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M15 > 15.4 Compressors > Flashcards

15.4 Compressors Flashcards

1
Q

What is the primary function of the compressor?

A

To increase static pressure and discharge it to the combustion chamber.

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2
Q

What is the secondary purpose of the compressor?

A

To supply bleed air from operation.

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3
Q

What are the 2 main components of the compressor stage ?

A

Stators and rotors.

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4
Q

What is the purpose of the rotor blades?

A

To suck in air and push it to the outlet side.

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5
Q

What effect do rotors have on velocity and pressure in a compressor?

A

V increases due to divergent duct.
P increases.

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6
Q

What effect do stators have on velocity and pressure?

A

V decreases.
P increases.

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7
Q

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

A

IGVs.

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8
Q

What is the purpose of IGVs?

A

They increase airflow into the first compressor stage.

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9
Q

What are the stator vanes in the fan duct called?

A

Outlet guide vanes.

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10
Q

What is the purpose of the outlet guide vanes?

A

They improve airflow into the fan nozzle.

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11
Q

What is the purpose of the exit vanes?

A

To discharge compressor discharge pressure.

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12
Q

Where are nozzle guide vane found?

A

At the rear of the combustion chamber.

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13
Q

What are the 2 compressor types?

A

Axial flow and centrifugal flow.

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14
Q

What does a centrifugal compressor consist of?

A

Impeller (rotor)
Diffuser (stator)

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15
Q

In a centrifugal compressor what is the high pressure rise per stage?

A

8:1

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16
Q

How many stages can a centrifugal compressor have?

A

Limited to 2 due to efficiency.

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17
Q

What is the purpose of the impeller in a centrifugal compressor?

A

To accelerate air to the diffuser at high operation speeds.

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18
Q

On what engines is the centrifugal compressor used?

A

Turboprop and turboshaft.

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19
Q

Describe the blades of an axial flow compressor?

A

Rotating aero foil shaped blades.

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20
Q

What is located in between the blades in an axial flow compressor?

A

Stators mounted to the casing.

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21
Q

Why is there a gradual decrease in diameter in an axial flow compressor?

A

To maintain a steady axial velocity.

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22
Q

How does a multi spool axial flow compressor differ from a single spool?

A

There are 2 or 3 rotor drums, each driven by a separate turbine segment.

23
Q

What occurs to bypassed air in an axial flow compressor?

A

It is merged with the core flow in the exhaust.

24
Q

What is the purpose of diffusion in an axial flow compressor?

A

It is a driving mechanism of compression.

25
Q

How do the stators contribute to diffusion?

A

By correcting the angle of flow between rotors.

26
Q

How is a high compression rate achieved in an axial flow compressor?

A

By a large number of stages.

27
Q

What is the compression ration in an axial flow compressor and why?

A

Small. 1:1 to 1:2.

To minimise losses due to flow separation and stall.

28
Q

What are the advantage of a centrifugal flow compressor?

A

High pressure rise per stage.
Efficient over a wide rotational speed.
Simple to manufacture and low cost.
Low weight.
Low starting power requirement.

29
Q

What are the disadvantages of a centrifugal flow compressor?

A

Large frontal area for given airflow.
Losses in turns between stages.

30
Q

What are the advantages of an axial flow compressor?

A

High peak efficiencies.
Small frontal area for given airflow.
Straight through flow allows for high RAM efficiency.
Increases pressure by increasing stages with negligible loss.

31
Q

What are the disadvantages of an axial flow compressor?

A

Good efficiencies over only a narrow rotational speed.
Difficult to manufacture and high cost.
High weight.
High starting power required.

32
Q

When fan balancing what does the centrifugal force depend on ?

A

Rotor speed.
Blade mass.
Distance of CG of fan blade from rotational axis.

33
Q

What is the moment weight?

A

Product of blade mass and distance of CG

34
Q

Where can the moment weight of a blade be found?

A

At the blade root.

35
Q

How are blades distributed?

A

The individual blade moments are matched by computerised balancing.

36
Q

How is blade 1 identified?

A

By an index mark on the fan disc or an offset hole.

37
Q

What are the 2 methods of changing a fan blade?

A

Replace with a new blade that has a moment weight as close as possible to old one.

Change pair of blades with similar moment weights.

38
Q

When does normal flow in an axial compressor occur?

A

At only 1 optimum operating condition for pressure ratio, rotational speed and airflow.

39
Q

What effect does rotor speed have on the AOA?

A

Rotor speed increases = AOA increases.

Rotor speed decreases = AOA decreases.

Directly proportional.

40
Q

What effect does aircraft forward speed have on the AOA?

A

Fwd speed increases = AOA decreases.

Fwd speed decreases = AOA increases.

Inversely proportional.

41
Q

What is airflow separation also called?

A

Compressor stall.

42
Q

What is compressor surge?

A

A severe form of compressor stall.

43
Q

What does compressor stall/ surge cause?

A

A reverse in airflow direction.

44
Q

What causes a compressor stall or surge?

A

Dents.
Ice.
Dirty compressor.

45
Q

What are the 2 operational reasons for a stall or surge?

A

If engine speed is too far below design speed.
Incorrect acceleration or deceleration.

46
Q

What are the 3 methods to prevent a stall or surge?

A

Using compressor bleed valves.
Dual or triple spool rotors.
Variable compressor stator vanes.

47
Q

How do bleed valve work?

A

They open at low engine speed so excessive airflow from fwd stage can escape.

48
Q

Where are bleed valves found?

A

In the mid section or aft section of the compressor.

49
Q

What is the most effective stall prevention method?

A

Using variable IGVs/ variable stator vanes.

50
Q

Where are variable IGVs found?

A

In the fwd stage of the HP compressor.

51
Q

What is the advantage of variable IGVs?

A

The optimum AOA can be reached at every engine speed.

52
Q

What is the disadvantage of the variable IGVs?

A

Very complicated control mechanism.

53
Q

What is the axial flow compressor pressure ratio at every stage ?

A

1:3:1

54
Q

What is the purpose of many stages in a compressor?

A

To raise static pressure to a high value for efficient combustion.

M15 (18 decks)

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