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Module 14 - Part 1 > Chapter 1 - Gas Turbines > Flashcards

Chapter 1 - Gas Turbines Flashcards

1
Q

What is the definition of mass?

A

Force required to overcome the inertia of a body

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2
Q

What is the definition of momentum?

A

Measure of the quantity of motion

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3
Q

What is the definition of power?

A

Rate of changing energy

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4
Q

What is the equation for power?

A

Energy / Time

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5
Q

Why is heat regarded as ‘disorderly energy’?

A

Because of its random patterns and the difficulty of channelling it efficiently

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6
Q

What is important to know about pressure energy?

A

A fluid under compression will attempt to flow from a high pressure to a low pressure, and in so doing, will or can do work

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7
Q

How do you convert Fahrenheit from Centigrade?

A

(9/5 x °C) + 32

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8
Q

What is Newton’s First Law of Motion?

A

Every object is compelled to change it’s state of motion by an external force acting upon it

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9
Q

What is Newton’s Second Law of Motion?

A

Rate of change of momentum of a body is proportional to the total force acting upon it

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10
Q

What is Newton’s Third Law of Motion?

A

For every action there is an equal and opposite reaction

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11
Q

What does Bernoulli’s theory state?

A

Sum of the pressure energy, the potential energy and the kinetic energy is constant through a tube

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12
Q

What is the difference between the gas turbine engine and the piston engine?

A

Gas turbine is continuous

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13
Q

Why can the ignition device be switched off after ignition is achieved in a modern jet engine?

A

The flame will be continuous, providing fuel is supplied

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14
Q

Why can the starter be switched off after ignition is achieved in a modern jet engine?

A

The hot gases crossing the turbine produce torque to drive the compressor

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15
Q

Ideally, what should the compression and exhaust phases be?

A

Isothermal - temperature of the working fluid should remain constant

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16
Q

Ideally, what should the combustion process be?

A

Isobaric - constant pressure

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17
Q

Ideally, what should the adding of energy to the working fluid be?

A

Adiabatic - no heat is lost from the mass flow ensuring that all the heat added remains to do useful work

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18
Q

What must air into the intake be?

A

‘Clean’, free from debris and also free from turbulence

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19
Q

What are all intakes subject to during flight manoeuvres?

A

‘Blanking off’

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20
Q

Why are perfectly round intakes preferred?

A

Little chance of setting up low-pressure areas within the intake which can then induce vortices into the airflow

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21
Q

How can the ‘Ram effect’ be induced in an intake?

A

The gas first passes through a convergent duct, then a divergent duct into the compressor inlet

Initially accelerates the gas, increasing the mass flow of air, then slowing this increased mass in order to raise its temperature

Increase in pressure can be achieved due purely to the forward speed of the aircraft

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22
Q

Why are pitot intakes not suitable for transonic velocities?

A

Shock wave forms at the forward lip of the intake

Disrupts airflow into the intake

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23
Q

What kind of intake is used at supersonic speeds?

A

Variable geometry which slow the air down and spill off excess air

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24
Q

What is the purpose of the dump valve in a supersonic intake?

A

Pressure obtained at the inlet duct may be excessive, so dump valves rid the intake of unwanted air

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25
Q

What are the two types of compressor?

A
  • Centrifugal vane
  • Axial flow
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26
Q

What is stationed around the disc or impeller of the centrifugal type of compressor?

A

‘Diffuser’ ring

Ring of stationary vanes formed with divergent cross section between them

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27
Q

Why is the centrifugal compressor’s efficiency considered to be relatively low?

A

High speed of rotation and drastic changes in the air-flow direction leading to a high temperature increase

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28
Q

What is the limit to the pressure ratio and tip speed of a centrifugal compressor?

A

4.5:1

1600 ft/sec

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29
Q

What are the main advantages of the centrifugal compressor?

A
  • Simple and comparatively cheap to manufacture
  • Robust in construction and less vulnerable to damage
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30
Q

What are the main disadvantages of the centrifugal compressor?

A
  • High speed of rotation required
  • Large frontal area
  • Limited pressure ratio
  • High temperature increase
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31
Q

What forms a ‘stage’ of a compressor?

A

Row of stators and a row of rotors

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32
Q

What does the convergent passage through an axial flow compressor ensure?

A

Uniform axial velocity

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33
Q

What is the difference between the single-spool and multi-spool compressor?

A

Single-spool - consists of one rotor assembly

Multi-spool - consists of two or more rotor assemblies, each driven by their own turbine at an optimum speed

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34
Q

What is the advantage of a multi-spool compressor?

A
  • Greater pressure ratios
  • Greater operating flexibility
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35
Q

Where does the ‘cold’ stream pass in a large fan fitted to the front of a single or twin-spool compressor?

A

Bypasses the other compressors and is ducted to atmosphere through the cold stream nozzle

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36
Q

What is the cross sectional area between each adjacent stator and rotor blade in an axial compressor?

A

Divergent

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37
Q

Why is the area between each stator and rotor blade divergent?

