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Module 15 > 15.6 > Flashcards

15.6 Flashcards

1
Q

What is the sole purpose of the gas turbine?

A

To absorb 60-70% of the energy

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2
Q

Two types of turbine on GTE?

A

Axial flow
Radial Flow

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3
Q

What is always the first main component of a turbine?

A

Stationary vanes

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4
Q

What is another name for these stationary vanes?

A

Turbine nozzle guide vanes

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5
Q

What are the rotors connected to?

A

Turbine disc

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6
Q

What turbines are always single stage?

A

Radial flow

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7
Q

Where are radial flow turbines normally used?

A

Small GTE such as the APU

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8
Q

Advantages of Radial flow turbines?

A

They are simple design and easy to manufacture

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9
Q

What are the the disadvantages of Radial turbines and what does this mean?

A

Only allow small airflows so are less efficient

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10
Q

Why in the radial flow turbine is only small amounts of airflow allowed making it inefficient?

A

High Aerodynamic losses and airflow must enter against centrtifugal fores

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11
Q

Where are axial flow turbines mainly used?

A

In a modern GTE

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12
Q

How many stages are axial flow turbines limited to?

A

As many as is necessary to operate compressor

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13
Q

What is an advantage of axial turbines?

A

They allow high amounts airflow to create high thrust

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14
Q

What does the turbine convert the gas energy to?

A

Torque

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15
Q

What is used to accelerate the gas flow from the combustion chamber?

A

Convergent duct

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16
Q

Where is the convergent duct?

A

Between nozzle guide vanes

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17
Q

What is an impulse turbine and how is it recognised?

A

A turbine which makes the rotation only by the impact of the gas flow on the rotor blades, recognisable by the special shape of the rotor blades

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18
Q

What are the 3 types of rotor blades?

A

Pure Impulse configuration
Pure Reaction configuration
Reaction-Impulse configuration

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19
Q

In a pure impulse type turbine where does the total pressure drop occur and why?

A

The fixed nozzle guide vanes because of their convergent shape

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20
Q

What does all the edgy convert to after hitting the rotor blade?

A

Kinetic energy

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21
Q

In a reaction configuration what are the fixed nozzle guide vanes designed to do?

A

Alter the gas flow direction without changing pressure

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22
Q

What (to do with the gas) causes the rotor to spin in a pure reaction type turbine?

A

Expansion and Acceleration of the gas

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23
Q

What type of duct is involved in Reaction impulse (reaction) turbine?

A

Convergent

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24
Q

What happens to the velocity and pressure in a reaction impulse (reaction) turbine?

A

The velocity increases and pressure drops. The direction is what changes

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25
Q

Where does the Impulsive reaction hit the aerofoil on the rotor blade?

A

On the root

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26
Q

Where does the Reaction aspect hit the aerofoil on the rotor blade?

A

The tip

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27
Q

What are rotor blades made out of?

A

Precision cast

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28
Q

Why are turbine blades constructed of a single crystal?

A

Strong and heat properties

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29
Q

What do heat barrier coatings and ceramic/airflow coatings ensure about the turbine?

A

It is cooler

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30
Q

What does a cool turbine ensure about the efficiency of the engine and why?

A

The exhaust temp is higher increasing efficiency

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31
Q

What is the benefit of shrouded turbines?

A

Vibration resistance
Allows lighter stage weights
Efficiency

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32
Q

Disadvantage of shrouded turbines?

A

Limits turbine speed so you require more blades

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33
Q

What do shrouded turbines form around the perimeter of the turbine rotor?

A

A band

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34
Q

What does the rotor element of a turbine section consist of?

A

Shaft and a wheel

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35
Q

What type of balance is required on a turbine wheel?

A

Dynamically balancing.

36
Q

What does dynamic balancing consist of?

A

Blades that have a rotating disc

37
Q

The rotational speeds cause what loads on a turbine?

A

Centrifugal loads

38
Q

When does a disc become a turbine wheel?

A

When it has blades attatched

39
Q

What do the blades transmit their energy to?

A

The rotor shaft

40
Q

How is the disc attached to the shaft?

A

Bolted or welded

41
Q

What does the disc rim absorb?

A

Heat from the gases and turbine blades

42
Q

How can you ensure that the disc rim isn’t subject to high temperatures?

A

Bleed cooling air back onto the face of the disc

43
Q

What allows attachment of turbine blades to the disc?

