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Module 15 Richard > 15.06 Turbine Section > Flashcards

15.06 Turbine Section Flashcards

1
Q

What is the purpose of the turbine

A

The turbine converts the gas from the combustion chamber into torque

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2
Q

What causes the turbine to rotate

A

The impact of the gas flow on the turbine rotor blades

This makes the torque to drive the turbine shaft

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3
Q

What is a turbine which makes the rotation only by the impact of the gas flow on the rotor blades called

A

An impulse turbine

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4
Q

How is an impulse turbine identified

A

The turbine type is recognisable by the special shape of the rotor blades

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5
Q

Where are impulse turbines normally found

A

Generally found on very old gas turbine engines

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6
Q

What is an impulse reaction turbine

A

It is named this because the force which operates the turbine is the sum of the force caused by the impulse of the gas flow on the rotor blades and of the force caused by the reaction of the gas flow which leaves the rotor stage

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7
Q

What two main conditions must turbine blades withstand

A

Extremely high temperatures and high centrifugal forces

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8
Q

What two factors that reduce the service life of a turbine blade

A
  • Material fatigue caused by many power cycles. It can only be reduced by correct engine operation
  • Corrosion caused by sulphuric acid. This mainly occurs because of the sulphur in fuel and high gas temperatures. It is only prevented by using the correct kind of jet fuel
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9
Q

What is creep deformation

A

Material deformation known as creep occurs when centrifugal loads are applied over a long period of time, specially with high material temperatures

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10
Q

When does creep deformation start

A

When the engine is operated at maximum power

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11
Q

What are the two opposing factors that make it hard to prevent material deformation

A

High turbine inlet temperatures are required for optimum internal efficiency, but low turbine material temperatures are needed to prevent plastic deformation

Due to these factors, plastic deformation can only be limited by efficient cooling of the turbine materials

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12
Q

What are the 2 main reasons that turbines are cooled on modern engines

A
  • To increase their service life. This is done by cooling the internal turbine components like nozzle guide vanes and rotor blades
  • For better turbine efficiency. This is done by cooling the outer turbine castings
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13
Q

What is the easiest cooling method for high pressure turbines

A

Convection cooling

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14
Q

How does convection cooling take place

A

Cooling airflow passes through the hollow turbine nozzle vanes and rotor blades

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15
Q

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

A

Impingement cooling

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16
Q

How does impingement cooling take place

A

The cooling air first flows into an insert fixed inside the hollow turbine aerofoil

The cooling air is forced through tiny jet nozzles and impacts on the inner walls of the aerofoils. This improves the contact between cooling air and turbine materials and therefore heat transfer

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17
Q

What is film cooling

A

The most effective method of cooling turbine nozzle guide vanes and at the rotor blades.

Reaches a maximum cooling effect but with a minimum of cooling air, this means more air is available to drive the turbine

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18
Q

How does film cooling take place

A

Cooling air is blown in to the hot gas flow via small drill holes in the turbine aerofoils

The gas stream detects the cooling air and forms a thin air film on the outer walls of the turbine blades and vanes. This cooling film prevents the direct contact of hot gas flow with turbine materials

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19
Q

What is a disadvantage of Film cooling

A

The small drill holes are very difficult to make and therefore very expensive

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20
Q

What is the purpose of active clearance control systems

A

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

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21
Q

What is the biggest downfall of engines with out a clearance control system

A

Fuel consumption can be up to 4 times higher

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22
Q

What is the reason that a turbine case expands faster than the turbine rotor

A
  • It is thinner than the rotor

- It is in contact with higher temperatures

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23
Q

Is the expansion of a material greater though thermal expansion or through centrifugal force

A

Centrifugal force

24
Q

Is tip clearance bigger or smaller at high engine speeds

A

Smaller due to centrifugal force

25
Q

What is normally used for cooling turbine cases

A

Fan air. Some engines use compressor air

26
Q

What are the 2 methods of turbine clearance control

A
  • Active Clearance Control

- Passive Clearance Control

27
Q

What is the Passive Clearance control method

A

The turbine cases are cooled by a continuous airflow which is not regulated

HP turbine - Uses air from HP compressor
LP turbine - Uses fan air as cooling source

