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M15 > COMPRESSORS > Flashcards

COMPRESSORS Flashcards

1
Q

What is a low pressure compressor module also called?

A

A booster module.

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2
Q

What are the two ways to compress air in the engines?

A
  • Decrease volume in cylinder by a piston

- Bernoulli’s principle to get compressed air (used in compressor of gas turbine engines)

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3
Q

What are the main components of a compressor?

A

Rotor

Stator

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4
Q

What is a rotor In a compressor?

A

Blade which rotates around compressor and sucks air to the outlet side of the compressor.
(Increases energy of the airflow)

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5
Q

What is a stator in a compressor?

A

Fixed component, blades called stator vains which guide and slow airflow to increase pressure.

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6
Q

What are inlet guide vains?

A

Improve the airflow into the first compressor stage.

Either fixed or moveable

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7
Q

What are stator vains In the fan duct usually called?

A

Outlet guide vains.

Improve airflow into the fan nozzle

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8
Q

What is the purpose of the inlet/outlet case on the compressor?

A

Provide support to the compressor rotor bearings.

Also improve and straighten airflow into and out of the compressor.

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9
Q

What are the two types of compressor?

A
  • axial flow compressor

- centrifugal flow compressor

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10
Q

Where are centrifugal flow compressors usually found?

A

Small gas turbine engines such as APUs

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11
Q

What does a centrifugal flow compressor consist of?

A
  • impeller
  • diffuser
  • compressor manifold
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12
Q

Describe the operation of an centrifugal flow compressor.

A
  • impeller rotate, air sucked to centre
  • rotation pushes air outward increasing air velocity and energy
  • divergent ducts on impeller some velocity increase becomes static pressure
  • airflow through diffuser velocity decrease pressure increased
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13
Q

What ratio can centrifugal compressors give?

A

5:1

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14
Q

How many compressor stages are deemed as most efficient in centrifugal compressors?

A

2

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15
Q

What is a double entry centrifugal compressor?

A

Two impellers mounted face to face.

  • air can enter from both sides decreasing frontal area.(saves weight)
  • no higher pressure but higher airflow.
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16
Q

What are the channels between rotors?

A

Diffusers.

- increases velocity but also slows it to create higher static pressure

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17
Q

What ratio are axial flow compressors usually?

A

1:3:1

Air enters with 10 psi, air pressure is 13 psi on outer side.

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18
Q

Why is compressor cross section gradually decreased?

A

Air pressure increases and density increases which means the air needs less volume

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19
Q

At optimum compressor speeds, what type of compressor is more efficient?

A

Axial flow compressor.

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20
Q

What type of compressor will cause less drag?

A

Axial flow. Smaller frontal areas.

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21
Q

Centrifugal flow compressors gain higher or lower pressure per stage?

A

Gain high pressure per stage.

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22
Q

In terms of weight saving, which type of compressor is lighter?

A

Centrifugal flow

Simpler to manufacture and lower cost

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23
Q

What type of compressor will need less power for engine start?

A

Centrifugal flow.

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24
Q

What conditions create the ‘design point’?

A

Compressors which have one optimum operating conditions for pressure ratio, rotation speed and air flow

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25
Q

What is required for smooth airflow into the rotor?

A

The correct relationship between the inlet velocity and rotor speed.

(Direction of airflow = resultant rotor inlet velocity)

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26
Q

In terms of rotors, what will determine the outlet direction of the airflow?

A

The shape of the rotor.

Speed of airflow at inlet is same as rotor outlet

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27
Q

What is the purpose of stator vains in compressors?

A

Decelerate and direct airflow into the next rotor stage.

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28
Q

When will the angle of attack change on stator vains?

A

If the rotor speed and inlet velocity changes

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29
Q

What is compressor stall?

A

Angle of attack increases and inlet air velocity decreases. Airflow separates from compressor aerofoil and causes a turbulent vortex.

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30
Q

What can compressor stall cause?

