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Resource Group Mod 15 2024 > 15.4 Compressor > Flashcards

15.4 Compressor Flashcards

1
Q

What is the primary function of the compressor?

A

To supply a sufficient quantity of air to satisfy the requirements of the combustion chamber.

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2
Q

What is the secondary function of the compressor?

A

To supply air to the systems that require bleed air for operation.

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3
Q

What are the two main components of a compressor?

A
  • Rotors.
  • Stators.
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4
Q

What’s the purpose of the Rotors?

A

To suck in air and push it to the outlet side of the rotor. This action increases the energy of the airflow.

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5
Q

What’s the purpose of the Stators/Stator Vanes?

A

The stator vanes guide and slow the airflow to cause an increase in pressure.

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6
Q

What is classed as a single stage of the compressor?

A

One pair of stators and rotors.

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7
Q

What component is always the first part of a compressor stage ?

A

The Rotor.

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8
Q

What is the name of stator vanes found just in front of the compressor inlet?

A

Inlet guide vanes (IGVs).

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9
Q

What’s the purpose of IGVs?

A

Improve the airflow into the first compressor stage.

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10
Q

Are IGVs fixed, or moveable?

A

Can be EITHER fixed or moveable!

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11
Q

What are Oulet guide vanes (OGVs) and where are they located?

A

Located in the fan duct, and improve airflow into the fan nozzle.

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12
Q

What is the purpose of the compressor inlet and outlet cases?

A
  • To give support for the compressor rotor bearings.
  • Improve and straighten the airflow into and out of the compressor.
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13
Q

What are the two types of compressor used in GTEs?

A
  • Centrifugal flow.
  • Axial Flow.
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14
Q

What are the main components of a Centrifugal flow compressor? (3)

A
  • Impeller (rotor)
  • Diffuser (stator)
  • Compresssor manifold.
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15
Q

What is the pressure rise ratio per stage in a Centrifugal compressor? (It’s classed as high).

A

8:1.

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16
Q

How many stages are Centrifugal compressors limited to, and why?

A

2 stages, due to efficiency concerns.

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17
Q

What material is the impeller usually made from?

A

Forged aluminium alloy, heat treated, machined, and smoothed for minimum flow restriction and turbulence.

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18
Q

What is the function of the impeller?

A

To pick up and accelerate the air outwardly to the diffuser.

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19
Q

What are the two types of impeller?

A
  • Single entry.
  • Double entry.
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20
Q

What are the features of a single entry impeller?

A
  • More efficient.
  • Simpler and more convenient ducting direct to diffuser.
  • But a larger overall diameter of the engine is required.
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21
Q

What are the features of a double entry impeller?

A
  • Smaller diameter.
  • Higher rotational speed.
  • More complex ducting design through the impeller to its rear side.
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22
Q

What’s a unique feature of the double entry impeller?

A

It has a plenum chamber included in the ducting.

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23
Q

What’s the purpose of the plenum chamber in the double entry impeller?

A

The air must enter the engine at almost right angles to the engine axis. The plenum chamber creates this effect.

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24
Q

How is positive flow into the compressor inlet achieved? (Regarding pressure).

A

Postive pressure surrounding the compressor.

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25
Q

How do ‘Blow-in doors’ within the plenum chamber assist with positive pressure?

A

The doors open automatically whenever engine compartment pressure drops below atmospheric pressure.

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26
Q

How are the Blow-in doors held shut when the engine is not operating?

A

Via a spring.

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27
Q

What also assists in holding the blow-in doors shut? (During T/O and flight).

A

Ram air.

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27
Q

What is the design and function of the diffuser within a centrifugal compressor?

A

An annular chamber with vanes that create divergent passages into the compressor manifold, from the impeller.

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28
Q

What is the purpose of the Compressor Manifold?

A

Diverts flow of air from the diffuser into the combustion chamber, with one outlet duct per chamber for even distribution.

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29
Q

What important action do the manifold outlet ducts peform?

A

Turn the radial airflow into axial airflow for the combustion chamber.

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30
Q

What assists the outlet ducts in turning the airflow from radial to axial?

A

Cascade (turning) vanes. Present a smooth turning face.

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31
Q

In what engine-type are centrifugal compressors most common?

A

Turboprop and turboshaft engines.

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32
Q

What are the main design features of an axial flow compressor?

A

Multi-stage, consisting of rotors that rotate around fixed stator vanes, that are fixed to the compressor case.

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33
Q

Why do axial flow compressors gradually diverge (reduce in diameter)?

