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M15 > TURBINES > Flashcards

TURBINES Flashcards

1
Q

What is a turbine for?

A

Provide the power necessary to drive the engine compressor and engine gearbox.

(Extracts hot gases from combustion chamber)

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2
Q

On engines with a hydro-mechanical control unit, when will the active clearance operate during flight?

A

Climb and cruise.

clearance control most effective if engine is working at high power for long periods of time

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3
Q

What two signals does a hydro-mechanical control unit need?

A

Core engine speed (cruise power usually in range of 80-95% N2)
Aircraft altitude

(Signals open and close clearance control valves)

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4
Q

What are the 2 types of turbines on gas turbine engines?

A

Radial flow turbine

Axial flow turbine

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5
Q

What are the stationary vanes named?

A

Turbine nozzle guide vanes.

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6
Q

What are the 2 main components of a turbine?

A

Stator vanes
Moving rotor blades on turbine disc.

(Stator and rotor)

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7
Q

What type of turbine is always single stage?

A

Radial flow turbines.

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8
Q

What two perimeters does the ECU use to calculate tip clearance?

A

Rotor size
Case size

(Also calculates necessary cooling air)

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9
Q

What is the purpose of the turbine frame?

A

Behind the low pressure turbine, carries bearing loads at the aft end of the LPT and transmits loads to the rear engine mount.

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10
Q

What does a typical turbine stator consist of?

A
  • stator case
  • nozzle guide vanes
  • Sealing and wall segments
  • clearance control and manifolds
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11
Q

What is an advantage of axial gas turbines?

A

Can be built with lots of stages of compressor.

Very high airflow for high thrust.

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12
Q

What does the turbine covert gas energy from the combustion chamber into?

A

Torque.

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13
Q

What happens to the gas flow after the combustion chamber?

A

Accelerates because of the convergent duct between the stator vanes and the rotor vanes.

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14
Q

What is an impulse turbine?

A

A turbine which makes rotation only by impact of gas flow.

Very old gas turbines

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15
Q

What is a reaction impulse engine?

A

Force of the turbine rotor outlet acts in a opposite direction to the discharged gas flow.

Creates thrust force.

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16
Q

What must turbine materials withstand?

A

Extremely high temps and centrifugal forces.

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17
Q

What 2 factors can reduce service life of a turbine?

A

Material fatigue by many power cycles.

Corrosion caused by sulphuric acid from high levels of sulphur in fuel and high gas temps.

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18
Q

Why do the blades become longer at take off power?

A

Heat increase in turbine.

Remain elastic, return to original state

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19
Q

When does the material deformation such as CREEP occur?

A

When centrifugal forces are applied over a long period of time. Starts at maximum power. Also increased with heat.

(Does not return to original shape)

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20
Q

How is plastic deformation limited?

A

Efficient cooling of turbine materials.

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21
Q

What are the 2 main reasons turbines are cooled?

A

Increase service life. (Cooling vanes)

Better turbine efficiency (cooling outer turbine casings)

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22
Q

How are high pressure turbine nozzle guide vanes and rotor blades cooled?

A

Air from the high pressure compressor.

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23
Q

What is the easiest method of cooling in high pressure turbines?

A

Convection cooling.

Cooling air passes through hollow vanes and blades, then exits at trailing edges and mixes with hot air

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24
Q

What is impingement cooling better for?

A

Turbine nozzle guide vanes and rotor blades.

Cooling air enters through inserts to hollow vanes and blades which have many holes which act as jet nozzles

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25
Q

What does impingement cooling improve?

A

The contact between the cooling air and turbine materials and therefore heat transfer.

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26
Q

What is the most efficient method of cooling?

A

Film cooling.

Maximum cooling effect with minimum cooling air. More air available to drive the turbine.

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27
Q

What is film cooling?

A

Cooling air blown into the hot gas flow via small drill holes in the turbine aerofoils.

For a thin film of air on outer walls or vanes and blades.

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28
Q

What is a disadvantage of film cooling?

A

Small drill holes very difficult to make and very expensive.

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29
Q

How are second stage rotor blades normally cooled?

A

Convection cooling.

30
Q

How are second stage nozzle guide vanes normally cooled?

A

Convection or impingement.

31
Q

How are the first stage compressor stages normally cooled?

A

Impingement, convection or film cooling.

32
Q

Why does large tip clearance decrease efficiency?

A

More gas passes through which does not drive the turbine, requiring additional fuel to keep desired rotor speed.

33
Q

How much does the fuel consumption increase by with a clearance increase of 0.25mm (0.01 inch)

A

1%

30,000 kg extra fuel per year per engine

34
Q

How much does fuel consumption increase by without a turbine clearance control system?

A

4 times.

120,000 kg fuel per engine per year

35
Q

What does the time taken for a material to expand depend mainly on?

