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Resource Group Mod 15 2024 > 15.6 Turbine > Flashcards

15.6 Turbine Flashcards

1
Q

What is the primary purpose of the turbine?

A

Transform a portion of the exhaust gas kinetic energy, into mechnical energy to drive the compressor and its accessories. (about 60-70% of total pressure energy from exhaust).

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2
Q

What is the amount of energy absorption at the turbine, determined by?

A

Determined by the load the turbine is driving. (Compressor size, type, number of accessories).

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3
Q

Where is the turbine usually located within the GTE?

A

Aft (downstream) of the combustion chamber.

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4
Q

What are the two main components of the turbine?

A
  • Turbine Nozzle Guide Vanes (stator).
  • Rotor section, blades attached to a rotating turbine disc).
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5
Q

How many stages can a turbine have?

A

One or more, defined by number of pairs of rotors and stators, just like compressors.

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6
Q

What are the two types of turbine?

A
  • Radial flow.
  • Axial flow.
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7
Q

How many stages does a radial flow turbine have?

A

Always 1.

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8
Q

Where are radial flow turbines normally used?

A

Only in small GTEs, such as an APU.

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9
Q

What is the advantage of a radial flow turbine?

A

Simple design and easy manufacture.

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10
Q

What are the disadvantages of radial flow turbines?

A
  • Only allows small airflow.
  • Less efficient.
  • High aerodyamic losses as air must pass through turbine opposing centrifugal forces.
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11
Q

Where are axial flow turbines used?

A

On modern GTEs and high bypass ratio engines.

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12
Q

Why are axial flow turbines the ideal choice for modern GTEs?

A
  • They create very high airflow, necessary for high thrust.
  • They allow for any number of stages necessary to operate the compressor and all the accessories.
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13
Q

How do the Nozzle Guide Vanes accelerate and direct the gas flow from the combustion chamber?

A
  • They have a convergent shape to accelerate the gas flow.
  • The shape and angle helps direct the airflow in the direction of rotation of the rotor blades.
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14
Q

What causes the turbine blades to rotate?

A

The impact of the gas flow.

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15
Q

What are the 3 types of turbine blade used?

A
  • Impulse configuration.
  • Reaction configuration.
  • Reaction - Impulse configuration.
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16
Q

What is the key point of the impulse-type turbine blade?

A

The blades experience an impulse force caused by the impact of the gas on the blades.

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17
Q

With the impulse-type blades, where does the total pressure drop of each stage occur?

A

Occurs in the fixed nozzle guide vanes, which increases velocity as pressure decreases.

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18
Q

What type of energy is the pressure energy converted to, in an impulse-type blade configuration?

A

Kinetic energy.

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19
Q

How does the impulse force react?

A

It’s resolved into 2 components.

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20
Q

In a Reaction configuration, what do the fixed nozzle guide vanes do?

A

They alter the gas flow direction, without changing the pressure.

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21
Q

How do the converging blade passages experience a reaction force?

A

By the expansion and acceleration of the gas.

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22
Q

Which of the blade configuration types is normally used in GTEs?

A

Impulse-Reaction configuration.

Pure impulse/reaction configurations alone, are not normally used.

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23
Q

What is experienced at the root of the blades, in a reaction-impulse configuration?

A

Impulse.

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24
Q

What is experienced at the tip of the blades, in a reaction-impulse configuration?

A

Reaction.

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25
Q

What is the benefit of the reaction-impulse configuration?

A

Blade exit pressure can be held relatively constant.

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26
Q

How are turbine blades normally constructed?

A

Either forged or cast, but are normally precision cast.

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27
Q

Turbine blades cast as a ‘single crystal’ gives what benefits?

A

Better blade strength and heat properties.

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28
Q

What coating helps keeps the turbine blades and inlet nozzles resistant from heat?

A

Ceramic coating.

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29
Q

What is the MAIN benefit of having heat resistant turbine blades and inlet nozzles?

A

Allows for higher exhaust temperatures, which increases engine efficiency.

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30
Q

What other method cools the turbine blades and inlet nozzles?

A

By airflow cooling.

31
Q

Where are most turbine sections ‘open’?

A

At the outer perimeter of the blades.

32
Q

What is a ‘shrouded turbine’?

A

A band is formed around the outer perimter of the turbine wheel.

33
Q

What are the advantages of a shrouded turbine?

A
  • Improves efficiency.
  • Improves vibration characteristics.
  • Permits lighter stage weights.
34
Q

What are the disadvantages of a shrouded turbine?

A
  • Limited Turbine speed.
  • Requires more blades in total.
35
Q

Why are additional stages normally required?

A

As a single turbine wheel cannot absorb enough power to drive all the components alone.

36
Q

What are the main components of the rotor element of the turbine?

