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Engines > 15.04 COMPRESSORS > Flashcards

15.04 COMPRESSORS Flashcards

1
Q

What 2 ways can you compress air in an engine?

A
  • Decrease the volume in a cylinder by a piston

- Make use of Bernoulli’s principle to get compressed air

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2
Q

What principle is used to compress air in gas turbine engines?

A

Bernoulli’s principle

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3
Q

What does the compressor supply a continuous flow to?

A

The combustion chamber

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4
Q

What are the two main components of the compressor?

A

Compressor rotor

Compressor stator

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5
Q

What is the function of the rotor?

A

It rotates around the axis of the compressor

Sucks in air and pushes it to the outlet side of the rotor

Increases the energy of the airflow

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6
Q

What is the function of the stator?

A

A fixed component with stator blades attached known as STATOR VANES

Guide and slow the airflow to increase pressure

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7
Q

What is always the first part of the compressor stage?

A

The rotor

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8
Q

What are a set of stator vanes infront of the compressor stage known as?

A

Inlet Guide Vanes (IGV)

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9
Q

What do IGVs do?

A

Improve the airflow into the first compressor stage

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10
Q

Are IGV fixed or movable?

A

Either

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11
Q

What are Outlet Guide Vanes?

A

Stator Vanes in the fan duct that improve airflow into the fan nozzle

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12
Q

What are the components on a compressor?

A

Rotor
Stator
Inlet case
Outlet Case

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13
Q

What do compressor cases provide?

A
  • Support for the compressor rotor bearings

- improve and straighten airflow into and out the compressor

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14
Q

What are two types of compressors?

A

Axial Flow

Centrifugal flow

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15
Q

Where are Centrifugal compressors mainly used?

A

Small GTEs such as APUs

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16
Q

What are the main components of a Centrifugal compressor?

A
  • Impeller (like a rotor)
  • Diffuser (like a stator)
  • Compressor manifold
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17
Q

Explain how a Centrifugal Compressor works

A
  • Impeller rotates, sucks air in horizontally near center of impeller
  • Rotation pushed out by centrifugal force, increases airflow velocity
  • Goes through divergent ducts formed by impeller vanes and velocity converted into static pressure
  • passes through diffuser where velocity decreases but pressure increases
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18
Q

What pressure ratio do centrifugal compressors give?

A

5:1

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19
Q

Why do some engines use multiple compressor stages?

A

To allow for higher pressures to be produced

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20
Q

What does the manifold on a centrifugal compressor do?

A

increase the pressure

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21
Q

How many stages does a centrifugal compressor have and why?

A

Maximum of 2 stages

- pressure decreases due to a change in direction between the stages

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22
Q

What are the 2 ways you can increase the efficiency in a centrifugal compressor?

A

Increase stages

Double entry type impellers

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23
Q

What is a double entry impeller?

A

Two impellers mounted face to face

  • gives a higher AIRFLOW because the air can enter from both sides
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24
Q

What is an advantage of the double entry impeller?

A

Saves weight and gives a smaller engine frontal area

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25
Q

How is airflow made in an Axial Flow Compressor?

A

By the rotor blades

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26
Q

Explain why the rotor and stator blades are aerodynamically shaped?

A

Smooths the airflow through the compressor stages

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27
Q

How does the pressure increase in an axial compressor?

A

Rotor increases velocity, diffuser-like shaped stator vanes convert the high energy from the airflow into static pressure

Channels between rotor blades act as diffusers so can increase the velocity but slow it down enough to increase the static pressure

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28
Q

What is the pressure ratio of each stage of an axial flow compressor?

A

1.3:1

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29
Q

What is the value of the air pressure when it first enters and when it exits?

A

enters at 10psi

exits at 13psi

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30
Q

If each stage has a small increase in pressure, what is done to increase it further?

A

Increase the amount of stages

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31
Q

What else increases as pressure increases? - what does this mean for the air?

A

density increases meaning air needs less volume

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32
Q

What is done to keep the compressor efficient as density and pressure increase through the stages?

A

compressor cross section gradually decreases - this keeps a constant velocity of the airflow through the compressor

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33
Q

What are some advantages of the Axial Flow compressor? (3)

A
  • at OPTIMUM COMPRESSOR SPEED, it is more efficient than a Centrifugal Flow Compressor
  • Cause less drag than centrifugal flow compressors because of the smaller frontal areas
  • Can reach high total pressures due to multiple stages
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34
Q

What are some advantages of the Centrifugal Flow Compressor? (5)

A
  • Good efficiency over a wide speed range
  • High pressure rise per stage
  • Simpler to manufacture and lower in cost
  • Less weight
  • Need less power for engine start
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35
Q

What do Axial and Centrifugal flow compressors have in common?

