Gas Turbines - Compressors

Question 1

What is the inlet to a gas turbine

Answer 1

Pitot inlet
Ensures correct quantity of air
Uniform pressure delivery
Free from turbulence
Shiny and smooth

Question 2

Lips of engine

Answer 2

Shaped to reduce flow seperation with cross winds/high angles of attack

Question 3

Throat

Answer 3

Convergent duct to suck

Question 4

Diffuser

Answer 4

Divergent duct to generate increased pressure

Question 5

How does a centrifugal compressor operate

Answer 5

Air enters the eye of the impeller and propelled from centre via centrifugal forces

Moves outwards velocity and pressure increases via impeller vanes

Air leaves impeller tangentially and enters the diffuser vanes

Kinetic energy added to flow is converted into pressure energy

50% of pressure rise across impeller and 50% in the diffuser section

Question 6

Centrifugal Compressor use on

Answer 6

Turbo prop
Air cycle machines
Turbo chargers

Question 7

Advantages to a centrifugal compressor

Answer 7

Simple
Robust
Short length
Less prone to stalls and surges

Question 8

Disadvantages to a centrifugal compressor

Answer 8

Big frontal area
Low mass flow
Poor compression ratio
High specific fuel consumption

Question 9

Axial Flow Compressor operations

Answer 9

Multiple stages consisting of rotors and stator

Rotors accelerate air in the direction of rotation and add kinetic energy to airflow - increase in pressure

Passed air to stator which are slightly more divergent than rotors so convert kinetic energy to pressure energy. Decrease in velocity as stator are fixed

Overall stage: constant velocity and pressure increase
10/20% pressure rise through each stage

Question 10

Role of a rotor in axial flow compressor

Answer 10

Adds energy
Increases velocity
Pressure increases

Question 11

Role of a stator in a axial flow compressor

Answer 11

Forms a divergent duct
Increases pressure
Velocity reduces (fixed)

Question 12

Convergent Duct

Answer 12

Maintains the axial velocity as pressure is lower at inlet than inside compressor

Question 13

Advantages of axial flow compressor

Answer 13

Small frontal area
High mass flow
High CPR (Compressor pressure ratio)
Low SFC (specific fuel consumption)

Question 14

Disadvantages of a axial flow compressor

Answer 14

Expensive/complex design
Prone to stalls and surges
Easily damaged

Question 15

Stall of a compressor - Causes/Symptoms

Answer 15

Partial blockage of air (reduction in airflow)

Causes:
Low RPM
Excessive fuel flow
Turbulent airflow
Contamination
Excessive lean mixture

Symptoms:
High gas temperature
Reduced thrust
Rumbling/banging
Fluctuating rpm/fuel flow

Question 16

Surge on compressor

Answer 16

A stall across all stages

Produces flames in both inlet and exhaust
Air is pushed back forward

Causes:
Over fueling

Question 17

Axial/Centrifugal risk of stall surge

Answer 17

Axial
Risk of stall and surge

Centrifugal
Risk of surge
Cannot stall

Question 18

Stall recovery of compressors

Answer 18

Close throttle to 2/3 stages
Consider shutdown

Question 19

Stall Prevention - Variable Inlet Guide Vanes

Answer 19

Direct airflow onto first set of blades
Adjustable angle to ensure air directed across first stage at correct AOA across operating conditions

Question 20

Compressor Stall Definition

Answer 20

Occurs when axial velocity is too low resulting in large angle of attack
Low velocity results when volume of air increases which blocks forward movement of air

Question 21

Stall Prevention - Variable Stator Vanes

Answer 21

Adjust stator angle inside engine to direct air at precise angle

• close at high OAT to decrease volume preventing velocity decrease
• close at low RPMs to reduce volume
• fully open at high RPMs

Closed to maintain optimal AOA with decreased velocity

Question 22

Stall Prevention - Inter Stage Bleed Valve

Answer 22

Vent pressure during start to improve axial flow across compressor
Bring load in line with working line

Question 23

Bleed air extraction

Answer 23

Taken from IP and HP stage of HPC

Reduced thrust
Increased EGT
Increase SFC

Question 24

Diffuser

Answer 24

Located between compressor and combustion chamber
Decrease in velocity in order to prevent flame extinction
Further increase pressure of airflow
Point of highest pressure in engine

Question 25

Inlet Guide Vanes Operation and location

Answer 25

Located after the fan but before first stage rotor
Role is to direct airflow onto the first set of rotor blades in

Question 26

Compression Ratio of a axial flow compressor

Answer 26

35:1

Each stage raises pressure by 10/20%

Question 27

Pressure ratio of one stage of a centrifugal compressor

Answer 27

4:1

Question 28

What happens to air inlet of high bypass turbofan

Answer 28

Speed decreases and total pressure remains constant

Question 29

Ice protection of air intake is

Answer 29

Anti iced using hot air from the HP compressor

Question 30

How does air pass between rotors and stator

Answer 30

Rotors increase air velocity/pressure and temp

Stator reduce velocity and increase pressure and temperature

Question 31

Axial flow compressor ratio

Answer 31

35:1 to 50:1

Question 32

Stagger Angle is

Answer 32

The difference between blade angle at the root and tip

Higher angle at root than tip

Question 33

Fan blade tip clearance purpose

Answer 33

To ensure no adverse turbulent flow from drag from fan blades

Casing tight around each stage

Question 34

Inter Stage bleed role

Answer 34

Inter stage bleed valves open to vent excess pressure into bypass duct on start to reduce load on compressor

Question 35

Blades in axial flow compressor decrease in size from

Answer 35

Low pressure to high pressure stages to sustain axial velocity

Question 36

How are the rotor blade mounted

Answer 36

Via the mountings, the aerodynamic force/centrifugal force

Question 37

Where does the stall tend to occur first in an axial compressor

Answer 37

Tendency to stall in front stages