A

To maintain a constant axial flow velocity, as pressure and density increase.

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34
Q

What is a single spool engine?

A

A single compressor drum with multiple stators, driven by a seperate turbine.

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35
Q

What is a twin spool engine?

A
  • A LP compressor (fan) driven by an LP turbine (N1).
  • And a HP compressor driven by a HP turbine (N2).
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36
Q

What is a triple/multispool engine?

A
  • A LP compressor (fan) driven by an LP turbine (N1).
  • A HP compressor driven by a HP turbine (N2).
  • And an IP compressor driven by an IP turbine (N3).
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37
Q

What is the ‘driving mechanism’ for compression?

A

The diffusion of air between the stators and rotors.

38
Q

What is the approximate compression ratio between stages of an axial flow compressor?

A

Between 1:1 and 1:2. Hence, many stages are required.

39
Q

Why do axial flow compressors use many stages of a small pressure increase?

A

Reduces the pressure losses, and increases performance and efficiency.

40
Q

Advantages of Centrifugal flow compressors?

A
  • High pressure rise per stage.
  • Efficiency over a wide rotational speed range.
  • Simplicity of manufacture and low cost.
  • Low weight.
  • Low starting power requirements.
41
Q

Disadvantages of Centrifugal flow compressors?

A
  • Its large frontal area for a given airflow.
  • Losses in turns between stages.
42
Q

Advantages of axial flow compressors?

A
  • High peak efficiencies.
  • Small frontal area for given airflow.
  • Straight through flow, allowing high ram efficiency.
  • Increased pressure rise by increasing the number of stages, with negligible losses.
43
Q

Disadvantages of axial flow compressors?

A
  • Good efficiencies over only a narrow rotational speed range.
  • Difficulty of manufacture and high cost.
  • Relatively high weight.
  • High starting power requirements (partially overcome by split compressors).
44
Q

What is a requirement of all fan blades?

A

They must be balanced.

45
Q

What force of the fan blades needs to be balanced?

A

The centrifugal force.

46
Q

What 3 parameters does the fan blade’s centrigual force depend on?

A
  • The rotor speed.
  • The blade mass.
  • The distance of the centre of gravity of the fan blade from the rotational axis.
47
Q

What is the ‘moment weight’ of a fan blade?

A

The product of the blade mass and the distance of the centre of gravity from the rotational axis.

48
Q

What are the 3 units for measuring moment weight?

A
  • gram inches.
  • ounce inches.
  • cm.g
49
Q

Where can the moment weight value of a blade be found?

A

Stamped on the root of the blade, alongside the P/N and S/N.

50
Q

What can cause a change in the moment weight value?

A

Bending. The old value will be crossed-out, and a new value stamped on.

51
Q

How are blade positions around the engine, numbered?

A

Normally in the direction of rotation of the engine. But it can also be the opposite.

52
Q

How are fan blade positions first assigned and balanced?

A

Using computerised balancing, where a computer uses weight values to determine best location around the spinner.

(Think a bit like wheel-balancing weights on a car wheel)

53
Q

What is the BEST method for replacing a fan blade?

A

Replace the blade with a new blade that has the closest possible moment weight to the old blade.

54
Q

What is the other method for replacing a fan blade?

A

Replace both the damaged blade, and the blade opposite to this blade.

Both blades should be replaced with blades of a moment weight as close as possible to the blades removed.

55
Q

What can be added to assist in fan balancing?

A
  • Balance weights, placed in the spinner or fan-disc.
  • Balance screws.
56
Q

What is the defintion of ‘Design Point’ of the compressor?

A

Optimum operating condition for the compressor where the airflow, pressure ratio, and speed are perfectly matched.

57
Q

What is the ‘Resultant Rotor Inlet Velocity’?

A

The direction of the airflow, as a result of the relationship between inlet velocity and rotor speed.

58
Q

What is important for smooth airflow into a rotor?

A

The Angle of Attack (AoA) of the airflow to the rotor blades must be small.

59
Q

What determines the direction of the airflow into the following rotor stage?

A

The stator outlet velocity and the rotor speed.

60
Q

Optimum compressor condition is only achievable at a single….?

A

Speed.

61
Q

What affects the AoA onto the rotor blades?

A
  • Chnages in rotor speed.
  • Changes in inlet velocity.
62
Q

What causes an increase in the AoA onto the rotor blades?

A

A decrease in inlet velocity.

63
Q

How does an increase in airflow AoA on the rotor blades, intiate a stall in the compressor?

A

The airflow seperates from the aerofoils in the compressor, causing turbulent flow.

