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Part 4 Systems > Final Exam - Basic Gas Turbine > Flashcards

Final Exam - Basic Gas Turbine Flashcards

1
Q

What is the Brayton Cycle?

A

A constant pressure cycle where air is the working fluid of the engine
Made up of 4 continuous Induction, compression, combustion, exhaust cycles

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2
Q

Describe the Brayton Cycle in terms of Pressure and Volume? Where do they occur in the engine?

A

Compressor: Volume decreases and pressure increases
Combustion: Volume increases and Pressure decreases slightly
Turbine/Exhaust: Volume increases and pressure decreases
Exhaust exit: Pressure is ambient and volume is greater than ambient, air is mixed to atmosphere to return to normal volume

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3
Q

What are the advantages of an axial flow compressor?

A
  • Consumes more air for same frontal area
  • Can attain higher pressure ratios
  • More thrust for the same frontal area
  • Can add stages to further increase pressure ratios
  • Improved efficiency
  • Better SFC for a given thrust
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4
Q

What are the advantages of centrifugal compressors? When are they used?

A
  • More robust
  • Easier to develop and manufacture

Favoured for small engines that require ruggedness and simplicity

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5
Q

What is the purpose of the inlet guide vanes, rotor blades and stator blades in the compressor? Describe their physical design?

A

IGV: stationary and designed to direct airflow onto the first rotor at the most desirable angle
Rotor: Move air rearward, they are aerodynamically designed with some blade twist
Stator: Receives the high velocity air from rotor blades and act as a diffuser. Changes kinetic energy to potential energy. Aerodynamic design

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6
Q

What is the Bypass ratio? What engines have it?

A

Ratio of mass airflow through the bypass duct to the mass airflow through the primary gas path in the same timeframe.
Found on all turbofan and some turbojet aircraft

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7
Q

What does a bypass ratio of 5:1 represent?

A

5 parts of bypass air for every 1 part of core air

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8
Q

What is bypass ratio a function of?

A

Size of the bypass duct and core intake

Velocity of the air

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9
Q

What bypass ratios are being obtained on modern engines?

A

30:1

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10
Q

What are the benefits of a multi-spool axial compressor?

A
  • Operational flexibility
  • High compression ratios
  • Quick acceleration
  • Better control of stall
  • Allows LP compressor (N1) to speed up as air density reduces, while the N2 and N3 compressors remain at optimum RPM
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11
Q

What isa compressor stall?

What symptoms or warnings may be present?

A 
The abrupt loss of efficiency of the axial flow compressor when the AoA of the compressor blades becomes excessive.
No warnings until the stall:
-Engine sneeze/bang
-Vibration
-High temp
-Fluctuating fuel flow & thrust
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12
Q

Generally, what is a compressor stall caused by?

A

Imbalance between the engine RPM and the Inlet velocity

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13
Q

What happens inside the compressor when it stalls? How do they vary in severity?

A

-Airflow slows down, stops or reverses direction

Transient stall is mild and will not normally damage engine, and can correct after several pulses
Hung stall is severe and can cause damage, loss of power, and reduced performance

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14
Q

What happens on the compressor blades when the AoA is too high or too low?

A
  • High AoA: Turbulent airflow separates from blade

- Low AoA: Very little or no low pressure zone is created

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15
Q

What are the causes of a compressor stall? (6)

A
  • Turbulent/disrupted inlet airflow (Reduce gas velocity)
  • Excessive fuel flow due to engine accelerations (Reduced gas velocity, Increased combustion back pressure)
  • Excessive lean mixture caused by abrupt deceleration (Increased gas velocity, Reduced combustion back pressure)
  • Damaged or contaminated compressors (Increased gas velocity by reducing compression)
  • Damaged turbine components, causing loss of power to compressor (increase gas velocity by reducing compression)
  • Operation outside RPM envelop
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16
Q

What situations may cause a compressor stall? How can the be corrected?

A
  • Side slipping with rear engines: Reduced inlet velocity, pilot reduce power and RPM will match inlet velocity
  • FOD, or FCU malfunction, back pressure may bend blades, causing them to touch and creating total engine failure.
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17
Q

What is the purpose of the turbine? What does it do to the airflow?

A
  • Function is to drive the compressor, accessories and drive shaft
  • Converts kinetic and heat energy into mechanical work
  • Extracts energy by reducing the pressure through convergent nozzles at the rear of stators and rotors
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18
Q

Describe the impulse blade design in the turbine?

A
  • Stators form convergent ducts and reduce pressure and increase velocity
  • Turbine blades form straight ducts and are rotated due to the reaction to the impulse of the airflow.
  • Total pressure drop is in stator stage
19
Q

Describe the Reaction blade design in the turbine?

A
  • Stators form straight ducts and only change direction
  • Rotors form convergent ducts and accelerate and expand the gas creating a reaction force that turns the rotor
  • All pressure reduction is done in the rotors
20
Q

Describe the Impulse-Reaction blade design in the turbine? What is the best ratio to use?

A

-Most common
-Combination of Impulse and reaction principle.
Rotors have compound curvature with some blade twist to evenly distribute load
-Exit pressure and velocity at rotor trailing edge is uniform along blade length
-Best ratio to use is 50:50 impulse to reaction.
Reaction greatest at the tip and impulse greatest at the root

21
Q

Describe the airflow through the propelling nozzle? what is its shape?

A
  • Airflow velocity increases due to converging duct
  • Airflow exits at subsonic speeds at low power settings only
  • Normally airflow velocity will reach sonic and airflow velocity cannot be increased further
  • When shockwave forms the propelling nozzle is choked and speed can only be increased by increasing temp.
22
Q

Describe how additional thrust is obtained from the propelling nozzle?