A

Causes the remaining velocity energy to be converted into pressure and temperature

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38
Q

What is the purpose of the stators between each rotor?

A

Present the air at the correct angle to the next stage of rotor blades

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39
Q

What is the reason for the small pressure increase through each stage?

A

To prevent air breakaway at the blades, and subsequent blade stall

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40
Q

What is the purpose of the ‘twist’ of each axial compressor blade?

A

Give the correct angle of incidence at each point of the blade, to maintain a fairly uniform axial velocity of the air

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41
Q

Why are stator blades usually shrouded in the front stages?

A

Minimise the vibrational effect of flow variations on the longer blades

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42
Q

How much engine thrust can the high by-pass ratio fan produce?

A

75%

43
Q

What is the purpose of the containment ring?

A

Prevent a detached fan blade to penetrate the engine casing and hazard the aircraft

Shaped to ‘flip’ the blade so that it will be swept down the fan duct before the next fan blade can hit it

44
Q

What prevents blade flutter in typical fan blades?

A

Mid-span shrouds (also called snubber or clappers)

45
Q

What are the advantages of Wide Chord Fan Blades?

A
  • Lighter
  • Shorter
  • Move more air for a given RPM
  • Better able to centrifuge FOD into the fan duct
  • More rigid, and thus do not require mid-span shrouds
46
Q

What are the signs of compressor stall?

A
  • Vibration
  • Rumbling noise
  • Inability of the engine to accelerate
  • Rapid rise in exhaust gas temperature
47
Q

What is surge?

A

Air at high pressure at the back of the compressor will travel to the low pressure zone at the front of the engine

Recognised by loud bang or banging in the intake

48
Q

At what speed are fixed stator axial compressors designed to work most efficiently at?

A

At the engine’s maximum cruise RPM

49
Q

What is the purpose of Inlet Guide Vanes (IGV)/Variable Inlet Guide Vanes (VIGV)?

A

Give ‘whirl’ to the air entering the front of the compressor

Air is presented to the first stage rotor at the most efficient angle of attack

50
Q

What will likely control the Variable Angle Stator Vanes (VSV)?

A

Full Authority Digital Engine Control (FADEC)

51
Q

What is the purpose of a bleed valve?

A

Bleed off excess pressure towards the rear of the compressor, preventing stall and surge

52
Q

What are the open and closed conditions of the bleed valve?

A

Open at idle RPM

Closed at higher RPM

53
Q

What are the two main types of fuel nozzles/burners?

A
  • Vaporising
  • Atomising
54
Q

Describe a vaporising type of fuel nozzle/burner?

A

Vaporises fuel by pre-heating it prior to combustion, as the burner nozzle is forward facing so that the flame is heating the fuel delivery pipe and vaporising the fuel

Known as ‘hockey-stick’

Raise problems when engine starting

55
Q

Describe the atomising type of fuel nozzle/burner?

A

Passes through a swirl chamber where tangentially disposed holes or slots impart a swirl to the fuel by converting its pressure to kinetic energy

Fuel is atomised to form a cone shaped spray

56
Q

How much air flow is used for cooling and at what temperatures can burning the fuel reach?

A

80%

2,000°C

57
Q

How much air is required for the complete combustion of fuel?

A

15 lbs of air to 1 lb of fuel

15:1 by weight

58
Q

What are the three main flows of airflow into the flame tube?

A
  • Primary air flow
  • Secondary air flow
  • Tertiary airflow
59
Q

What is the purpose of primary air flow?

A

Mixing with the fuel and to support combustion

60
Q

What is the purpose of secondary air flow?

A

Shape the flame and complete combustion

61
Q

What is the purpose of tertiary air flow?

A

Cut off the flame and reduce gas temperature to a figure acceptable to the turbine

62
Q

What is mnemonic steel?

A

Special heat resistant steel

63
Q

What does the secondary and tertiary air flow also form?

A

A boundary flow on the inside and outside of the flame tube

64
Q

What are the four main types of combustion system?

A
  • Multiple chamber
  • Tube-annular (can-annular)
  • Annular
  • Reverse flow
65
Q

What is the purpose of the interconnectors in a multiple chamber of combustion chamber?

A
  • Equalise the pressure generated throughout the system during combustion
  • Propagates the flame around all the chambers
66
Q

What is the advantage of a reverse flow combustion system?

A

Significantly reduced engine length

67
Q

What are the stationary guide vanes called in the turbine section?

A

Nozzle guide vanes (NGV)

68
Q

What are the two basic types of turbine used to produce torque?

A
  • Impulse type
  • Reaction type
69
Q

Describe the cross sectional geometry of impulse/reaction turbine blades?

A

Aerofoil shape with a partial ‘cup’ shape at the forward part of the blade

70
Q

How does the impulse/reaction turbine extract energy from the gas?

A

Partial cup shape absorbs some of the kinetic energy from the air

Adjacent blades form a convergent duct which accelerates the gas flow and causes a further reaction, producing even greater rotational force

71
Q

What is the purpose of the NGVs?

