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Module 15 Richard > 15.04 Compressors > Flashcards

15.04 Compressors Flashcards

1
Q

What is a combination of a rotor and a stator known as?

A

A compressor stage

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2
Q

What is always the first part of a compressor stage?

A

The rotor

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3
Q

What is the purpose of the stator?

A

It guides and slows the airflow - increasing pressure

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4
Q

What are the set of stator vanes in front of the compressor stage called?

A

Inlet guide vanes (IGVs)

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5
Q

What is the purpose of the IGVs?

A

They improve the airflow into the compressor

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6
Q

What is the purpose of the compressor casings?

A
  • they support the bearings

- they improve and straighten the airflow

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7
Q

What are the two types of compressors?

A
  • axial flow compressors

- centrifugal flow compressors

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8
Q

What pressure ratio can centrifugal flow compressors give?

A

A pressure ratio of 5:1

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9
Q

On a centrifugal flow compressor, what has the same function as a rotor and stator on an axial flow compressor?

A

The impeller and diffuser

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10
Q

How many stages of a centrifugal flow compressor are the most efficient?

A

2 stages - due to a pressure reduction when the airflow changes direction in more than 2 stages

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11
Q

What is the double entry type impeller?

A

Two impellers mounted face to face. Also known as the double-face impeller

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12
Q

What is the advantage of a double-face impeller?

A

It saves weight and gives the engine a smaller frontal area

Higher airflow at given diameter, not higher pressures

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13
Q

In an axial flow compressor, what direction is the airflow?

A

Along the horizontal axis of the compressor

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14
Q

What is the purpose of the channels (diffusers) inbetween the rotor blades?

A

They slow the airflow increasing the static pressure

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15
Q

What pressure ratio does each stage of an axial flow compressor have?

A

1.3:1

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16
Q

What happens as the air pressure increases in the compressor stages?

A

The density increases and the volume decreases

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17
Q

What is the purpose of the compressor cross-section gradually decreasing?

A

It keeps the velocity of the airflow constant

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18
Q

What are the advantages to axial flow compressors?

A
  • at optimum compressor speed, they are more efficient than centrifugal flow compressors
  • smaller frontal area which causes less drag
  • higher pressures due to more compressor stages
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19
Q

What are the advantages to centrifugal flow compressors?

A
  • smaller and cheaper to manufacture
  • lighter in weight
  • need less power to start
  • high pressure rise per stage
  • good efficiency over a wide range of speeds
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20
Q

Axial and centrifugal flow compressors have one thing in common. What is this?

A

They are very sensitive to changes in the airflow passing through the compressor due to the compressors only having one optimum operating condition for a given rotational speed, airflow and pressure ratio.

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21
Q

What is the name given to the optimum operating conditions of a compressor?

A

The design point

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22
Q

The airflow into the compressor is smooth as long as…

A

The angle of attack of the rotor blades is small

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23
Q

What is the purpose of the stator vanes?

A

They decelerate the airflow and guide it to the next rotor stage of the compressor

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24
Q

What is a compressor stall?

A

The airflow separation from the compressor blades causing a turbulent vortex

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25
Q

If a compressor stall affects all compressor stages, what is this known as?

A

A compressor surge

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26
Q

What occurs in a compressor surge?

A

Low pressure zones occur and the airflow comes to a stop and reverses in direction. The airflow collapses and builds up again in short intervals along with heavy vibrations and loud banging noises.

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27
Q

A surge in the forward part of the compressor isn’t dangerous, why is this?

A

The stall only occurs on part of the blades as they are larger

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28
Q

Why is a surge in the aft part of the compressor dangerous?

A

The airflow decreases rapidly and causes a strong reverse flow which can be from the combustion chamber to the inlet.

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29
Q

What are reasons for a compressor surge?

A
  • dents or ice on inlet
  • damaged rotor or stator vanes
  • dirty compressor blades/combustion components
  • damaged combustion components
  • damaged turbine components
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30
Q

What are the two operational reasons a compressor can stall or surge?

A
  • speed of the engine is too low below the design speed

- incorrect acceleration or deceleration

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31
Q

The rotor speed of the HP compressor decreases faster than….

A

The rotor speed of the LP compressor. This means the LP compressor can supply too much air which cannot pass through the HP compressor

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32
Q

What happens if the engine is accelerated too quickly?

A

Excess fuel in the combustion chamber increases the back pressure and can stall the compressor

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33
Q

What are the 3 methods used to prevent compressor stall and surge?

A
  • VSVs
  • Compressor bleed valves
  • Double/Triple spool engine’s
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34
Q

Where are compressor bleed valves usually found?

A

In the mid and sometimes the aft parts of the compressor

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35
Q

When are the compressor bleed valves open?

A

They open at low speeds so that excessive airflow can escape

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36
Q

What is the disadvantage of using compressor bleed valves?

A

They reduce efficiency

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37
Q

What is the advantage of double and triple spool engines?

A

It reduces the weight of the shafts and therefore the rotors can be speed-matched more easily.
This means they have excellent acceleration capabilities

38
Q

What are is the most effective way of preventing compressor stall?

A

Variable Stator Vanes

39
Q

What is the disadvantage of VSVs?

A

They have a complicated control mechanism

40
Q

How are major compressor modules usually built up of?

A

They usually are built up from sub-modules

41
Q

What is the spinner cone also known as?

A

The nose cone or compressor inlet cone

42
Q

What is the low pressure compressor rotor also known as?

A

The booster rotor

43
Q

What two designs are spinner cones usually?

A
  • sharp tip

- curved tip

44
Q

What ice protection measures can the spinner cone have?

A
  • heated nose cone which removes ice

- soft tip that causes imbalance and removes ice

45
Q

What are the spirals or white markings on the spinner cone used for?

