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Module 15 > 15.4 > Flashcards

15.4 Flashcards

1
Q

What is the main role for a compressor section of a GTE?

A

To supply sufficient quantity of air to satisfy 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

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3
Q

What is the location of the bleed ports dependant on?

A

The pressure and temperature of the required systems

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4
Q

Two main components of a compressor?

A

Stator and Rotor

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5
Q

What do the rotors do and why?

A

They contain rotor blades that suck in air and push it to the outlet side of the rotor to increase the energy of the airflow

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6
Q

What is the name given to the blades on a stator?

A

Stator vanes

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7
Q

What do stator vanes do?

A

Guide and slow the airflow which increases static pressure

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8
Q

What is the combination of a rotor and a stator called?

A

Compressor stage

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9
Q

What is always the first part of the compressor stage?

A

Rotor

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10
Q

What is an inlet guide vane?

A

A set of stator vanes infront of compressor stage

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11
Q

What do the IGV’s do?

A

Improve airflow into the first compressor stage

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12
Q

Are IGV’s fixed or moveable?

A

Both

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13
Q

What are the stator vanes in the fan duct called?

A

Outlet guide vanes or OGV’S

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14
Q

What do OGV’S do?

A

Improve airflow into the fan nozzle

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15
Q

Two types of compressors used in gas turbine engines?

A

Centrifugal and Axial

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16
Q

What does the centrifugal flow compressor consist of?

A

Impeller (Rotor)
Diffuser (Stator)
Compressor manifold

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17
Q

What is the ratio of centrifugal compressors and what does this say about it?

A

8:1 so a High-pressure rise per stage

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18
Q

How many stages are centrifugal compressors limited to and why?

A

2 due to concerns in efficiency

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19
Q

What is the impeller made out of?

A

Aluminium alloy

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20
Q

How many entries can the impeller have?

A

One or two

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21
Q

Because an impeller has a smaller diameter, what does this mean for the rotational speed?

A

It must be higher to ensure sufficient airflow

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22
Q

What does the single entry impeller ensure and what comes after the eye?

A

Direct ducting through the eye to inducer vanes (easier than the double entry which is more complicated)

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23
Q

How do axial compressors compress air and why is this good?

A

They compress the air in its direction of flow which reduces energy lost due to turns

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24
Q

Where are axial flow compressors found?

A

turbofan engines

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25
Q

What does the axial flow compressor consist of?

A

One or more rotating aerofoil blades

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26
Q

Where are centrifugal flow compressors used?

A

Turboprop and turboshaft

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27
Q

Why do you need multiple stages in a compressor?

A

Because the energy output (pressure increase) from a single stage is small

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28
Q

Where are the stationary vanes (stators) mounted in an axial flow compressor?

A

To the compressor casing

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28
Q

What does a stage refer to?

A

A rotor-stator pair

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29
Q

What duct is included in the axial compressor?

A

Convergent

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30
Q

Why is there a gradual decrease in the diameter of the rotating drum and casing? (Convergent)

A

To maintain a steady axial velocity as pressure and density increase

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31
Q

What are multi-spool compressors driven by?

A

An independent turbine

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32
Q

What happens to the remainder of the flow in axial compressors?

A

It is bypassed and mixes in the exhaust

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33
Q

What does the fan act as?

A

A Low pressure compression stage

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34
Q

What is core flow/hot air?

A

The smaller proportion of flow

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35
Q

Where is core flow passed through?

A

The intermediate pressure and high pressure compressor

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36
Q

What is the compression ratio of axial flow?

A

Between 1:1 and 1:2

36
Q

Why is the rate of pressure increase per stage purposelly low?

A

Because you don’t want air to move to quick so that it losses efficiency and stalls

37
Q

How do you achieve a high compression rate?

A

Increasing stages

38
Q

How many stages does the fan consist of?

A

One

39
Q

Advantages of cenrtigiugal compressors?

A

High pressure rise per stage
Efficiency over wide rotational speed range
Simple
Low cost
Low starting power

40
Q

Disadvantages of centrifugal compressors?

A

Large frontal area
Losses in turns between stages

41
Q

Advantages of axial flow compressors?

A

High peak efficiency
Smaller frontal area
Straight through flow allowing high RAM efficiency
Increase pressure rise by increasing stages with negligible losses

42
Q

Axial flow compressors disadvantages?

A

Only efficient at slight tolerances (narrow window)
High cost
Difficult to manufacture
Relatively high weight
High starting power (partially overcome by split compressors)

43
Q

What is beneficial about balancing fan blades?

