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M15 > M15.06 Turbines > Flashcards

M15.06 Turbines Flashcards

1
Q

What is the main function of a turbine?

A

. Provide power necessary to drive the engine compressor and accessory gear box

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2
Q

What are the two different types of turbines in gas turbine engines?

A

. Radial flow turbine
. Axial flow turbine

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3
Q

What is the first main component of a turbine?

A

. Set of stationary vanes

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4
Q

What are the stationary vanes called??

A

. Turbine nozzle guide vanes

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5
Q

What stage turbines are radial flows?

A

. Single stage turbines

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6
Q

Where are radial flow turbines used?

A

APU
as only allow small airflows but are small and easy

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7
Q

Where are axial flow turbines used?

A

On modern gas turbine engines

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8
Q

How many stages are there on an axial flow turbine?

A

Many stages

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9
Q

What is the basic operation of a turbine?

A

Turbine converts the gas energy from the combustion chamber into torque

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10
Q

The gas flow from the combustion to the turbine why?

A

Because of the convergent shaped ducts between the nozzle guide vanes (first) to the turbine blades

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11
Q

The impact of the gas flow on the turbine blades causes what?

A

Causes the turbine to rotate, which makes the torque to drive the turbine shaft

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12
Q

How does an impulse turbine work?

A

Makes the rotation of the turbine only by the impact of gas flow into the rotor blades
(Very old)

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13
Q

Impluse reaction turbines?….?…. Find out how tf they work

A
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14
Q

An impluse reactuin turbine has another name, what is it??

A

Reaction turbine

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15
Q

The loads carried by a turbine component determine what??

A

How long they remain in service

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16
Q

What loads must turbine materials withstand??

A

. Extremely high temperatures
. Very Hugh centrifugal forces at high rotational speeds

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17
Q

What are the two factors that reduce service life of a turbine??

A

. Material fatigue by power cycles (which is reduced by correct engine operation)
. corrosion by sulphuric acid (use the correct jet fuel)

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18
Q

At take off each turbine must withstand what?

A

Hugh centrifugal forces of several tonnes

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19
Q

What is turbine creep??

A

When centrifugal loads are applied over a long time especially at high temps - causing material deformation

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20
Q

When would creep start developing?

A

When the engine is operated at max power

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21
Q

How would you get a large increase in the rate of creep??

A

. High energise speed and high temperatures

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22
Q

How can the pilot prevent creep?

A

Keep max power for a short amount of time only

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23
Q

What method is used to reduce creep?

A

Decrease the temperature/ cooling

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24
Q

Two reasons why turbines are cooled?

A

. Increases service life
. Better turbine efficiency

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25
Q

How are HP turbine nozzle guide vanes and rotor blades cooled??

A

. With air from the engine HO compressor

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26
Q

Which turbine cooling method is the easiest?

A

. Convection cooling

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27
Q

How does convention cooling work?

A

. Cooling air from compressor enters through holes at the bottom of blades and flow through the channels, cooling them

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28
Q

What cooling method is better for turbine nozzle vanes and rotor blades?

A

. Impingement cooling

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29
Q

How does film cooling work?

A

. Cooling air is blown into hot gas flow via small drill holes in the turbine aerofoil,which forms a thin air film in the outer walls

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30
Q

What method of cooling is the most effective?

A

Film cooling

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31
Q

What is the main disadvantage of film cooling?

A

. Very expensive

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32
Q

The first stage of nozzle vanes and rotor blades are cooled by??

A

. Convection
. Impingement
. Film cooling
. Because very high gas temperatures from the combustion chamber

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33
Q

The second stage nozzle guide vanes are cooled by??

A

. Convection
. Impingement
. Because gas temperature is cooler at this stage

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34
Q

The second stage rotor blades are cooled by??

A

. Convection only
. Because a lot cooler gas temperatures

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35
Q

Modern gas turbines have an active clearance control system, why??

A

. To keep the tip clearance as small as possible in all operating conditions

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36
Q

What is the tip clearance??

A

. The gal between the tip of the rotor blades and the casing of the compressor of turbine

37
Q

Why do too clearances change?

A

. Be Jane if the different expansions if engine materials at different engine operations

38
Q

Why do you want a small turbine gap clearance?

A

. Because a large gal clearance decreases the efficieny of the gurbine stages as a larger proportion of the gas would pass through the gap of the turbine casing, which would not help drive the turbine.

39
Q

Why does the turbine casing expand faster than turbine rotors blades?

A

. Case is thinner than the rotor
. Case is in contact with high temperatures

40
Q

When the rotor speeds get faster, it increases the centrifugal force, which does what to tip clearances?

A

. Reduce tip clearance because rotor blades expand more

41
Q

When would tip clearance decrease?

A

Engine shut down, turbine rotor and casing decrease

42
Q

What is used to cool the turbine casings??

A

. Cold fan air
. Sometimes compressor bleed air

43
Q

Why do we have active clearance control systems?