A

A series of grooves and notches that have the same contour as the blade

44
Q

What compensates for thermal expansion in the attachment of the blade root to the notch?

A

A clearance

45
Q

What causes the blade root to fit tightly in the disc rim?

A

Operation of engines and thermal expansion

46
Q

What is the turbine shaft made from?

A

Alloy steel

47
Q

What are the 2 methods of joining the shaft to the disc?

A

Bolting and welding (which had a butt or protrusion)

48
Q

What design is common in the attachment of the turbine blades?

A

Fir tree

49
Q

What must be capable of withstanding torque loads?

A

Shaft

50
Q

What must the turbine shaft have a means of connection to?

A

Compressor rotor hub

51
Q

How do you attach the Turbine shaft to Compressor Rotor Hub?

A

Using spline cuts on the forward end of the shaft

52
Q

What does the spline fit into?

A

A coupling device

53
Q

What if a coupling device is not used?

A

Then the turbine shaft may fit into a splined recess

54
Q

Where is a splined coupling method used?

A

Centrifugal compressor engines

55
Q

What method does axial use?

A

A splined recess

56
Q

How are the bases retained in their respective grooves?

A

Peening, welding, lock tabs and riveting

57
Q

Explain the peening method (used frequently)

A

It requires a small notch to be ground before blade installation
After blade is inserted the metal flows into the notch by a small punch mark

58
Q

What does the blade root being made as a stop ensure?

A

The blade can be removed and inserted in one direction only.

59
Q

When the blade root is being used as a stop, what is on the opposite side of removal and application?

A

A tang that is bent to secure the blade in the disc.

60
Q

Where are the turbine inlet nozzle guide vanes located?

A

Aft of the combustion chamber and forward of the turbine wheel

61
Q

Where does the inlet nozzle guide vane experience high temperature?

A

Aft of the combustion chamber but forwards of the turbine wheel

62
Q

What does the inlet nozzle convert heat and pressure energy to?

A

Velocity so it can be transferred to mechanical energy

63
Q

How do nozzles convert heat efficiently?

A

They deflect gasses at a certain angle

64
Q

What is the second purpose of the turbine inlet nozzle?
Why?

A

To direct airflow to the direction of turbine rotation
The gas must enter the turbine whilst it is still spinning

65
Q

What does the inlet nozzle assembly consist of?

A

An inner and an outer shroud

66
Q

Why must the nozzle vanes be contrasted to allow for thermal expansion?

A

There can be severe warping or distortion otherwise

67
Q

How can you compensate for thermal expansion?

A

Loose assembly of the shorts (The slots area slightly larger than the vanes)

68
Q

What increases strength rigidity and support in nozzle guide vanes?

A

Inner and outer support rings

69
Q

What do the support rings also facilitate?

A

Easy removal of nozzle vanes as a unit (without them, the vanes could fall out as you remove the shroud)

70
Q

What allows for thermal expansion when the vanes are riveted or welded onto the inner and outer shroudes?

A

The inner and outer shrouds should be cut into segments.

71
Q

Are cracks permitted?
How is detection mostly carried out?

A

No, they need to be replaced
Visually

72
Q

What is a structural inspection technique of cracking in a turbine?

A

Dye penetrant

73
Q

What can be done to pitting raised by FOD?

A

It can be polished by blending out or stoning

74
Q

What two general equipment can be used to carry out an inspection more efficiently?

A

Light and magnifying glass

75
Q

How do stress rupture cracks normally appear as?

A

Hairline cracks that occur on leading/trialing edge at right angles to edge length. (Not less than 1/16)

76
Q

How does deformation caused by overheating appear as?

A

Wavy

77
Q

On first stage turbine blades, what must you automatically suspect when any stress crack deformation has occurred?

A

Overheat condition

78
Q

What materials experience creep?

A

Metals and plastics

79
Q

What is Creep?

A

A slow deformation on a molecular level caused by prolonged exposure to heat.

80
Q

What promotes creep?

A

High temperatures

81
Q

What can creep cause concludingly?

A

Dimensional inconsistency

82
Q

What may scoring identify?

A

The length of the blade is out of tolerance

83
Q

Why must you number the blades before removal?

A

To ensure they’re put in the correct place that they were taken off of

84
Q

What is another name for the rotor?

A

Turbine wheel

85
Q

What proceeds first, the nozzle guide vane or the rotor/turbine wheel?

A

Nozzle guide vane

Module 15 (18 decks)

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