15.4

Question 1

What is the main role for a compressor section of a GTE?

Answer 1

To supply sufficient quantity of air to satisfy the combustion chamber

Question 2

What is the secondary function of the compressor?

Answer 2

To supply air to the systems that require bleed air

Question 3

What is the location of the bleed ports dependant on?

Answer 3

The pressure and temperature of the required systems

Question 4

Two main components of a compressor?

Answer 4

Stator and Rotor

Question 5

What do the rotors do and why?

Answer 5

They contain rotor blades that suck in air and push it to the outlet side of the rotor to increase the energy of the airflow

Question 6

What is the name given to the blades on a stator?

Answer 6

Stator vanes

Question 7

What do stator vanes do?

Answer 7

Guide and slow the airflow which increases static pressure

Question 8

What is the combination of a rotor and a stator called?

Answer 8

Compressor stage

Question 9

What is always the first part of the compressor stage?

Answer 9

Rotor

Question 10

What is an inlet guide vane?

Answer 10

A set of stator vanes infront of compressor stage

Question 11

What do the IGV’s do?

Answer 11

Improve airflow into the first compressor stage

Question 12

Are IGV’s fixed or moveable?

Answer 12

Both

Question 13

What are the stator vanes in the fan duct called?

Answer 13

Outlet guide vanes or OGV’S

Question 14

What do OGV’S do?

Answer 14

Improve airflow into the fan nozzle

Question 15

Two types of compressors used in gas turbine engines?

Answer 15

Centrifugal and Axial

Question 16

What does the centrifugal flow compressor consist of?

Answer 16

Impeller (Rotor)
Diffuser (Stator)
Compressor manifold

Question 17

What is the ratio of centrifugal compressors and what does this say about it?

Answer 17

8:1 so a High-pressure rise per stage

Question 18

How many stages are centrifugal compressors limited to and why?

Answer 18

2 due to concerns in efficiency

Question 19

What is the impeller made out of?

Answer 19

Aluminium alloy

Question 20

How many entries can the impeller have?

Answer 20

One or two

Question 21

Because an impeller has a smaller diameter, what does this mean for the rotational speed?

Answer 21

It must be higher to ensure sufficient airflow

Question 22

What does the single entry impeller ensure and what comes after the eye?

Answer 22

Direct ducting through the eye to inducer vanes (easier than the double entry which is more complicated)

Question 23

How do axial compressors compress air and why is this good?

Answer 23

They compress the air in its direction of flow which reduces energy lost due to turns

Question 24

Where are axial flow compressors found?

Answer 24

turbofan engines

Question 25

What does the axial flow compressor consist of?

Answer 25

One or more rotating aerofoil blades

Question 26

Where are centrifugal flow compressors used?

Answer 26

Turboprop and turboshaft

Question 27

Why do you need multiple stages in a compressor?

Answer 27

Because the energy output (pressure increase) from a single stage is small

Question 28

Where are the stationary vanes (stators) mounted in an axial flow compressor?

Answer 28

To the compressor casing

Question 28

What does a stage refer to?

Answer 28

A rotor-stator pair

Question 29

What duct is included in the axial compressor?

Answer 29

Convergent

Question 30

Why is there a gradual decrease in the diameter of the rotating drum and casing? (Convergent)

Answer 30

To maintain a steady axial velocity as pressure and density increase

Question 31

What are multi-spool compressors driven by?

Answer 31

An independent turbine

Question 32

What happens to the remainder of the flow in axial compressors?

Answer 32

It is bypassed and mixes in the exhaust

Question 33

What does the fan act as?

Answer 33

A Low pressure compression stage

Question 34

What is core flow/hot air?

Answer 34

The smaller proportion of flow

Question 35

Where is core flow passed through?

Answer 35

The intermediate pressure and high pressure compressor

Question 36

What is the compression ratio of axial flow?

Answer 36

Between 1:1 and 1:2

Question 36

Why is the rate of pressure increase per stage purposelly low?

Answer 36

Because you don't want air to move to quick so that it losses efficiency and stalls

Question 37

How do you achieve a high compression rate?

Answer 37

Increasing stages

Question 38

How many stages does the fan consist of?

Answer 38

One

Question 39

Advantages of cenrtigiugal compressors?

Answer 39

High pressure rise per stage Efficiency over wide rotational speed range Simple Low cost Low starting power

Question 40

Disadvantages of centrifugal compressors?

Answer 40

Large frontal area Losses in turns between stages

Question 41

Advantages of axial flow compressors?

Answer 41

High peak efficiency Smaller frontal area Straight through flow allowing high RAM efficiency Increase pressure rise by increasing stages with negligible losses

Question 42

Axial flow compressors disadvantages?

Answer 42

Only efficient at slight tolerances (narrow window) High cost Difficult to manufacture Relatively high weight High starting power (partially overcome by split compressors)

Question 43

What is beneficial about balancing fan blades?