A

Thickness of material.

Thin materials heated much faster

36
Q

Does the turbine casing expand faster or slower than the rotor?

A

Faster.

37
Q

Is the expansion larger by centrifugal force or heat expansion?

A

Centrifugal force.

38
Q

What air is usually used to cool turbine casings?

A

Fan air.

39
Q

What are the 2 types of clearance control?

A

Passive

Active

40
Q

What does passive clearance control do?

A

Used on HP and LP turbines. AIRFLOW NOT REGULATED.

Uses air from last stages of HP compressor as cooling source for HP turbine, and fan air for LP turbine.

41
Q

What does an active control system do?

A

Same as passive, but QUANTITY OF AIR IS REGULATED.

42
Q

What shapes are the HPT and LPT cooling tubes?

A

HPT- rectangular (bleed holes at edges, makes sure turbine case expands and shrinks evenly. Blows air to thickest part of materials first)

LPT- circular

43
Q

What are spray rings for on turbine clearance control systems?

A

Distribute air to individual tube assemblies and cases.

44
Q

Which clearance control system always have one or more clearance control valves?

A

Active.

45
Q

What do the wall segments do in the turbine case consist of?

A

Abradable Ceramic surface which gives thermal resistance

Can be worn off quickly if in contact with turbine rotor blades

46
Q

How many nozzle guide vanes does a vane segment have?

A

2

47
Q

What is the air seal called which is on the inner ring on the vane segments?

A

Interstage seal

Prevents gas flow through the gap between the stator and rotor

48
Q

How does the clearance control system minimise the tip clearance between the turbine rotor blades and stator case?

A

Feeds cold air to the outer surface of the case

49
Q

How are low pressure turbine cases built on modern engines?

A

One whole piece

Split stator cases on some engines

50
Q

How is the low pressure turbine case cooled?

A

Air tubes for the active control system.

51
Q

What does the high pressure turbine rotor consist of?

A
  • turbine discs
  • rotor blades
  • rotating interstage seal
  • fwd and read blade retainers
52
Q

What is the purpose of the rotor discs?

A

Hold the rotor blades and transfer the torque from the blades to the rear compressor shaft

53
Q

Why should the cross sectional area of the turbine discs be large?

A

Discs must withstand high centrifugal loads in high operating temperatures

54
Q

What does the interstage seal act as?

A

A spacer between the two disks

Some high pressure turbines require the seal to transfer torque stage 2 of the disc to stage 1

55
Q

What are blade retainers designed to do?

A

Designed to hold the turbine blades in place so that they do not move out of their seats during gas loads.

56
Q

Where are strip-type blade retainers usually found?

A

Low pressure turbines - slotted into dovetail slots and bent up at each end.

57
Q

What is the root of rotor blades on high pressure turbines called?

A

Fir tree root because of its shape.

58
Q

How is the service life for rotor components made longer?

A

Fir tree root has many contact surfaces so forces from the blade are smoothly transferred to the disc.

59
Q

What are the two types of blade used in modern gas turbine engines?

A

Open end - (high rotational speeds such as HP turbines)

Shrouded - (some HP turbines)

60
Q

What do the shrouds on shrouded blades help to reduce?

A

Vibration

61
Q

What does the knife edge seal do on shrouded blades?

A

Help to reduce air loss at the Tip of the blade

62
Q

Shrouds on turbine blades add more weight to each rotor blade. Because of this, where are they used?

A

Low pressure turbines with lower rotational speeds.

63
Q

How can turbine discs be connected to a drive shaft?

A

Directly splined

Bolted together and connected to a common drive shaft.

64
Q

How are high pressure turbine blades cooled?

A

Internally cool by air from the HPC

65
Q

Where does cooling air enter rotor blades?

A

A slot between the lower end of the blade root and lower end of the fir tree slot.

66
Q

What components will the turbine frame provide support to?

A

Turbine rotors, jet nozzle and exhaust cone

67
Q

What is a typical turbine frame built up of?

A
  • frame hub with bearing support
  • outer frame casing
  • set of struts which connect the hub with outer casing
68
Q

The struts of a turbine frame are aerodynamically shaped and hollow inside. What does this design allow the use of?

A

Room for oil supply lines, scavenge lines and oil drain lines.

(Also provides better support agains bending and torsion loads)

69
Q

What are the two different arrangements for frame struts?

A

Radial struts - designed as short as possible. Minimises expansion and stress on frame hub low.

Tangential struts - minimise thermal expansion, hub rotates slightly if expanded

70
Q

What is an advantage of radial flow turbine?

A

Simple design and easy to manufacture.

Used on small gas turbines, such as APU’s

71
Q

What are disadvantages of axial turbines to radial turbines?

A

Only allow small air flows and less efficient

High aerodynamic losses, airflow must pass through

M15 (18 decks)

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