A
  • Accelerate the gas stream to as high a velocity as possible by utilising a convergent shape
  • Direct the gas at the most efficient angle of attack onto the turbine assembly
72
Q

What is the primary factor that limits the amount of power a gas turbine can produce?

A

Maximum temperature that can be tolerated at the turbine inlet (Turbine Inlet Temperature TIT)

73
Q

What other factor must be considered where there is more than one turbine stage?

A

Interstage (or Intermediate) Turbine Temperature (ITT)

74
Q

In a twin spool engine, what will the forward most turbine drive?

A

The HP compressor

The first turbine the air will encounter

75
Q

What is a free power turbine (Power Turbine, Free Turbine or Gas Coupled Turbine)?

A

No direct link to any of the engine compressors

76
Q

How is blade cooling of turbine blades achieved?

A

Percentage of mass flow of air is passed through holes formed in the blades

Air comes from compressor stage, which is relatively cool (400°C)

77
Q

Why must blade cooling air come from the rear of the HP compressor?

A

Colder air could cause thermal shock leading to stress cracks forming

78
Q

What is the most common type of blade attachment?

A

‘Fir tree’ fixing

79
Q

Describe ‘fir tree’ fixing?

A

Very accurate machining is required to ensure that loading is shared by all serrations

Free in the serrations when the turbine is stationary, and is stiffened in the root by centrifugal loading when the turbine is rotating

80
Q

At what speed does gas enter the exhaust system from the turbine section?

A

750 - 1200 ft/sec

81
Q

Why is there a divergent area between an exhaust cone and the outer wall?

A

To decrease the speed of the airflow from the turbine section, to reduce the effect of high friction losses

82
Q

What is the secondary purpose of the exhaust cone?

A

Prevents hot gases from flowing across the rear face of the turbine disc

83
Q

What are the three methods of reducing the noise from a jet engine?

A
  1. Use a smaller throttle setting (thrust/power) for take-off
  2. Wrap the engine in a noise insulating material
  3. Reduce the velocity of the jet efflux or increase the velocity of the surrounding (ambient) air at the exhaust
84
Q

Where is the noisiest part of a high by-pass engine?

A

Fan

85
Q

What are the requirements of a thrust reverse system?

A
  • Not affect engine operation, either when in use or not
  • Be able to withstand high temperatures
  • Be mechanically strong but light in weight
  • Be ‘fail safe’
86
Q

What is the difference between the propeller thrust reverse system and the typical turbo engine?

A

Propeller - reversal of thrust is 180° forward
Turbo - 45° to the horizontal in the forward direction

87
Q

What is the actual amount of useful reverse thrust?

A

Approximately 50% of normal forward thrust

88
Q

What are the main types of thrust reverser?

A
  • Clamshell door
  • Retractable ejector
  • Cold stream cascade reverser
89
Q

How is the retractable ejector actuated?

A

Hydraulically

90
Q

What is another name for the retractable ejector?

A

Bucket target type

91
Q

Describe cold stream reversers?

A

Hydraulically or pneumatically operated doors, called translation cowls, that uncover a set of cascade vanes

Blocker doors also simultaneously block off the cold stream and divert it through those vanes

92
Q

How is it ensured that high velocity engine exhaust gases will not damage the thrust reverse system as it deploys?

A

Throttle lever must be in the minimum power position (fully to the rear) prior to selecting reverse thrust

93
Q

What is the main disadvantage of a single spool engine?

A

Takes too long to accelerate (spool up)

94
Q

What is the tip speed usually in a high by-pass turbofan engine?

A

Around Mach 1.3

95
Q

What is the weight reduction of a typical low by-pass engine over a pure jet of similar mass flow?

A

20%

96
Q

How can a by-pass engine obtain the same power at the turbine to drive the compressors and the accessories than a pure jet?

A

Turbine inlet temperature (TIT) is elevated and the pressure ratio is also increased

97
Q

How much energy is available for jet thrust in a turbo-prop engine?

A

10%

98
Q

What is the main difference between a turbo-prop and turbo-jet engine?

A

Turbo-prop engine usually has more stages

99
Q

How does Boyles’ and Charles’ Law relate to how a gas turbine engine works?

A

If a mass of air which is not confined in volume is heated, there will also be a subsequent increase in the volume of the gas while the pressure remains approximately constant

100
Q

What is the Brayton Cycle?

A

The working cycle of the gas turbine engine which goes through the stages of compression, combustion and expansion plotted on a graph of volume against pressure

101
Q

What are the two types of centrifugal compressor?

A

Single and double-entry

102
Q

Approximately what percentage of compressor delivery air is used for initial combustion?

A

20%

103
Q

What safety features are there to protect against asymmetry of the thrust reverser system?

A

If the operating system of one thrust reverser were to fail, a mechanical lock would prevent it from deploying until it is restored, and in any case not until WOW switch is made

The operational reverser will be automatically isolated

104
Q

What are the types of by-pass engines?

A

Low by-pass - ratio of 1:1

High by-pass - ratio of over 1:1 to more than 6.5:1

Module 14 - Part 1 (1 decks)

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