Turbine Engine Compressors

Question 1

What are the advantages of a centrifugal compressor over an axial flow compressor?

Answer 1

1. Cheaper and easier to manufacture.
2. More robust.
3. Better able to handle FOD ingestion.
4. Less prone to stalling and surging.
5. Relatively lower in weight.

Question 2

What is an impellor?

Answer 2

The internal rotating component of a centrifugal compressor, which takes inlet/intake air at its centre and compresses it by centrifugal force. Sometimes known as the rotor.

Question 3

What is the purpose of the diffuser?

Answer 3

To decrease the air’s velocity and to increase its pressure.

Question 4

Where does the air pressure increase as it passes through a centrifugal compressor?

Answer 4

Air pressure increases progressively through the impeller and increases rapidly through the diffuser.

Question 5

What is configuration /description of a centrifugal compressor?

Answer 5

Short in length, spoke like design, easy to manufacture, with high compression per stage.

Question 6

How does velocity and pressure vary through a centrifugal compressor?

Answer 6

Velocity and pressure both increase through the impeller, however through the diffuser the velocity decreases and the pressure increases.

Question 7

What is the configuration/ description of an axial flow compressor?

Answer 7

Multiple stages of rotating aerofoil blades and fixed vanes (stators), with a larger mass flow capacity with high efficiency.

Question 8

What are the advantages of an axial flow compressor over a centrifugal compressor of the same frontal diameter? 2

Answer 8

Higher compressor pressure ratios and greater mass airflow.

Question 9

What are the advantages of an axial flow compressor over a centrifugal compressor? 4

Answer 9

1. A lower frontal area for a given mass airflow, which allows for a smaller diameter engine.
2. Achieves higher compression ratios more efficiently.
3. A lower pressure rise per stage.
4. A much lower airflow exit velocity compared to its inlet velocity.

Question 10

What is the definition of the compressor pressure ratio?

Answer 10

The ratio of the compressor outlet pressure to the compressor inlet pressure.

Question 11

What is the typical total compressor pressure ratio of a single stage centrifugal compressor?

Answer 11

Up to about 7.5 :1

Question 12

What is the typical total compressor pressure ratio of a double stage centrifugal compressor?

Answer 12

Up to about 15:1

Question 13

What is the typical total compressor pressure ratio of a modern axial flow compressor?

Answer 13

20 : 1, but up to 30 : 1 in some cases.

Question 14

How does the temperature, pressure, and velocity of the air change as it passes through each stage of an axial flow compressor?

Answer 14

Temperature and pressure progressively increase and the axial velocity increases across the rotors and decreases across the stators, but overall velocity decreases, especially when considering the effect of the diffuser.

Question 15

What is the function of the inlet guide vanes?

Answer 15

The inlet guide vanes are designed to direct the airflow onto the first stage rotors at an appropriate angle of attack.

Question 16

What is the function of variable inlet guide vanes?

Answer 16

To change angle depending on flow characteristics, directing the airflow onto the first stage rotors at the optimal angle of attack, thereby reducing the chance of a compressor stall during operation off design (at low) rpm.

Question 17

What is the function of the variable stators in an axial flow compressor?

Answer 17

The variable stators are designed to direct the airflow onto the subsequent stage of rotors at the optimum angle of attack, thereby increasing the compressor pressure ratio.

Question 18

What is the primary function of each row of stator blades?

Answer 18

To convert the kinetic energy of the airflow into pressure energy.

Question 19

What is the secondary function of the stator blades?

Answer 19

To stop swirl in the air thereby maintaining axial flow as it passes through the compressor.

Question 20

What are the function of the rotors and stators in an axial flow compressor?

Answer 20

The rotors accelerate the airflow and the stators decelerate the flow to increase the air’s pressure.

Question 21

What shape does an axial flow compressor form towards the combustion chamber?

Answer 21

The annulus area of an axial flow compressor reduces towards the high pressure end, forming a convergent duct.

Question 22

Why does the air annulus area decrease towards the high pressure end of an axial flow compressor?

Answer 22

To allow for the reduction in the volume of the air as its pressure progressively increases and to maintain the axial velocity of the air constant as density increases.

Question 23

Why is there only a small pressure increase across each stage of an axial flow compressor?

Answer 23

Because the aerofoil blades are, by necessity, quite small and consequently only produce a small amount of lift at each stage.

Question 24

What effect does the diffuser have on the air leaving the compressor?

Answer 24

A diffuser reduces the velocity and increases the pressure of the airflow.

Question 25

What is likely to result from unstable airflow through a compressor?

Answer 25

A compressor stall.

Question 26

What determines the angle of attack of a compressor blade at a given blade angle at any given time?

Answer 26

The rpm of the compressor and TAS of the airflow.

Question 27

What is the condition that leads to compressor stalls?

Answer 27

The axial velocity of the air flow is too low relative to the rpm.

Question 28

What are the effects of a dirty or FOD damaged compressor?

Answer 28

Lower compression ratio/output and/or a compressor which is more prone to stalling.

Question 29

What are the indications of a compressor stall? 3

Answer 29

1. A surging/popping noise
2. A rise in gas temperature (e.g. EGT/ITT)
3. Fluctuations in rpm.

Question 30

What is the principle effect/purpose of the bleed valves/bands in the compressor?

Answer 30

To offload the compressors (LP and HP) and increase the mass airflow through the early stages of the compressor, during acceleration from low engine speed thereby reducing the possibility of compressor stall and surge at low rpm.

Question 31

What is the function of the bleed valves/bands in the compressor?

Answer 31

To increase the mass airflow through the early stages of the compressor at low rpm.

Question 32

What positions are the surge/stall bleed valves or bleed bands during engine acceleration?

Answer 32

Open at low and increasing rpm, progressively closing as rpm increases to design rpm.

Question 33

Do turbine engines use the same compressor bleed outlets for surge/stall protections as they do for environmental control systems?

Answer 33

No

Question 34

What is the normal source of bleed air at high and moderate (climb and cruise) power settings?

Answer 34

The low pressure compressor.

Question 35

What is the normal source of bleed air at low (decent) power settings?

Answer 35

Both low and the high pressure compressors in a two spool engine, and all three of the low, intermediate and high pressure compressors in a three spool engine.

Question 36

What are the effects of bleeding air off the compressor for systems such as anti-icing?

Answer 36

Gas temperature increases, thrust reduces and SFC increases.

Question 37

What is the main disadvantage of bleed significant amounts of air from a turbine engine’s compressor?

Answer 37

The mass flow of air through the compressor reduces and consequently the efficiency of the engine reduces.