of the liner and skin are the same.

64
Q

What causes the noise generated by the compressor and turbine?

A

Comes from the interaction of pressure fields and turbulent wakes from rotating blades and stationary blades.

65
Q

What are the two groups of frequencies produced by compressors and turbines?

A

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.

66
Q

What is a hush kit and where is it used?

A

A silencer placed on the exhaust to reduce the lower frequency waves from travelling.
- Used on low bypass engines

67
Q

What is a suppressor nozzle?

A

Increases the mixing rate of the hot stream and cold air
- Increases the frequency which is quickly attenuated.

Gas Turbines (5 decks)

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