A

To keep the tip clearance as small as possible

44
Q

Where is passive clearance control method used??

A

. HP and LP turbines

45
Q

What is used to cool turbine cases using passive clearance control system??

A

Cooled by continuous airflow that is not regulated

46
Q

Where does the HO turbine cooling air for a passive clearance system come from??

A

. Uses air from the last stages of HP compressors as cooling source

47
Q

What does the LP turbines uses for passive clearance cooling source?

A

Fan air

48
Q

Explain how active clearance control systems work?

A

Use the same cooling air as passive clearance control systems but can also control the quantity of air

49
Q

What shape are LPT cooling tubes??

A

Circular shaped tubes
(Which point directly to turbine case)

50
Q

What shape are the HPT cooling tubes??

A

. Rectangular shaped
(Bleed holes are at the edges)

51
Q

The clearance control valves are always separate for?

A

. Valves for HPT
. Valves for LPT

52
Q

How are the clearance control valves controlled?

A

. By engine control unit and actuated by fuel pressure from hydro mechanical unit

53
Q

What does the tip clearance control save??

A

Saves a lot of fuel usage

54
Q

On engines with hydro mechanical control units, the active clearance control operates when??

A

. Only during climb and cruise

55
Q

The hydro mechanical engine control unit uses two signals to active the clearance control system, what are they??

A

. Core engine speed
. Aircraft Altitude from pressure port

56
Q

Cruise power operation is usually in the range of?

A

80 - 95% N2

57
Q

With the two signals the hydro mechanical engine unit can?

A

Open and close the clearance control valve

58
Q

On engines with FADEC system, tip clearance can be controlled??

A

For all operating conditions

59
Q

A typical twin spool engine has?

A

. HP turbine (HPT)
. LP turbine (LPT)
. A turbine frame

60
Q

Where the the HPT rotor fitted?

A

To the HP compressor rear shaft

61
Q

What is the LPT surrounded by?

A

The LP turbine case

62
Q

What is the typical turbine stator assembly?

A

. Stator case
. Nozzle guide vanes
. Sealing and wall segments
. Clearance control air manifolds

63
Q

What is the purpose of the HPT case?

A

. Supports the nozzle guide vanes and wall segments opposite the rotor blades

64
Q

How many pieces are HPT cases normally made from??

A

1

65
Q

What is special about the wall segments in a turbine cases?

A

Have an abradable case ceramic surface which give optimal thermal resistance

66
Q

What are the nozzle guide vanes held in place by?

A

Nozzle guide vanes support

67
Q

The nozzle guide vanes are made of?

A

Many vane segments

68
Q

What prevents gas flow through stator and rotors gap??

A

Interstate seal

69
Q

On the HP turbine case there are cooling tubes for?

A

Active clearance control

70
Q

There are ports in the turbine case for??

A

For borescope, inspecting the rotor blades

71
Q

The stator case of the LP turbine is bolted to the?

A

Rear flange of the HP turbine

72
Q

LPT are usally made from how many pieces?

A

1

73
Q

The HP root has what 4components?

A

. Turbine discs
. Rotor blades
. Rotating interstage seal
. FWD and AFT blade retainers

74
Q

What is the purpose of the turbine disc?

A

Hold the rotor blades and transfer the torque

75
Q

What is the purpose of the interstage seal on the rotor??

A

. Acts as a spacer in between the two discs

76
Q

What is the function of the blade retainers?

A

. Holds the turbine blades in one place so that it doesn’t move from their seats due to gas loads

77
Q

Pg 81

A
78
Q

The Rory or blades on HP turbine have a special shape?

A

Special shape root - known as fir tree root

79
Q

What are the two types of turbine blades used??

A

. Open end blade
. Shrouded blade

80
Q

Where would open end blades be used?

A

On turbines with High rotational speed - HP turbines

81
Q

Where would shrouded blades be used??

A

. Mainly LP turbines
Sometimes HP

82
Q

What do shrouded blades help with??

A

. Reduce vibrations
. Also knife edge seat on outer surface

83
Q

What is a disadvantage of a shrouded blade?

A

. Adds more weight to each rotor blade
(Therefore mainly LP with lower rotational speeds)

84
Q

HP blades are cooled internally by air from the HP compressor, how?

A

Fed into gap from the front or rear of turbine disc

85
Q

Where is the turbine frame?

A

The aft major structural compartment of the engine

86
Q

What are the two functions of the turbine frame??

A

. Supports the turbine rotors
. Provides attachment points for rear engine mount

87
Q

What are the two different frame struts??

A

. Radial strut
. Tangential strut

88
Q

How does a radial strut work??

A

. As short as possible
. Minimises expansion so keeps the stress on frame hub low

89
Q

How does a tangential strut work??

A

. Minimises stress caused by thermal expansion because if expand, the hub will rotate slightly

M15 (10 decks)

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