A
  • Airflow to slow down, stop or even reverse.
  • engine runs roughly and decrease rotor speed
  • if stall is strong, can affect all stages (compressor surge)
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31
Q

What is compressor surge?

A

A very severe form of compressor stall.

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32
Q

How is compressor surge generated?

A
  • Rapid decrease of airflow

- stall causes blockage of airflow stronger stall other stages

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33
Q

Compressor surge at the front stages of the compressor will cause what?

A

Usually only affects part of the compressor blades due to the bigger diameter of the blades.
(Not dangerous at this stage)

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34
Q

What will surge in the aft part of the compressor cause?

A

Airflow decreases rapidly and causes strong reverse flow.

Reverse flow could be from the combustion chamber to the inlet

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35
Q

What is the attitude of the air during compressor stall?

A

Airflow will collapse and build up in short intervals.

- causes loud banging and vibration

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36
Q

All primary engine components can cause compressor stall or surge. What issues can arise to creat this?

A
  • dents or ice
  • damaged rotor blades
  • dirty rotors
  • damaged combustion components
  • damaged turbine
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37
Q

What issues will fast deceleration cause?

A

Compressor stall or surge as the HP compressor slows faster.

LP compressor withdraws air which cannot pass through the HP compressor.

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38
Q

Why will too fast acceleration cause compressor stall or surge?

A

Too much fuel in combustion chamber causing excess back pressure for a while.

39
Q

What systems are In place to prevent compressor stall or surge?

A
  • compressor bleed valves
  • double or triple spool rotors
    Variable compressor stator vains
40
Q

What is the purpose of compressor bleed valves?

A

(Mid or aft section of compressor)

  • Open at low engine speeds so excess air can go overboard.
  • prevents blockage in aft compressor stage
41
Q

What is a major disadvantage of compressor bleed valves?

A

Air which is pushed overboard compressed which causes a major loss of efficiency

42
Q

What is the most effective method of stall prevention?

A

Variable stator vains

43
Q

What is the function of the fan blades?

A

Provide initial compression and pass air through primary and secondary gas paths

44
Q

What does the LPC/LPR concern?

A

Attached and secures the LPC rotor to the LPT shaft.

45
Q

What is the fan stator case?

A

Provides flow path for fan discharge air and connects engine with necelle.

46
Q

What is the intermediate fan case?

A

Major structural component. Engine mounting location.

47
Q

What is the rotor drum for in a low compressor module?

A

Attached to the fan disc which has dovetail slots for rotor blades.

48
Q

What does a knife edge which can cut in creating a small gap between the rotor assembly and the shroud improve?

A

Efficiency of the compressor.

49
Q

What are the 2 construction methods for a compressor stator case?

A

Closed stator case (all stator vanes fitted to shrouds between rotor stages)

Split stator case. (Stator vanes installed in halves and bolted together afterwards)

50
Q

What does the fan case assembly house?

A

The fan rotor and low pressure compressor.

51
Q

What do flanges on the outer surface of the case help?

A

Stiffen the case and prevent deformation.

52
Q

What are the lining elements for on a fan case?

A

Found on the inner surface, providing a smooth surface for the airflow in the fan duct.

53
Q

What are the very small holes for on the surface of the lining element in the fan case?

A

Reduce noise created by the fan

54
Q

What is the shrouded area on the fan case for?

A

Made of soft material which prevents damage to the fan blades if they come in contact

(Also named abraidable shroud)

55
Q

What is the ring element made up of which is called the fan blade containment?

A

Honeycomb construction covered by layers of Kevlar.

56
Q

What are the outlet guide vanes in the fan case assembly made of?

A

Aluminium or composite materials.

57
Q

What is the module between the low pressure compressor and high pressure compressor?

A

Fan frame.

58
Q

What do the variable bleed valves do for the low pressure compressor?

A

Protect it against surge. Bleed off excessive air from the low pressure compressor into fan discharge duct.

59
Q

What do the fan frame struts do?

A

Carry loads from the engine bearings to the engine mount.

Always hollow to save weight

60
Q

What does the centrifugal force of fan blades depend on?

