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Module 15 > 15.4 Compressors > Flashcards

15.4 Compressors Flashcards

1
Q

What is the compressors role?

A

Must increase pressure of the mass of air received from the air inlet duct and then discharge it to the combustion change in the quantity that is required

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2
Q

What is the secondary function of the compressors?

A

To supply sir to the systems that require bleed air to operate

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3
Q

What are the two main components of the compressor?

A

Compressor rotor, compressor stators

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4
Q

What do the rotor blades on the rotor do?

A

Suck in air and push it to the outlet side of the rotor, this action increases the energy of airflow

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5
Q

What do the compressor stator vanes do?

A

Guide and slow the airflow to cause an increase in pressure

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6
Q

What comes first rotor or stator in the compressor stage?

A

Rotor

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7
Q

What are the two types of compressors currently used in the gas turbine?

A

Centrifugal flow compressor, axial flow compressor

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8
Q

What does a centrifugal flow compressor consist of?

A

Impeller (rotor), diffuser (stator) and a compressor manifold

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9
Q

Why are centrifugal compressors limited to two stages?

A

Concerns about efficiency

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10
Q

Is the impeller single or double entry and what does it do?

A

Can be either and is to accelerate the air outwardly to the diffuser

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11
Q

How does double entry and single entry work?

A

Double has a smaller diameter but is usually operated at higher rotational speeds, single impeller permits convenient ducting directly to the impeller eye

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12
Q

What does a axial flow compressor consist of

A

An axial compressor consists of lots of aerofoil blades in lots or rows and made up of stators and rotors

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13
Q

Where are centrifugal compressors usually found?

A

Used in turboprop and turboshaft

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14
Q

What do compressors usually consist of?

A

Multiple stages

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15
Q

Why is a gradual decrease in diameter happening on the rotating drum?

A

To maintain a steady axial velocity, as the pressure of the air increases

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16
Q

What does the total number of stages depend on in a compressor?

A

Desired pressure increases

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17
Q

What are the advantages of centrifugal flow compressors?

A

High pressure rise per stage
Efficiency over a wide rotational speed range
The simplicity of manufacture and low cost
Low weight
Low starting power requirements

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18
Q

What are the disadvantages of centrifugal flow compressor?

A

Its large frontal area for a given airflow
Losses in turns between stages

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19
Q

What are the advantages of the axial flow compressor?

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

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20
Q

What are the 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)

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21
Q

What are the only part of the engine that can be balanced without disassembly?

A

Fan blades

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22
Q

What are the three parameters the centrifugal force depends on?

A

Rotor speed, blade mass, distance of the centre of gravity of the fan blade from rotational axis

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23
Q

What units can moment weight be given in ?

A
  • gram inches
  • cm,g
  • ounce inches
24
Q

Where can the moment weight usually be found ?

A

On the blade’s root

25
Q

How is vibration minimised from the fan blades ?

A

Individual moment weights are generally matched in pairs on the fan disk by computerised balancing

26
Q

How can fan blade number 1’s position be identified on a fan disc ?

A

An index mark or by an offset hole

27
Q

When replacing a fan blade, if the moment weight is too high, what should be done?

A

Balance weights can be used to equalise fan blade moment weight

28
Q

What is moment weight?

A

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

29
Q

What can be different about a fan blade even if they have the same mass?

A

Moment weight

30
Q

What numbers can be found on the blade root?

A

Part number
Serial number
Moment weight

31
Q

Why are both compressors sensitive to change in airflow?

A

Because the engine compressors have only one optimum operating condition for a given pressure ratio, rotational speed and airflow

32
Q

When replacing a fan blade if you can’t find a fan blade the same weight what should you do?

A

You should find a fan blade most similar to the weight or replace the pair fine to roughly the same weight

33
Q

If rotor speed increases what happens to angle of attack?

A

Angle of attack increases

34
Q

If rotor speed decreases what happens to angle of attack?

A

Angle of attack decreases

35
Q

If airspeed increases what happens to angle of attack?

A

Angle of attack decreases

36
Q

If airspeed decreases what happens to angle attack?

A

Angle of attack increases

37
Q

Why do you want a small angle of attack?

A

So airflow into the rotor is smooth

38
Q

What happens when airflow separation in the engine?

A

Compressor stall

39
Q

What does a compressor stall do?

A

Changes the proper airflow in the compressor causes airflow to slow down stop or even reverse its direction

40
Q

What is a compressor surge?

A

A very severe form of compressor stall

41
Q

How is a compressor surge generated?

A

Rapid decreased of airflow causes stall on some blades or stages
Stall causes a blockage in the airflow which leads to a stronger stall
Causes low pressure zones in which the airflow comes to a stop and reverses its direction

42
Q

Where can compressors surge occur?

A

Surge can happen on all engine compressors. It can occur in the forward or the aft compressor stages

43
Q

What happens on forward stage surge?

A

Surge in the forward stages usually affects only a part of the compressor blades. This is due to the bigger dimensions of the blades. The effect on the compressor operation is not very dangerous

44
Q

What happens on an aft stage surge?

A

Surge in the aft stages develops very rapidly over a large part of the compressor. This behaviour is supported by the high pressure in the aft stages and by the short compressor blades

45
Q

Why do compressor stalls and surges happen?

A

Compressor stall and surge is caused by problems in engine operation or by damaged engine components

46
Q

What primary engine components can cause compressor stall if they are damaged?

A

1)Dents or ice
2)Damaged rotor blades or stator vanes
3)Dirty compressor blades or stator vanes can cause stall
4)Damaged or broken combustor components can cause a blockage in the airflow and decrease the velocity in the compressor.
5)Damaged turbine components will decrease the airflow and dents in the jet nozzle or broken objects in the exhaust system can also cause a blockage to the airflow.

47
Q

What are the two operational reasons for compressor stall and surge?

A

The compressor can stall if the speed of the engine is too far below the design speed
It can also stall at incorrect acceleration or deceleration

48
Q

What are the three different design methods to prevent compressor stalls and surges?

A

Compressor bleed valves
Use of dual or triple spool rotors instead of single spool rotors
variable compressor stator valves

49
Q

When are engine bleed valves open?

A

They are open at low engine speeds so that the excessive airflow from the forward stages can escape overboard

50
Q

What is the most effective method to prevent stall?

A

Variable stator vanes

51
Q

What’s the main advantage of variable stator vanes?

A

At an optimum AOA the following rotor blades can be reached for every engine speed

52
Q

What is another name for the low- pressure compressor ?

A

N1 compressor

53
Q

What is another name for the high pressure compressor ?

A

N2 compressor

54
Q

How is each compressor rotor driven in a double or triple spool rotor ?

A

By a separate turbine at the exhaust section of the engine

55
Q

What is the pressure ratio of each stage or an axial flow compressor ?

A

1.3:1

56
Q

Why must many stages be used in an axial compressor?

A

One stage only raises a small amount, so many are needed for the correct pressure

Module 15 (18 decks)

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