15.4 Compressors

Question 1

What is the primary function of the compressor?

Answer 1

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

Question 2

What is the secondary purpose of the compressor?

Answer 2

To supply bleed air from operation.

Question 3

What are the 2 main components of the compressor stage ?

Answer 3

Stators and rotors.

Question 4

What is the purpose of the rotor blades?

Answer 4

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

Question 5

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

Answer 5

V increases due to divergent duct.
P increases.

Question 6

What effect do stators have on velocity and pressure?

Answer 6

V decreases.
P increases.

Question 7

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

Answer 7

IGVs.

Question 8

What is the purpose of IGVs?

Answer 8

They increase airflow into the first compressor stage.

Question 9

What are the stator vanes in the fan duct called?

Answer 9

Outlet guide vanes.

Question 10

What is the purpose of the outlet guide vanes?

Answer 10

They improve airflow into the fan nozzle.

Question 11

What is the purpose of the exit vanes?

Answer 11

To discharge compressor discharge pressure.

Question 12

Where are nozzle guide vane found?

Answer 12

At the rear of the combustion chamber.

Question 13

What are the 2 compressor types?

Answer 13

Axial flow and centrifugal flow.

Question 14

What does a centrifugal compressor consist of?

Answer 14

Impeller (rotor)
Diffuser (stator)

Question 15

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

Answer 15

8:1

Question 16

How many stages can a centrifugal compressor have?

Answer 16

Limited to 2 due to efficiency.

Question 17

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

Answer 17

To accelerate air to the diffuser at high operation speeds.

Question 18

On what engines is the centrifugal compressor used?

Answer 18

Turboprop and turboshaft.

Question 19

Describe the blades of an axial flow compressor?

Answer 19

Rotating aero foil shaped blades.

Question 20

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

Answer 20

Stators mounted to the casing.

Question 21

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

Answer 21

To maintain a steady axial velocity.

Question 22

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

Answer 22

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

Question 23

What occurs to bypassed air in an axial flow compressor?

Answer 23

It is merged with the core flow in the exhaust.

Question 24

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

Answer 24

It is a driving mechanism of compression.

Question 25

How do the stators contribute to diffusion?

Answer 25

By correcting the angle of flow between rotors.

Question 26

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

Answer 26

By a large number of stages.

Question 27

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

Answer 27

Small. 1:1 to 1:2. To minimise losses due to flow separation and stall.

Question 28

What are the advantage of a centrifugal flow compressor?

Answer 28

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

Question 29

What are the disadvantages of a centrifugal flow compressor?

Answer 29

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

Question 30

What are the advantages of an axial flow compressor?

Answer 30

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.

Question 31

What are the disadvantages of an axial flow compressor?

Answer 31

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

Question 32

When fan balancing what does the centrifugal force depend on ?

Answer 32

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

Question 33

What is the moment weight?

Answer 33

Product of blade mass and distance of CG

Question 34

Where can the moment weight of a blade be found?

Answer 34

At the blade root.

Question 35

How are blades distributed?

Answer 35

The individual blade moments are matched by computerised balancing.

Question 36

How is blade 1 identified?

Answer 36

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

Question 37

What are the 2 methods of changing a fan blade?

Answer 37

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.

Question 38

When does normal flow in an axial compressor occur?

Answer 38

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

Question 39

What effect does rotor speed have on the AOA?

Answer 39

Rotor speed increases = AOA increases. Rotor speed decreases = AOA decreases. Directly proportional.

Question 40

What effect does aircraft forward speed have on the AOA?

Answer 40

Fwd speed increases = AOA decreases. Fwd speed decreases = AOA increases. Inversely proportional.

Question 41

What is airflow separation also called?

Answer 41

Compressor stall.

Question 42

What is compressor surge?

Answer 42

A severe form of compressor stall.

Question 43

What does compressor stall/ surge cause?

Answer 43

A reverse in airflow direction.

Question 44

What causes a compressor stall or surge?

Answer 44

Dents. Ice. Dirty compressor.

Question 45

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

Answer 45

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

Question 46

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

Answer 46

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

Question 47

How do bleed valve work?

Answer 47

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

Question 48

Where are bleed valves found?

Answer 48

In the mid section or aft section of the compressor.

Question 49

What is the most effective stall prevention method?

Answer 49

Using variable IGVs/ variable stator vanes.

Question 50

Where are variable IGVs found?

Answer 50

In the fwd stage of the HP compressor.

Question 51

What is the advantage of variable IGVs?

Answer 51

The optimum AOA can be reached at every engine speed.

Question 52

What is the disadvantage of the variable IGVs?

Answer 52

Very complicated control mechanism.

Question 53

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

Answer 53

1:3:1

Question 54

What is the purpose of many stages in a compressor?

Answer 54

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