A
  • Rotor speed
  • Blade mass
  • Distance of centre of gravity of fan blade from rotational axis
61
Q

What does the moment weight of the fan blade change with?

A

Blade mass and centre of gravity of fan blade.

Blade mass x distance

62
Q

Can fan blades me manufactured to have the same moment weight?

A

No.

63
Q

Where can the moment weight be found?

A

On the root of the blade.

Part number, serial number, moment weight

64
Q

How is fan blade no. 1 identified?

A

Index mark on the fan disc or an offset hole.

65
Q

What are used to balance the difference in moment weight of old and new fan blades?

A

Balance weights.

Installed on fan disc or in the spinner cone [balance screws]

66
Q

What is the compressor outlet case often named?

A

The diffuser case, because the gas path is a divergent duct.

67
Q

Where is the rear bearing of the high pressure compressor usually?

A

In the combustion case between the compressor and turbine.

68
Q

How many stages do modern high pressure compressors have?

A

9-14

69
Q

What does a labyrinth seal do in the stator vanes on a high pressure compressor?

A

Prevents reverse airflow through the gap between the rotor and the stator.

70
Q

What does a CDP seal (behind the last high pressure compressor stage) do?

A

Prevents compressor discharge airflow from escaping into the bearing area below the combustion chamber.

71
Q

How many different ways are there to fit rotor blades to a HP compressor rotor?

A

3.

72
Q

What movement of blades is prevented by using blade retainers?

A

Axial.

73
Q

What is attachment method of using bolts for connecting blades to the HP rotor usually for?

A

Used for long blades at the first stage of compressor.

74
Q

What does the front compressor case for a high pressure compressor support structural loads of?

A

Bending and torsion.

75
Q

Why do the inner ends of vane segments in stator vanes have an inner platform?

A

Minimise vibration caused by airflow.

76
Q

What do variable stator vanes have at each end?

A

A trunnion in each end.

77
Q

What are VSV’s?

A

Variable stator vanes.

Installed together with an actuation lever

78
Q

What is the HP compressor a source of?

A

Pressurised air for the pneumatic system.

Compressor stator has openings to supply bleed air.

79
Q

Where is the bleed air supplied through on a HP compressor?

A

Hollow stator vanes or hollow holes around the case.

80
Q

What do bleed manifolds do?

A

Collect bleed air from pick up holes and supply it to consumers.

81
Q

What is the spinner cone also known as?

A

Inlet cone or nose cone.

82
Q

What sections is a fan rotor divided into?

A
  • spinner come
  • fan disk
  • fan blades
  • low pressure compressor rotor (booster rotor)
83
Q

What two designs are there for spinner cones?

A

Sharp tip

Curved tip

84
Q

What are the white spiral markings on spinner cones used for?

A

Scare birds away.

85
Q

Where can balance weights be fitted to the spinner cone?

A
  • Attached to the mounting bolts

- installed on into threaded inserts at the outer surface of the spinner

86
Q

What type of fan blade fitting is found on most engines?

A

A dovetailed fitting

87
Q

A loose fit is necessary when fitting rotor blades. Why are rotor blades fit loose?

A

All sudden forces or torque loads do not exceed the maximum allowable stress limits of the fan blades

88
Q

What loads are on rotor blades?

A

Centrifugal

Air loads

89
Q

What system do long rotor blades employ to keep the loading on them safe?

A

Shroud which forms a ring

90
Q

What are the shrouds on rotor blades also known as depending on the manufacturers?

A

Mid-span shroud

Part-span shroud

91
Q

What method protects rotor blade shrouds from rubbing against each other and breaking?

A

A special coating

Shrouds transfer blade flutter to friction

92
Q

What type of fan blade do most aircraft use?

A

Wide chord fan blade

93
Q

Why would wide chord fan blades be used over conventional blades?

A

They are more efficient

94
Q

Wide chord fan blades are able to withstand FOD damage better than conventional. Why is this?

A

High flexibility, able to absorb impact energy

M15 (18 decks)

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