64
Q

What can happen to the airflow when it stalls in a compressor stage?

A

It can slow down, stop, and even reverse direction.

65
Q

What is a compressor surge?

A

A very severe form of compressor stall.

66
Q

How is a surge generated, in 3 steps?

A

1) A rapid decrease of airflow causes stall on some blades or stages.

2) The stall causes a blockage in the airflow which leads to a stronger stall in the subsequent stages.

3) This causes low-pressure zones in which the airflow comes to a stop and reverses its direction.

67
Q

Where in the compressor can surges occur?

A

Forward or Aft stages.

68
Q

What types of compressors can experience a surge?

A

ALL types.

69
Q

What are the characteristics of a surge in the Forward section of a compressor?

A
  • The blades are larger, and thus only part of the blades are affected.
  • The effects are not as dangerous as an Aft section stall.
70
Q

What are the characteristics of a surge in the Aft section of a compressor?

A
  • Shorter compressor blades and higher pressures results in rapid stall development.
  • Rapid decrease in airflow that can reverse.
71
Q

What is the most extreme effect of a compressor surge in the Aft section?

A

Reverse flow from the combustion chamber, back to the engine inlet.

72
Q

What are the observed effects of compressor surge?

A
  • Heavy vibrations.
  • Loud banging.
73
Q

What are the effects of compressor surge on the aircraft’s performance?

A
  • Thrust decreases.
  • Engine speed fluctuates.
  • EGT increases.
74
Q

What are the causes of compressor surge and stall, in relation to the compressor components?

A
  • Dents or ice on the engine inlet cause turbulent or disrupted airflow.
  • Damaged rotor blades or stator vanes.
  • Dirty compressor blades or stator vanes.
  • Damaged or broken combustor components can cause a blockage in the airflow and decrease the velocity in the compressor.
  • Damaged turbine components .
  • Dents in the jet nozzle.
  • Broken objects in the exhaust system.
75
Q

What are the causes of compressor surge and stall, in relation to how the aircraft is operated?

A
  • Engine speed is too far below design intended speed.
  • Incorrect acceleration or deceleration.
76
Q

How do very low speeds on an axial flow compressor, potentially cause a stall?

A

The forward stages of the compressor are more efficient than the aft stages, so the air backs-up in the aft section and reverses.

77
Q

How can fast deceleration cause a compressor stall?

A

The rotor speed of the HP compressor (aft) decreases at a faster rate than the LP compressor (fwd).

(Same effect as very slow speeds, where air backs-up in the aft section).

78
Q

How can fast acceleration cause a compressor stall?

A

Results in too much fuel in the combustion chamber, causing an increase in back-pressure.

79
Q

What are the 3 methods for preventing compressor stall and surge?

A
  • Compressor bleed valves.
  • Dual/triple spool.
  • Variable compressor stator vanes.
80
Q

Which of the 3 methods is most effective at preventing stalls and surges?

A

The variable compressor stator vanes.

81
Q

Where are bleed valves found in the compressor?

A

Mainly the mid-section, but sometimes the aft section.

82
Q

How do bleed valves help prevent compressor stall?

A

They open at low speeds to allow excessive airflow from the forward stages to escape, preventing blockages in the aft section.

83
Q

What’s the disadvantage of compressor bleed valves?

A

Large efficiency losses, as some of the air is compressed and then bled off.

84
Q

How does adding dual/triple spools help prevent compressor stall?

A

The compressors are split and driven by their own turbines.

85
Q

What’s another advantage of dual and triple spool engines, unrelated to stall prevention?

A

Excellent acceleration capabillities, due to smaller mass of rotors.

86
Q

How do variable compressor IGVs/stator vanes help prevent a stall?

A

Optimal AoA on the following rotor blades can be achieved at every engine speed. (Maxium efficiency).

87
Q

Where are variable stator vanes usually located?

A

The forward stages of the HP compressor.

88
Q

What is the only axis that variable stator vanes are moveable along?

A

ONLY the vertical axis.

89
Q

What’s the disadvantage of variable stator vanes?

A

Very complex control mechanism.

90
Q

What is the approximate compressor ratio value for every stage in an axial flow compressor?

A

1.3:1

(Air enters stage at 10psi, exits stage at 13psi.

91
Q

What happens to pressure and density as air flows through the compressor stages?

A

Pressure and density both increase.

92
Q

How is a constant velocity maintained through the compressor, whilst pressure and density are increasing?

A

By gradually reducing the diameter of the compressor per stage.

Resource Group Mod 15 2024 (13 decks)

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