Answer 43

They can be balanced without dissassembly of the engine

Question 44

What needs to have identical centrifugal forces on an engine to prevent vibrating rotors?

Answer 44

The fan blades

Question 45

What three parameters do centrifugal forces depend on?

Answer 45

Rotor speed Blade mass Distance of CoG from fan blade to rotational axis

Question 46

What is the moment weight?

Answer 46

The product of the blade mass and the distance of the CofG from the rotational axis

Question 47

What is the moment weight measured by?

Answer 47

A special blade scale

Question 48

Why do fan blades have a different moment weight even if they have the same mass?

Answer 48

The CofG is different

Question 49

What can the moment weight be given in?

Answer 49

Gram inches cm.g Ounce inches

Question 50

Where can the moment weight, serial and part numbers be found?

Answer 50

The root

Question 51

What matches the blades on the fan disc using moment weight

Answer 51

Computerised balancing

Question 52

What is the best method to change a single fan blade?

Answer 52

To replace the blade with a new blade with a moment weight as close to the old one as possible

Question 53

What can be used if the difference in moment weight is larger than specified by the manufacturer?

Answer 53

Balance weights can be used

Question 54

Where are the balance weights installed?

Answer 54

On the fan disc or in the spinner cone

Question 55

What is another method for replacing a single fan blade?

Answer 55

To change a pair with similar moment weights even if one blade is damaged

Question 56

What are axial and centrifugal compressors sensitive to?

Answer 56

Changes in airflow passing through compressor

Question 57

What is the resultant rotor inlet velocity?

Answer 57

The correct relationship between the inlet velocity and the rotor speed which ensures smooth airflow

Question 58

The airflow to the rotor is smooth as long as what?

Answer 58

The AOA to the rotor blades is small

Question 59

What do stator vanes do? Also to speed?

Answer 59

Guide the airflow to the next rotor stage and decelerates

Question 60

The airflow in the axial compressor is smooth as long as what?

Answer 60

The AOA to the rotor blades and stator vanes is small

Question 61

What happens to the angle of attack when the inlet air velocity decreases?

Answer 61

Angle of attack increases

Question 62

What happens to the angle of attack when the inlet air velocity increases?

Answer 62

Angle of attack decreases

Question 63

What happens when the AOA increases? What is this known as?

Answer 63

Turbulent vortex caused because of airflow separation (Compressor stall)

Question 64

What does a compressor stall cause air to do?

Answer 64

Airflow to slow down, stop or reverse direction

Question 65

What is a compressor surge?

Answer 65

A severe form of compressor stall where airflow reverses, stops or slows

Question 66

How are pressure surges formed?

Answer 66

Rapid decrease of airflow Blockage in airflow Low pressure zones formed which reverses airflow

Question 67

Where do surges occur?

Answer 67

On the forward or aft compressor stages

Question 68

What do surges in the forward stages usually affect?

Answer 68

Only a part of the compressor blade

Question 69

When do surges develop very rapidly over a large part of compressor?

Answer 69

In the aft stage

Question 70

What are the two main reasons for a compressor stall and surge?

Answer 70

Engine is too far below design speed Incorrect acceleration or deceleration

Question 71

What are the three design methods to prevent compressor stall and surge?

Answer 71

Bleed valves Dual or triple spool rotors instead of single spool Variable compressor stator vanes

Question 72

Where are bleed valves found?

Answer 72

Mid section or aft of compressor

Question 73

When are bleed valves open?

Answer 73

During low speeds so excessive airflow can escape overboard

Question 74

On a dual spool engine, the forward part is named as what?

Answer 74

N1 or Low pressure compressor

Question 75

On a dual spool engine, the aft part is named as what?

Answer 75

N2 or the High pressure compressor

Question 76

Is each compressor rotor joined or does it use a separate turbine?

Answer 76

Separate turbine

Question 77

What is the most effective method to prevent stall?

Answer 77

Variable stator vanes

Question 78

What axis do variable stator vanes move along?

Answer 78

The vertical axis

Question 79

Where can variable stator vanes/variable inet guide vanes be found?

Answer 79

Forward stage of High pressure compressor

Question 80

What is an advantage of variable stator vanes?

Answer 80

Optimum angle of attack on rotor blades can be reached for every engine speed

Question 81

Disadvantage of variable stator vane?

Answer 81

Needs complicated control mechanism.

Question 82

What does the compression ratio represent?

Answer 82

Ratio of volume of combustion chamber from largest and smallest capacity

Question 83

What remains constant from the front to the rear compressor?

Answer 83

Axial velocity of gas

Question 84

What is compression ratio not proportional to?

Answer 84

RPM but does vary with it

Question 85

Each stage of an axial flow compressor has a ratio of what?

Answer 85

1.3:1

Question 86

Because the pressure increase at each stage is small, what needs to be in place to raise the pressure for efficient combustion?

Answer 86

Multiple stages