A
  • The turbine shaft.
  • The tubine wheel.
37
Q

What does the turbine wheel consist of?

A

A dynamically balanced unit of turbine blades fitted to a rotating disc.

38
Q

How does the turbine disc abosorb heat?

A

Via conduction.

39
Q

How is the turbine disc cooled?

A

Bleeding cool air back onto the face of the disc.

40
Q

What is the turbine disc attached to?

A

The power-transmitting shaft of the engine.

41
Q

What is the turbine shaft normally made of?

A

Alloy steel.

42
Q

In what two methods is the turbine disc attached to the shaft?

A
  • Welded.
  • Bolted through a shaft hub.
43
Q

What is the most common method for attaching the turbine disc to the shaft?

A

Bolted through a shaft hub.

44
Q

What causes the turbine shaft to rotate?

A

The blades extract power from the exhaust gas, and the kinetic energy rotates the disc, which is welded or bolted to the engine drive shaft.

45
Q

What else is the turbine shaft attached to?

A

The compressor shaft.

46
Q

How is the turbine shaft fitted to the compressor shaft?

A

Via splined ends.

47
Q

What must be controlled in order to keep the turbine within safe operating limits?

A

Engine speed and temperature.

48
Q

What stresses does the turbine section experience? (2)

A
  • Rotational (torque) stress.
  • Thermal stress.
49
Q

How do the turbine blades fit onto the turbine disc, with thermal expansion considered?

A

They fit loosely onto the turbine disc, so that they become tight when the blades expand under thermal expansion when the engine is operating.

50
Q

What are the two usual types of blade root design?

A
  • Bulb type.
  • Fir-tree.
51
Q

What are the two main design methods of blade retention on the turbine disc?

A
  • Peening.
  • Blade root stop.
52
Q

What is the peening method for turbine blade retention?

A

A small notch is ground in the edge of the fir-tree root before the blades are installed. Upon installation, the disc metal fills the notch and helps lock the blade in place.

53
Q

What is the ‘blade root stop’ method for turbine blade retention?

A

A stop on one end of the turbine blade root allows the blades to be installed or removed in only one direction. The other end is secured via a tang that can be bent over and locked in place.

54
Q

What are the 3 common names for the stator element of the turbine section?

A
  • Turbine inlet nozzle vanes.
  • Turbine inlet guide vanes.
  • Nozzle diaphragm.
55
Q

Where are the turbine inlet nozzles located?

A

Directly aft of the combustion chamber, immediately forward of the turbine wheel.

56
Q

What is true about the temperature of the turbine inlet nozzle vanes?

A

It’s usually the hottest area that comes into metal contact with the engine.

57
Q

Why are the turbine inlet nozzle vanes contoured, and at a specifc angle?

A

To convert a portion of the heat and pressure energy into velocity energy, which is then converted into mechanical energy.

58
Q

What’s a secondary purpose of the turbine inlet nozzles?

A

To deflect the gases in the direction of turbine rotation.

59
Q

How are the turbine inlet nozzles contructed to allow for thermal expansion?

A

They are assembled loosley into the inner and outer vane shrouds.

60
Q

What are support rings?

A

They allow for the removal of the turbine inlet nozzle vanes as a one complete unit.

61
Q

What other method can the turbine inlet nozzle vanes be secured to the shrouds by?

A

Riveted or welded.

62
Q

If the turbine inlet nozzle vanes are riveted or welded to the shrouds, how is thermal expansion compensated for?

A

The supporting rings are installed as segmented pieces.

63
Q

What type of damage is not permitted on blades?

A

Cracks.

64
Q

What can assist in the visual inspection of turbine blades?

A

Dye penetrant

65
Q

What must be done if a crack is found on a the turbine disc?

A

The disc and the turbine rotor must be replaced.

66
Q

What can be done if slight-pitting is found on the turbine blades?

A

They can be blended within limits.

67
Q

How do ‘stress rupture cracks’ usually appear?

A

As hairline cracks on the edges of the turbine blades, at 90 degree angles to the edge.

68
Q

What cause deformation of the turbine blades?

A

Overtemp conditions.

69
Q

How can deformation appear on turbine blades?

A

Waviness along the leading edges of the turbine blades.

70
Q

What must the condition of the turbine blade leading edges be?

A

Straight and uniform across the entire edge, except for areas that have had blended repairs.

71
Q

What is blade ‘creep’?

A

Slow structural deformation on a molecular level, by prolonged exposure to high stress.

72
Q

What promotes blade creep?

A

Extreme temperatures.

73
Q

How is blade creep identified?

A

Dimensional inconsistencies of the blades.

74
Q

What can ‘scores’ on the turbine case be an indication of?

A

The length of the turbine blades are out of tolerance. (Maybe due to creep or overtemp).

Resource Group Mod 15 2024 (13 decks)

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