A

To scare birds away

46
Q

Where is the spinner cone always attached?

A

To the front face of the fan disc

47
Q

Where are balanced weights attached?

A

To the mounting bolts of the spinner cone or in threaded inserts at outer surface of the spinner cone

48
Q

What is the fan rotor disc usually made from?

A

Titanium alloy

49
Q

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

A

The dovetailed fitting

50
Q

Why is a loose fit required for the fan blades?

A

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

51
Q

What do long fan blades have to keep the fan blade deflection within safe limits?

A

Shrouds - dampening blade fluttering into friction

52
Q

What is the coating for on the fan blade shrouds?

A

Is ensures the blades can easily slide on each other

53
Q

What are the advantages of modern wide chord fan blades?

A
  • more efficient than conventional blades (main advantage)
  • can be run at lower speeds (reducing noise)
  • flexible (absorbing impact damage better than older blades)
54
Q

What type of seal is located inbetween the dovetail slots of the fan blades?

A

A labyrinth seal

55
Q

What are the shroud segments filled with inside the stator assembly?

A

They are filled with an abradable material for the knife edges to cut in. This improves the efficiency of the compressor

56
Q

What are the two construction methods for the compressor stator case?

A
  • split stator case (VSVs fitted to shrouds and matched during assembly)
  • closed stator case (VSVs installed in two stator halves)
57
Q

What are flanges for on the fan case assembly?

A

They help stiffen the case and prevent deformation

58
Q

What are the lining elements of the fan case also known as?

A

Acoustic panels

59
Q

What are the holes for on the fan case acoustic lining?

A

They reduce noise created by the fan

60
Q

What is important to remember about the outer surface of the acoustic lining?

A

It is very thin to save weight and can be easily damaged by FOD, tools or incorrect maintenance

61
Q

There is a shrouded area on the fan case instead of acoustic panels. What is this for?

A

It is known as the abradable shroud - the tips of the blades can cut into this for maximum efficiency and smallest tip clearance

62
Q

What is located opposite the abradable shroud on the outer wall of the fan case?

A

Fan blade containment - usually made of honeycomb construction and covered by layers of Kevlar

63
Q

What are the outlet guide vanes (located in the fan duct) made from?

A

Aluminium or composite materials

64
Q

What is the fan frame?

A

It is the next main component behind the fan module between the LPC and the HPC

65
Q

What is the centre hub of the fan frame used for?

A
  • It supports the forward bearings of the engine rotors

- It houses the internal gearbox on the forward end of the N2 rotor shaft

66
Q

What would you find on the aft side of the fan frame?

A

The forward engine mount

67
Q

What is located on the outer surface of the fan frame?

A

Oil tanks, ignition exciters and computers

68
Q

What are the fan frame struts for?

A

They carry the loads from the engine bearings to the engine mount. They are always hollow to save weight.

69
Q

What do the centrifugal forces for balanced rotors depend on?

A
  • rotor speed
  • blade mass
  • the distance of the CofG from the rotational axis
70
Q

What is the moment weight?

A

The product of the fan blade mass and the distance of the CofG from the rotational axis

71
Q

Why do fan blades have a different moment weight even if they have the same mass?

A

The CofG of each blade is different due to manufacturing tolerances

72
Q

Where can the moment weight of each fan blade be found?

A

At the root of each blade. The information found:

  • Part Number
  • Serial Number
  • Moment weight
73
Q

If the moment weight of a fan blade changes because of blending, what happens to the moment weight information?

A

It is crossed out and the new moment weight is marked on the root next to it

74
Q

How can fan blade No.1 be identified?

A

An index mark on the fan disc or by an offset hole

The rest of the blades are usually numbered in direction of rotation

75
Q

If a fan blade needs to be replaced, what two methods are proposed by the engine manufacturers?

A
  • replacement with a blade with a moment weight as close as possible to the old one (preferred)
  • balancing weights can be used
76
Q

Where are balancing weights installed?

A

On the fan disc or the spinner cone

77
Q

What is the compressor outlet case (the case between the last compressor stage and the combustion section) also known as?

A

The diffuser case - because the gas path through the case is a divergent duct

78
Q

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

A

In the outlet case of the HP compressor (which is in between the compressor and the combustion chamber)

79
Q

Modern high pressure compressors contain how many stages?

A

Between 9 and 14 stages

80
Q

What is the purpose of the labyrinth seal?

A

It prevents reverse airflow through the gap between the rotor and the stator.

81
Q

Where is a CDP seal found?

A

It’s located behind the last compressor stage

82
Q

What is the purpose of the CDP seal?

A

It prevents compressor discharge airflow from escaping into the bearing area

83
Q

What do the retainers that hold the fan blades also do?

A

They prevent axial movement

84
Q

On small compressor blades the discs have circumferential dovetail slots. What do each of these slots also have?

A

A loading slot and two locking slots

85
Q

How are the compressor blades locked in place?

A

By a locking lug when all the blades from the first stage have been inserted

86
Q

What type of blade fitting is usually used for the longer blades at the first compressor stages?

A

Bolts inserted through the disc and into the blade root

87
Q

How are stator vanes usually installed?

A

Circumferential dovetail slots

88
Q

What do the variable stator vanes have installed at each end?

A

Trunnions

89
Q

How are all VSVs moved simultaneously?

A

Via the actuator ring

90
Q

How is bleed air supplied to the cooling or pneumatic system?

A

Through hollow stator vanes or through holes in the outer platform of the stator vanes. Bleed pickup holes are located around the whole compressor case

91
Q

Where does the bleed air from the pickup holes go to?

A

The bleed manifold supplying consumers

Module 15 Richard (18 decks)

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