A

They are sensitive to changes in the airflow which passes through the compressor

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36
Q

Why are compressors sensitive to changes in airflow?

A

They only have one optimum operating condition for a given pressure ratio, rotational speed and airflow

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37
Q

What is the resultant rotor inlet velocity?

A

the result of the air inlet velocity and the rotor speed

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38
Q

A small angle of attack to the rotor blades provides what sort of airflow into the rotor?

A

A smooth airflow

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39
Q

What do stator vanes do to the airflow?

A

Decelerate the airflow and guide it to the next rotor stage of the compressor

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40
Q

What determines the direction of the rotor blades?

A

the outlet velocity of the stator outlet and the rotor speed

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41
Q

In an axial compressor, the airflow will remain smooth as long as what?

A

the angle of attack on the rotor blades and stator vanes are small

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42
Q

What determines the angle of attack?

A

Rotor speed

Inlet velocity

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43
Q

What happens when the rotor speed increases to a certain point, making the inlet velocity decrease?

A

The airflow separates from the compressor aerofoils and causes a turbulent vortex (compressor stall)

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44
Q

Where can compressor stall occur?

A

It can occur at some aerofoils only - weak and unnoticeable
OR
Occurs at one or more compressor stage

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45
Q

If the compressor stall gets worse and affects all compressor stages, what is it called?

A

Compressor surge

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46
Q

How is a compressor surge generated?

A
  • Rapid decrease in airflow causes stall on some blades or stages
  • Stall causes blockage in airflow, leading to a stronger stall
  • Low pressure zone airflows stop and reverse their direction
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47
Q

Where can compressor surge occur?

A

In all stages of the compressor - forward or aft of it

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48
Q

What does compressor surge in the forward stages affect?

A

Only a part of the compressor blades due to the bigger dimensions of the blades.

Not very dangerous

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49
Q

What does compressor surge in the aft stages affect?

A

Airflow can decrease rapidly and cause a strong reverse flow

Comes with heavy vibrations and loud noises

Thrust decreases, engine speed fluctuates, EGT increases

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50
Q

Give reasons as to how a compressor can stall or surge (4)

A
  • Dents or ice on the engine inlet can cause turbulent or disrupted airflow which decrease the inlet velocity
  • Damaged OR dirty rotor blades/stator vanes disturb correct airflow and cause stall
  • Damaged or broken combustor components can cause a blockage in the airflow and decrease the velocity in the compressor
  • Damaged turbine components decrease airflow and dents in the nozzle or broken objects in the exhaust system can cause blockage to airflow
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51
Q

How can fast deceleration cause stall or surge in the compressor?

A

Rotor speed of the HP compressor decreases faster than the LP

LP compressor supplies more than the HP demands

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52
Q

What sort of engines create stall or surge when decelerating in a fast pace?

A

Dual or triple spool engines

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53
Q

How does fast acceleration lead to stall ?

A

Too much fuel in combustion chamber increases the back pressure in the combustion chamber

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54
Q

What methods are used to avoid stall and surge?

A
  • Compressor Bleed Valves
  • Dual or Triple Spool engines rather than a single spool
  • Variable Compressor stator vanes (most modern)
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55
Q

Where are compressor bleed valves found?

A

in the mid/aft section of the engine compressor

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56
Q

When are compressor bleed valves open?

A

At low engine speeds to prevent airflow blockage at the aft compressor stages

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57
Q

What is a big disadvantage of the compressor bleed valves?

A

Big loss in efficiency due to airflow being compressed then blown overboard

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58
Q

How can you guarantee safe operation using bleed valves?

A

Necessary at several positions to ensure all stages speeds match

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59
Q

What is the forward part of a dual spool engine called?

A

Low pressure Compressor

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60
Q

What is another name for the low pressure compressor?

A

N1 compressor

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61
Q

What is another name for the high pressure compressor?

A

N2 Compressor

62
Q

What is an advantage of muti-spool engines?

A

Has good acceleration capabilities due to smaller rotor mass of split rotors

63
Q

What is the most effective method to prevent stall?

A

Use of Variable Stator Vanes

64
Q

What axis are varibale stator vanes movable at?

A

vertical

65
Q

Where can VSVs and VIGVs be found?

A

Forward stages of the HP compressor

66
Q

What are some advantages of VSVs?

A

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

Compressor does not stall and always operates at an optimum efficiency

67
Q

What is a disadvantage of VSV?

A

Needs a very complicated control mechanism

68
Q

What drives the LPC?

A

The low pressure turbine

69
Q

What do Fan Blades provide?