A
  • Upstream pressure increases when the engine becomes choked
  • Pressure is greater than ambient and pressure thrust obtained
  • Acts over the area of the exit of the nozzle
23
Q

How does a divergent-convergent exhaust duct work? Why are they required?

A

Required to recover some wasted energy due to the gasses not expanding rapidly enough to immediately achieve outside air pressure
D-C exhaust duct will recover some energy to to create a further increase in gas velocity, thus increasing thrust

24
Q

What are the requirements of the oils in a gas turbine engine? Why is each required? What is temp range required? What oils achieve this?

A
  • Low Volatility: minimise evaporation
  • High flash point: reduce flammable vapours
  • Anti-foaming: provide positive lubrication
  • Low coke and lacquer deposits: minimise solid particulate formation
  • Wide temperature Range: -60˚F - 400˚F
  • Low pour point: flow easily in all temps
  • Film strength: molecules stick together under compression and centrifugal loads
  • High viscosity index: retains its viscosity well

All are achieved using synthetic anti-freeze oils

25
Q

What is the purpose of the magnetic chip and plug detectors in the oil system?

A
  • Collect any ferrous debris that oil may have collected

- Can be inspected to provide early warning of impending failures

26
Q

How do magnetic chip and plug detectors work? Where in the system are they located?

A

Magnetic plug: permanent magnet in oil flow, that can be inspected easily.
Chip detectors: magnetic plug with electronic device which transmits signal to cockpit in event of significant buildup, or when large debris is sensed.

Located at the return side of the oil system, after it has been through the engine

27
Q

What must a fuel system be able to do?

A
  • Provide engine with fuel in suitable form for combustion
  • Control the fuel flow to required quantity in all operating conditions
  • Allow easy starting, stable running and smooth acceleration
28
Q

How does the fuel system meets its requirements? With what equipment?

A
  • 1 or more fuel pumps deliver fuel
  • fuel spray nozzles deliver atomised spray to combustion chamber
  • Fully automatic controlling devices except for power selection
29
Q

What is the FMU? How can it be driven?

A

Fuel metering unit. Engine driven accessory which can operation mechanical, hydraulic, electric or pneumatic forces, or a combination (hydro-mechanical etc)

30
Q

What devices are designed into the FMU to limit engine from exceeding parameters?
What device is used to provide this control? What input does the pilot have?

A
  • Pressure control
  • Flow control
  • Pressure ratio control
  • acceleration control
  • Speed control

Achieved through the use of a governor.
Pilot does not have control of FMU, rather the aircraft computer does this task

31
Q

How is power of an engine controlled?

A

-By changing the amount of fuel delivered to the engine

32
Q

What components are there in a turbojet fuel system? What does each one do?

A
  • HP fuel pump
  • Fuel flow regulator: Regulated fuel flow by various inputs
  • LP shaft governor
  • HP shutoff cock
  • Temp control actuator: Actuated fuel flow regulator based on EGT amplifier
  • HP compressor delivery pressure limiter: Can limit fuel based on compressor delivery pressure
  • EGT amplifier: can limit fuel based on EGT
33
Q

What are some alternate names for the fuel nozzles? What is their task?

A

injectors
distributers

Must atomise or vaporise the fuel to ensure its rapid burning

34
Q

Describe the Simplex fuel nozzle?

A
  • Early system used
  • Fuel passes check valve and screens to enter swirl chamber
  • Chamber which induces swirl to fuel by applying it tangentially to open ring exit passage
  • Fixed area atomising orifice is the swirl exit passage
35
Q

Describe the Duplex fuel nozzle?

A
  • Has single fuel inlet line (or 2 in dual manifold variant)
  • Primary and main flow tube with independent orifices
  • Smaller orifice deals with lower flows
  • Larger orifice deals with higher fuel flows as pressure increases
36
Q

What is the principle of operation of after-burning? When is after-burning mainly used?

A
  • Introduction and burning of more fuel in the exhaust gasses between the turbine and propelling nozzle
  • Increased temp results in increased jet velocity and therefore more thrust
  • Mainly used during take-off and climb.
37
Q

How is it possible for combustion to occur a second time?

A

Because only 30-40% of air was burned during combustion.

Plenty of O2 still left to be burned and expanded

38
Q

How hot is the after-burning flame? How is cooling provided?

A

1700˚C +
-Spray bars arranged axially so only a portion of air is combusted, and a cooling layer along the walls of the jet pipe remains

39
Q

What is thrust of a gas turbine proportional to? What key factors will affect this?

A

Directly proportional to mass airflow

Temp will affect this, with hot temps and high altitudes will reduce mass airflow

40
Q

What is the principle of operation of water injection? What can it do to power output? What if not all the coolant is used during take-off?

A
  • Water or methanol can be added to to increase the air density through cooling
  • Can restore or increase engine power output
  • Methanol added to provide anti-freeze capability if not all water is used. during takeoff

-Water can spraying in the compressor inlet or into the combustor inlet

41
Q

What location is better for water injection? Why?

A

Into the combustor inlet

  • Better for axial flow engines because:
  • More even distribution
  • More coolant can be injected
  • Will increase mass airflow over turbine relative to compressor.
  • Temp drop across turbine is reduced and Jet pipe pressure is increased giving more thrust
  • The reduction in TIT due to water injection, will allow more fuel to be scheduled to increase flow and thus increasing rotational speed of engine
42
Q

What are the benefits of injecting water/methanol mix over just water?

A
  • If just water was injected, TIT would be decreased
  • Water methanol will cool the air increasing density and thrust, but by burning the methanol the TIT is restored, without having to adjust the fuel flow
43
Q

What are the components of the water injection system?

A
  • Coolant tank
  • Control unit
  • Metering valve
  • Air driven turbine pump
  • Spray nozzles

Part 4 Systems (4 decks)

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