28
Q

What is the active clearance control method

A

Use the same cooling air as Passive clearance control systems but they can control the quantity of air

29
Q

What are the turbine clearance control tube assemblies also called

A

Spray rings

30
Q

What shape are LPT cooling tubes

A

Circular

The bleed holes point directly to the turbine case

31
Q

What shape are HPT cooling tubes

A

They are often rectangular shaped tubes

Bleed holes are on the edges

32
Q

Why are the holes on the side of the HPT tubes

A

This is important as they blow the cooling air to the areas with the thickest materials first and make sure that the turbine case expands and shrinks evenly

33
Q

What is the purpose of the clearance control valves

A

They are just shut off valves on older engines but they are usually modulating valves on modern engines

34
Q

Can the HP turbine and LP turbine share the same clearance control valve

A

No they are separate valves however they can be combined in a common valve housing

35
Q

How are the clearance control valves controlled

A

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

36
Q

What two signals does a hydro mechanical engine control unit need to activate the clearance control system

A
  • Core engine speed

- Aircraft altitude

37
Q

When does the active clearance control system operate on systems with only a hydromechanical unit

A

Only during climb and cruise

38
Q

What is the advantage of FADEC controlled clearance control systems

A

The tip clearance can be controlled for all operating conditions, this further improves fuel saving

39
Q

What 3 main turbine components does a typical twin spool engine have

A
  • High Pressure Turbine (HPT)
  • Low Pressure Turbine (LPT)
  • A Turbine Frame
40
Q

What type of loads do turbine loads take

A

Carries the baring loads of the aft end of the low pressure turbine and transmits these loads to the rear engine mount on the turbine frame

41
Q

What components does a typical turbine stator have

A
  • The Stator Case
  • The Nozzle Guide Vanes
  • Sealing And Wall Segments
  • Clearance control air manifolds
42
Q

Are HP turbine cases made up from several pieces bolted together or are they one solid piece

A

Made in one piece

43
Q

What do turbine case wall segments have to help with thermal resistance

A

An abradable ceramic surface

44
Q

What is the disadvantage of the abradable ceramic surface on turbine case wall segments

A

The ceramic surface can wear off easily when it is in contact with the tips of the turbine rotor blades

45
Q

How are the nozzle guide vanes of the high pressure turbine held in place

A

They are held on by nozzle guide vane supports which are attached to the combustion case or the turbine case

46
Q

What are nozzle guide vanes made up of

A

Many vane segments. Each vane segment usually has two nozzle guide vanes

47
Q

How are the gaps between vane segments sealed

A

With metallic sealing strips which fit into slots in the outer and inner platform of the nozzle guide vane

48
Q

What is an interstage seal

A

On the inner ring of the segments of the nozzle guide vanes the inner ring is a sealed surface which belongs to an air seal. In this position the seal is known as the interstage seal

49
Q

Where is a stator case of a low pressure turbine fitted

A

The case is bolted to the rear flange of the high pressure turbine

50
Q

What do the outer seals form with the knife edges of the rotor blades

A

A labyrinth seal

51
Q

What is the high pressure turbine rotor attached to

A

The compressor rear shaft behind the rear bearing

52
Q

What components does the high pressure turbine rotor have

A
  • The turbine disks
  • The rotor blades
  • A rotating interstage seal
  • Forward and rear blade retainer
53
Q

The cross sectional area of the turbine discs is very large. Why is this important

A

This is necessary as the disc must withstand high centrifugal loads from the blades in high operating temperatures

54
Q

What is the purpose of the interstage seal

A

It acts as spacer between the two discs

On some high pressure turbines this seal must transfer some torque from the stage two disk to the stage one disk

55
Q

What are the rotor blade retainers designed to do

A

Hold the turbine blades in place so that they do not move from their seats due to the gas loads

Module 15 Richard (18 decks)

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