Module 15.4 Compressors

Question 1

What are the two main components of a compressor?

Answer 1

• Stator
• Rotor

Question 2

What are the two types of compressor?

Answer 2

• Axial flow
• Centrifugal flow

Question 3

What is the purpose of the stator vanes?

Answer 3

• Guide and slow airflow to increase pressure

Question 4

What are stator vanes in front of a compressor called?

Answer 4

Inlet Guide Vanes (IGV’s)

Question 5

Stator vanes in the fan duct are usually named what?

Answer 5

Outlet guide vanes

Question 6

How many stages are centrifugal compressors limited to due to efficiency concerns?

Answer 6

Two

Question 7

What are the two main functional components of a centrifugal compressor?

Answer 7

Impeller and diffuser

Question 8

In centrifugal compressors what can be said about the rise per stage?

Answer 8

High pressure rise

Question 9

What are the two types of impeller fan?

Answer 9

• Single entry
• Double entry

Question 10

What can be said about double entry impellers?

Answer 10

Smaller diameter but require a higher rpm

Question 11

Where are centrifugal compressors commonly used?

Answer 11

• Turboprop
• Turboshaft

Question 12

What shape are the compressor blades?

Answer 12

Aerofoil shaped

Question 13

What does a gradual decrease in diameter of the rotating drum maintain?

Answer 13

Steady axial velocity

Question 14

Two spools is much more suitable for what type of engine?

Answer 14

Dual flow

Question 15

In a 3 spool engine what does the fan also act as?

Answer 15

Low pressure compressor

Question 16

What is the purpose of the stators?

Answer 16

• Act as guide vanes
• Act as diffusers

Question 17

To achieve high compression in axial flow what is required?

Answer 17

Lots of stages

Question 18

Why is it beneficial to have a centrifugal compressor in an APU?

Answer 18

• Low start up power

Question 19

What is the compression ratio from one stage to the next in an axial compressor?

Answer 19

1.1 to 1.2 to 1.3 (1.3 mentioned in 15.5)

Question 20

Does fan blade balancing require engine disassembly?

Answer 20

No

Question 21

The centrifugal force depends on what?

Answer 21

• Rotation speed
• Mass of the object
• Distance between CoG and the rotational axis

Question 22

What is the moment weight?

Answer 22

Product of the blade’s mass and the distance of the CoG from the rotational axis

Question 23

Do blades have the same moment weight if they have the same mass?

Answer 23

No, not necessarily

Question 24

What numbers will be displayed on the root?

Answer 24

• Part no.
• Serial no.
• Moment weight

Question 25

How are the fan discs matched?

Answer 25

Computerised balancing

Question 26

No.1 fan blade position is identified by what?

Answer 26

- Index mark on fan disc - Offset hole

Question 27

What are the two methods of fan blade change?

Answer 27

- Replace blade with one of moment weight extremely close to the opposite fan blade (pair) - Replace the pair

Question 28

What effect does increasing rotor speed have on the angle of attack?

Answer 28

Increases the angle of attack

Question 29

If relative airflow speed increases what effect does this have on the AoA?

Answer 29

Decreases the AoA

Question 30

What is the AoA between in the compressor?

Answer 30

Chord line of the vane and resultant rotor inlet velocity

Question 31

A low AoA ensure what?

Answer 31

Airflow into the rotor is smooth

Question 32

What is a compressor stall?

Answer 32

Airflow separation on the vanes

Question 33

How do compressor stalls occur?

Answer 33

Rapid decrease of airflow causes stall on some blades or stages Stall causes a blockage in airflow Low pressure zones, airflow stops and reverses

Question 34

What are some reasons for stall and surge?

Answer 34

- Dents or ice - Damaged blades - Damaged or broken combustor - Damaged turbine components - Speed of engine below designed operating speed - Incorrect acceleration or deceleration

Question 35

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

Answer 35

- Compressor bleed valves - Dual or triple spool - Variable compressor stator vanes

Question 36

Where are bleed valves found?

Answer 36

Mid compressor section sometimes aft compressor section as well.

Question 37

When are engine bleed valves open?

Answer 37

Low engine speeds

Question 38

What is the most effective method of preventing compressor stall?

Answer 38

Variable stator vanes

Question 39

If the pressure ratio is 1.3:1 and the air pressure to stage 1 is 10 psi. What is the input pressure into stage 2?

Answer 39

13 psi