A

initial compression and then pass the compressed air to the engines primary and secondary gas paths

70
Q

What can the fan rotor be divided into?

A
  • Spinner cone
  • Fan disc
  • Fan Blades
  • LPC rotor (booster rotor)
71
Q

What is the spinner cone also known as?

A

Compressor inlet cone/ nose cone

72
Q

What does a spinner cone provide?

A

A cover for the end of the fan rotor disc and a smooth aerodynamic fairing at the engine inlet

73
Q

What sort of tips can a spinner cone have?

A

Curved or sharp

74
Q

What do curved and sharp tip spinner cones minimise?

A

Ice build-up on the outer surface of the spinner cone

75
Q

What do some spinner cones come with?

A

Anti-ice holes that warm air blows through to prevent ice build-up

**some engines have soft tips to create small imbalances during operation which prevents large ice build-up

76
Q

What is drawn on the outer surface of the spinner and why?

A

Spirals or white markings to scare birds away

77
Q

What is the spinner cone always attached to?

A

the front face of the fan disc

78
Q

What can be attached to the spinner cone?

A

Balance weights

79
Q

What is the fan rotor disc made of?

A

A large disc made of titanium alloy

80
Q

What is the fan rotor disc attached to?

A

The forward rotor shaft and the rotor drum of the LPC

81
Q

What does the fan rotor disc do?

A

Helps support the fan blades

82
Q

What sort of fitting is used the most to fit the fan blade onto the fan rotor disc?

A

dovetailed fitting

83
Q

What can fan blades be held in by?

A
  • Blade retainers
  • Bolts
  • Cylindrical pins
84
Q

What check is required when mounting the fan blades to the fan rotor disc?

A

check to make sure that there is movement between the blade root and the blade seat

85
Q

Why is movement/ a loose fitting between the blade root and the disc necessary?

A

So sudden forces and torque loads do not exceed the maximum allowable stress limits of the fan blades

86
Q

What is the most common force on rotor blades that causes it stress?

A

Centrifugal Force

87
Q

What alternative method can be used to keep within the stress limits on LONGER rotor blades?

A

Shrouds

88
Q

What do shrouds form?

A

A ring on the fan rotor

89
Q

What other names are given to shrouds on rotor blades?

A

mid-span shrouds

part-span shrouds

90
Q

What does a shroud do in regards to energy?

A

Converts fluttering energy to friction, thus dampens the blade flutter

91
Q

Why do mid-span shrouds have a special coating?

A

To allow them to slide over each other easily

92
Q

What types of blades are usually found on modern GTEs?

A

Wide Chord fan blades

93
Q

What does the Wide Chord Fan Blade not have that other blades do?

A

Shrouds

94
Q

What are the characteristics of Wide Chord Fan Blades?

A
  • Hollow
  • Made of titanium sheets bonded together
  • Made of a bonded titanium honeycomb construction
  • Made of a composite material with a titanium leading edge
95
Q

What is the main advantage of wide chord blades?

A

More efficient than conventional blades

96
Q

Why can wide chord fan blades withstand FOD better than conventional fan blades?

A

They have a higher flexibility

97
Q

Where is the impact energy absorbed on a wide chord fan blade?

A

the total blade span

98
Q

Where is the impact energy absorbed on a shrouded fan blade?

A

blade tip

99
Q

What is the low pressure compressor module otherwise known as in the fan module?

A

booster module

100
Q

What are the main components in an LPC Module?

A

Rotor drum
Stator Case
Airflow Splitter Fairing
Interstage seals

101
Q

What is the LPC rotor drum attached to?

A

Fan disc

102
Q

What is between the dovetail slots?

A

Rings with sharp edges for the labyrinth seals

103
Q

What is a labyrinth seal?

A

Made by the knife edge of the rotor drum and the opposing shrouds of the stator assembly

104
Q

What are shrouds in the stator assembly filled with?

A

soft abradable material

105
Q

What does the stator case assembly enclose and support?

A

Encloses the LPC rotor and supports the LPC stator vanes

106
Q

How can stator vanes be fitted to the stator case?

A

dovetail slots or bolts

107
Q

What are the 2 different construction methods for a compressor stator case?

A

closed stator case

split stator case

108
Q

How are the stator vanes fitted in a closed stator case?

A

Stator vanes fitted to the shrouds between the rotor stages

109
Q

What does the fan case assembly house?

A

Fan rotor and the LPC

110
Q

What do flanges do?

A

(on outer surface)

  • stiffen case
  • prevent deformation
111
Q

What are lining elements on the fan assembly otherwise known as?

A

Acoustic Panels

112
Q

Where are acoustic panels found?

A

On the inner surface of the fan case

113
Q

What do acoustic panels provide?

