15.4 Compressors

Question 1

What is the primary function of a compressor?

Answer 1

to supply a sufficient quantity of air to satisfy the requirements of the combustion chamber.

Question 2

What is the secondary function of a compressor?

Answer 2

to supply air to the systems that require bleed air for operation.

Question 3

What is the compressors two main components?

Answer 3

Compressor rotor

Compressor stator

Question 4

How does the rotor increase the energy of airflow?

Answer 4

By the action of the rotor blades sucking in air and push it to the outlet side of the rotor as it rotates in the compressor

Question 5

What are the blades on a stator called?

Answer 5

Stator vanes

Question 6

What type of component is a stator?

Answer 6

Fixed component

Question 7

What do stator vanes do?

Answer 7

increase the pressure of air by slowing the airflow

Question 8

What is the combo of a rotor and stator called?

Answer 8

A compressor stage

Question 9

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

Answer 9

IGVs

Question 10

Are IGVs fixed or moveable?

Answer 10

fixed or movable.

Question 11

What are the stator vanes in a fan duct called and what do they do?

Answer 11

Outlet Guide Vanes
improve the airflow into the fan nozzle.

Question 12

What are the two types of compressors used in GTEs?

Answer 12

The centrifugal flow compressor
The axial flow compressor

Question 13

What does a centrifugal flow compressor consist of?

Answer 13

Impeller (rotor)
Diffuser (stator)
Compressor manifold

Question 14

What is the energy rise per stage in a centrifugal compressor?

Answer 14

8:1
So high

Question 15

How many stages can a centrifugal compressor be?

Answer 15

Limited to 2 for efficency reasons

Question 16

What is the impeller made from? Designed for what?

Answer 16

Forged aluminium alloy
Minimum flow restriction

Question 17

What is the impellers function?

Answer 17

pick up and accelerate the air outwardly to the diffuser.

Question 18

What are the two types of impeller?

Answer 18

Single entry
Double entry

Question 19

What is the advantage of a single entry impeller?

Answer 19

Permits ducting directly to the impeller eye (inducer vanes)
Double entry ducting is more complicated

Question 20

How does axial flow compressor reduce energy loss?

Answer 20

Compresses the air in its original direction of flow, so no energy used on turning it.

Question 21

Where are axial compressors used?
Where are centrifugal compressors used?

Answer 21

Axial: Turbofan
Centrifugal: turboshaft or turboprop

Question 22

Whats the structure of an axial flow compressor?

Answer 22

Stationary vanes (stator) mounted to the compressor casing between the rotating blades (rotor)

Question 23

Axial flow compressor: Why is there a gradual decrease in the diameter of the rotating drum and the casing?

Answer 23

to maintain a steady axial velocity, as the pressure and density of the air increase.

decrease in diameter: increases the velocity

Question 24

What does a single spool consist of?

Answer 24

A single drum and multiple stators

Question 25

How are the two or three drums of rotors driven in a multi- spool compressor?

Answer 25

Each driven by a separate turbine segment at the optimum respective rotational speed

Question 26

What does the fan act as in triple- spool engine?

Answer 26

As a LP compression stage

Question 27

What do stators also do in axial compressors?

Answer 27

Correct the angle of flow between rotors?

Question 28

What is the compression ratio in axial compressor?

Answer 28

small, between 1:1 1:2 and 1:3 (1:3 mentioned in 15.5)

Question 29

Why is the compression ratio kept small in axial compressors?

Answer 29

Amount of diffusion and angle of deflection per stage is kept small to minimise flow separation and stall Adding more stages makes designing more difficult

Question 30

Why is it better to have fans at the front of an engine?

Answer 30

Safer to have such a large component in the cold part of the engine

Question 31

What are centrifugal compressors advantages:

Answer 31

- high pressure rise per stage - efficient over a wide rotational speed range - simplicity of manufacture - low weight - low starting power

Question 32

Centrifugal compressor disadvantages:

Answer 32

- Large frontal area for a given airflow - Losses in the turns between stages

Question 33

What gives an axial compressor high ram efficiency?

Answer 33

It's straight through flow

Question 34

Axial compressors efficiency:

Answer 34

High peak efficiencies, but only over a narrow rotational speed range

Question 35

Axial compressor disadvantages:

Answer 35

Difficulty of manufacture and high cost Relatively high weight High starting power requirements; (partially overcome by split compressors)

Question 36

Does the engine have to be disassembled for fan blade balancing?

Answer 36

no

Question 37

Why do high bypass fans require more attention on their balance during service?

Answer 37

because of their large diameter and high rotational forces.

Question 38

What is needed for no vibrations on the rotor of an engine?

Answer 38

fan blades that are opposite each other must have identical centrifugal forces.

Question 39

What does the centrifugal force of a blade depend on?

Answer 39

The rotor speed The blade mass The distance of the centre of gravity of the fan blade from the rotational axis

Question 40

What is a blades moment weight?

Answer 40

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

Question 41

Why do blades with the same mass have different moment weights?

Answer 41

the centre of gravity of each fan blade is different due to manufacturing tolerances

Question 42

What is moment weight given in?

Answer 42

gram inches cm.g ounce inches

Question 43

What is find on the blade root?

Answer 43

- part number - serial number - moment weight

Question 44

How are blades matched?

Answer 44

Computerized balancing

Question 45

How is fan blade no.1 identified?

Answer 45

an index mark on the fan disc or by an offset hole

Question 46

What are the two methods for a fan blade change:

Answer 46

Preferred method: replace the blade with a new blade that has a moment weight as close as possible to the old one. Other: change the pair of blades