A

They can be balanced without dissassembly of the engine

44
Q

What needs to have identical centrifugal forces on an engine to prevent vibrating rotors?

A

The fan blades

45
Q

What three parameters do centrifugal forces depend on?

A

Rotor speed
Blade mass
Distance of CoG from fan blade to rotational axis

46
Q

What is the moment weight?

A

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

47
Q

What is the moment weight measured by?

A

A special blade scale

48
Q

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

A

The CofG is different

49
Q

What can the moment weight be given in?

A

Gram inches
cm.g
Ounce inches

50
Q

Where can the moment weight, serial and part numbers be found?

A

The root

51
Q

What matches the blades on the fan disc using moment weight

A

Computerised balancing

52
Q

What is the best method to change a single fan blade?

A

To replace the blade with a new blade with a moment weight as close to the old one as possible

53
Q

What can be used if the difference in moment weight is larger than specified by the manufacturer?

A

Balance weights can be used

54
Q

Where are the balance weights installed?

A

On the fan disc or in the spinner cone

55
Q

What is another method for replacing a single fan blade?

A

To change a pair with similar moment weights even if one blade is damaged

56
Q

What are axial and centrifugal compressors sensitive to?

A

Changes in airflow passing through compressor

57
Q

What is the resultant rotor inlet velocity?

A

The correct relationship between the inlet velocity and the rotor speed which ensures smooth airflow

58
Q

The airflow to the rotor is smooth as long as what?

A

The AOA to the rotor blades is small

59
Q

What do stator vanes do? Also to speed?

A

Guide the airflow to the next rotor stage and decelerates

60
Q

The airflow in the axial compressor is smooth as long as what?

A

The AOA to the rotor blades and stator vanes is small

61
Q

What happens to the angle of attack when the inlet air velocity decreases?

A

Angle of attack increases

62
Q

What happens to the angle of attack when the inlet air velocity increases?

A

Angle of attack decreases

63
Q

What happens when the AOA increases? What is this known as?

A

Turbulent vortex caused because of airflow separation (Compressor stall)

64
Q

What does a compressor stall cause air to do?

A

Airflow to slow down, stop or reverse direction

65
Q

What is a compressor surge?

A

A severe form of compressor stall where airflow reverses, stops or slows

66
Q

How are pressure surges formed?

A

Rapid decrease of airflow
Blockage in airflow
Low pressure zones formed which reverses airflow

67
Q

Where do surges occur?

A

On the forward or aft compressor stages

68
Q

What do surges in the forward stages usually affect?

A

Only a part of the compressor blade

69
Q

When do surges develop very rapidly over a large part of compressor?

A

In the aft stage

70
Q

What are the two main reasons for a compressor stall and surge?

A

Engine is too far below design speed
Incorrect acceleration or deceleration

71
Q

What are the three design methods to prevent compressor stall and surge?

A

Bleed valves
Dual or triple spool rotors instead of single spool
Variable compressor stator vanes

72
Q

Where are bleed valves found?

A

Mid section or aft of compressor

73
Q

When are bleed valves open?

A

During low speeds so excessive airflow can escape overboard

74
Q

On a dual spool engine, the forward part is named as what?

A

N1 or Low pressure compressor

75
Q

On a dual spool engine, the aft part is named as what?

A

N2 or the High pressure compressor

76
Q

Is each compressor rotor joined or does it use a separate turbine?

A

Separate turbine

77
Q

What is the most effective method to prevent stall?

A

Variable stator vanes

78
Q

What axis do variable stator vanes move along?

A

The vertical axis

79
Q

Where can variable stator vanes/variable inet guide vanes be found?

A

Forward stage of High pressure compressor

80
Q

What is an advantage of variable stator vanes?

A

Optimum angle of attack on rotor blades can be reached for every engine speed

81
Q

Disadvantage of variable stator vane?

A

Needs complicated control mechanism.

82
Q

What does the compression ratio represent?

A

Ratio of volume of combustion chamber from largest and smallest capacity

83
Q

What remains constant from the front to the rear compressor?

A

Axial velocity of gas

84
Q

What is compression ratio not proportional to?

A

RPM but does vary with it

85
Q

Each stage of an axial flow compressor has a ratio of what?

A

1.3:1

86
Q

Because the pressure increase at each stage is small, what needs to be in place to raise the pressure for efficient combustion?

A

Multiple stages

87
Q
A
88
Q
A

Module 15 (18 decks)

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