A

A smooth surface of the airflow and reduce noise created by the fan

114
Q

How do acoustic linings reduce noise?

A

By small holds on the surface of the lining to absorb the noise energy

115
Q

What does the shrouded area on the fan case provide?

A

Provides a way to prevent damage to the fan blades if they come into contact with the shroud due to the surface being made of a soft material

116
Q

What is the shrouded area otherwise known as?

A

The abradable shroud

117
Q

What does the abradable shroud allow the fan to do?

A

Allows the fan blade to run with a very small tip clearance and maximum efficiency

118
Q

What is the fan blade containment made of?

A

Honeycomb construction covered by layers of Kevlar

119
Q

What does the fan blade containment prevent?

A

Prevents the fan blade from penetrating the fan case if it breaks during high speed operation

120
Q

Where are outlet guide vanes installed?

A

In the fan duct behind the fan blades

121
Q

What are outlet guide vanes usually made of?

A

Aluminium or composite materials

122
Q

What is the component between the LPC and the HPC?

A

Fan frame

123
Q

What does the centre hub support and house?

A

The forward bearings of the engine rotors

Houses the internal gearbox on the forward end of the N2 rotor shaft

124
Q

What are Variable Bleed Valves?

A

Used to protect the LPC against surge and bleed off excessive air from the LPC into the fan discharge duct

125
Q

What is an important characteristic about the fan frame struts?

A

Always hollow to save weight and so supply lines can be routed through these hollow struts

126
Q

How does imbalance on the fan rotor occur?

A
  • If material is moved from a blade

- Replacement of fan blades ( due to the difference in moment weights between the fan blades)

127
Q

How can a rotor run free of vibration?

A

If the fan blades that are opposite to each other create identical centrifugal forces

128
Q

What are the 3 parameters that centrifugal forces depend on?

A
  • Rotor Speed
  • Blade Mass
  • Distance of the centre of gravity of the fan blade from the rotational axis
129
Q

What is the moment weight?

A

blade mass x distance of the CoG from the rotational axis

130
Q

Between mass and moment weight, what can change on fan blades?

A

The moment weight can change even if the mass stays the same

131
Q

How can fan blades have a different moment weight but the same mass?

A

The CoG of each fan blade is different due to manufacturing tolerances

132
Q

What units can moment weight be given in?

A
  • gram inches
  • cmg
  • oz inches
133
Q

What information can be found on the blade root?

A
  • Part number (same for all fan blades on the engine type)
  • Serial number (identifies individual blades that have been manufactured)
  • Moment weight
134
Q

How can fan blade no. 1 be identified?

A

By an index mark on the fan disc or by an offset hole

135
Q

What 2 methods are proposed by the engine manufacturers to change single fan blades?

A
  • Replace the blade (best method)

- Balance weights

136
Q

Where are balance weights installed?

A

On the fan disc or in the spinner core

137
Q

What is another name for the compressor case?

A

The diffuser case because the gas path through the case is a divergent duct

138
Q

Which part of the engine is the diffuser a part of?

A

Combustion Case

139
Q

What do the knife edges formed by the stator vanes and the compressor rotor form?

A

A labyrinth seal

140
Q

What does the labyrinth seal formed by the stator vanes and compressor rotor prevent?

A

Prevents reverse airflow through the gap between the rotors and stators

141
Q

What is a CDP seal?

A

Compressor Discharge Pressure seal

142
Q

A CDP seal is always found, where?

A

In the HP compressor

143
Q

What does a CDP seal prevent?

A

prevents the compressor discharge airflow from escaping into the bearing area below the combustion chamber

144
Q

What ways can be used to fit rotor blades to the HP compressor rotor? (3)

A
  • Dove tail slots (Axial)
  • Dove tail slots (Circumferential)
  • Bolts
145
Q

What do blade retainers prevent?

A

Prevent axial movement

146
Q

When would bolts usually be used to fit rotor blades in the compressor?

A

Mainly used for the long blades at the first compressor stages

147
Q

What does the front compressor case suppoer?

A

Structural loads so that the inner rear compressor case does not become deformed

148
Q

How are stator vanes usually installed?

A

In circumferential dove tail slots

149
Q

What is used in stator vanes to minimise vibrations?

A

The inner ends on the vane segments have an inner platform

150
Q

What is the source of pressurised air for the pneumatic system?

A

The HP compressor

151
Q

Where is the bleed air supplied through? (2)

A

The hollow stator vanes or through holes in the outer platform of the stator vanes

152
Q

What do bleed manifolds do?

A

Collect the bleed air from the pick-up holes and supply it to the respective consumers

Engines (4 decks)

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