15.4 - Compressors

Question 1

Explain the purpose of the compressor in a gas turbine engine.

(primary secondary and other)

Answer 1

The primary purpose of a compressor is to increase the pressure of the mass of air entering the engine inlet and discharge it to the diffuser and then the combustion section at the correct velocity, temperature and pressure.

The secondary purpose of a compressor section is to supply Engine Service Bleed Air to cool hot section parts, to pressurise bearing seals, and to supply heated air inlet anti icing and fuel system heat for de-icing.

Another secondary purpose is to extract air for aircraft services (pack system). This is called Customer Service Bleed Air

Question 2

Name the two types of compressor.

Answer 2

1. Centrifugal Flow
2. Axial Flow

Question 3

Define compressor pressure ratio.

Answer 3

Compressor Pressure Ratio = Outlet Pressure / Inlet Pressure

It is the pressure of the air exiting the compressor over the pressure of the air entering the compressor.

Question 4

Describe the operation of the centrifugal compressor including its main components.

Answer 4

The centrifugal compressor allows air to enter at the impeller and as the vanes rotate, it picks up and accelerates the air outwards. The air if flung into the divergent vanes of the diffuser where air speed slows down and pressure increases. At the diffuser, kinetic energy is converted to pressure energy. 50% of the total pressure rise occurs in the impeller while the other half occurs in the vaneless space and the diffuser. A temperature rise also results. The impeller is the rotor part (moving) while the diffuser is the stator part ( stationary)

The air leaves the diffuser and is fed into the compressor manifold

Question 5

Where does the pressure rise take place in a centrifugal compressor?

Answer 5

50% of the pressure rise occurs in the impeller while the other 50% occurs in the vaneless space of the diffuser

Question 6

State three advantages of a centrifugal compressor.

Answer 6

1. High pressure rise per stage. Up to 10:1 and 15:1 in a dual stage.
2. Good effeciency over a wide rotational speed range
3. Simplicity of manufacture and low cost compared to axial engines

Question 7

Name the three types of axial flow compressor.

Answer 7

1. The Single Spool
2. The Dual Spool
3. The Triple Spool

Question 8

What is a spool?

Answer 8

A compressor being driven by a turbine is called a spool.

E.g A dual spool engine consists of two compressors each driven independently by two turbines. (N1 and N2)

Question 9

What does the term “N1” refer to?

Answer 9

On a Dual Spool Engine, N1 refers to the front compressor which is the low pressure compressor.

On a Triple Spool Engine, N1 refers to the fan which is the low pressure compressor.

Question 10

In a twin spool gas turbine engine, what drives the “N2”?

Answer 10

The N2 or high pressure compressor is driven independently by it’s own turbine.

(high pressure turbine)

Question 11

What does a stage consist of in an axial flow compressor?

Answer 11

A complete compressor stage consists of a row of rotor blades followed by a row of stator blades.

Question 12

Name the main components of an axial flow compressor.

Answer 12

The Rotor, Stator vanes (stators), the rotor shaft, rotor discs and rotor blades

Question 13

How are the compressor blades attached?

Answer 13

The blades are fastened in slots around the periphery of the rotor discs.

Question 14

State the advantage of an axial flow compressor.

Answer 14

Straight through (axial/parallel) flow of the air allowing high RAM efficiency

Question 15

Explain a compressor stall.

Answer 15

A compressor stall can occur in an axial flow compressor when an imbalance of compressor RPM or inet air velocity occurs resulting in an excessive angle of attack of one or more compressor blades disrupting the smooth airflow through the compressor.

Question 16

Name three types of anti-stall systems.

Answer 16

1. Multi-spool layout
2. Bleed valves/rings
3. Variable intake guide and compressor stator vanes

Question 17

How can air leakage be minimised around tips of compressor blades?

Answer 17

A cooling/heating schedule which sprays the appropriate temperature of air supply over the critical area of the compressor casing, thus contracting or expanding it as required to minimise tip clearance.

Question 18

Name the two compressor types?

Answer 18

Centrifugal Flow

Axial Flow

Question 19

What happens to temperature, pressure and velocity in a compressor?

Answer 19

Temperature and pressure increase while velocity decreases

(velocity increases at some points throughout the compressor such as impeller)

Question 20

What does the mass of air passing through a compressor depend on? (3)

Answer 20

• Air density
• Aircraft speed
• Engine R.P.M

Question 21

What is a typical pressure ratio for a centrifugal compressor?

Answer 21

Pressure ratio = Outlet pressure / inlet pressure

centrifugal typical pressure ratio is 15:1 or 15

Question 22

What effect has temperature got on pressure rise?

Answer 22

The lower the inlet temperature of air entering the impeller, the greater the pressure rise for a given amount of work put into the air by the impeller.

Lower temperature = greater pressure rise

Question 23

List the parts of a centrifugal compressor?

Answer 23

Impeller Rotor

Diffuser

Manifold

Question 24

Which engine normally has the higher pressure ratio (axial/centrifugal)?

Answer 24

Pressure ratio = outlet pressure / inlet pressure

Axial compressor

Question 25

What is a compressor stage?

Answer 25

A complete compressor stage is defined as a row of rotor blades followed by a row of stator blades

Question 26

Explain a rotor disc?

Answer 26

The rotor discs are either bolted or splined to the shaft. The disks are axially located by spacer rings; in some cases specific spacers carry air seals

Question 27

What is blade stagger?

Answer 27

The angle of twist is termed the stagger angle

The stagger angle ( or angle of attack to the relative airflow) decreases from root to tip.

This ensures that an equal amount of work is done on the air at all points along the blade.

Question 28

Explain the purpose of mid span shroud?

Answer 28

To relieve vibration and bending stress caused by unstable intake airflow.

Midspan shroud is formed about 60% up the blade length

Question 29

What type of roots can be used to retain a compressor blade? (2)

Answer 29

Fir Tree Root

Dove tail

Question 30

What can fixed structural inlet vanes be used to carry?

Answer 30

Direct airflow at the correct angle onto the first stage rotor blades

Question 31

Why are stator vanes shrouded at their inner ends?

Answer 31

To prevent vibration and may also form an interstage seal

Question 32

Explain why stator vanes are divergent?

Answer 32

The air is then allowed to expand into a divergent duct called a diffuser and according to Bernoulli’s principle, as air spreads out its speed slows down and increases its static pressure

It is here where the kinetic energy (from impeller) is converted into pressure energy.

Question 33

Explain an engine surge?

Answer 33

Reversal of airflow at the high-pressure end, often leading to complete airflow breakdown,

sometimes so severe as to cause a loud bang as air is spat out of the front of the compressor

Question 34

What is the advantage of a mutliple spool compressor?

Answer 34

It allows for airflow control and is a method of compressor stabilisation

(other two include: Bleed valves/rings and variable intake guide and compressor stator vanes)

Question 35

Explain the purpose of bleed valves?

Answer 35

They prevent engine surge (reverse airflow) by achieving a bleed-off of surplus low-pressure air supply at speeds below ‘‘design speed’’

Question 36

What types of muscle power can be used to activate bleed valves?

Answer 36

Pneumatic

Question 37

Explain tip clearance control?

Answer 37

A cooling/heating schedule which sprays the appropriate temperature of air supply over the critical area of the compressor casing, thus contracting or expanding it as required to minimise tip clearances minimising efficiency losses due to air leakage around the tips of compressor blades