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Systems Part 4 > Final Exam BGT content > Flashcards

Final Exam BGT content Flashcards

1
Q

What is the Brayton Cycle?

What is the Brayton Cycle widely known as?

A
  • Continuous thermodynamic cycle of the gas turbine engine.
  • Constant pressure cycle because pressure remains fairly constant across the combustion section as volume attempts to increase, increasing gas velocity.
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2
Q

Describe the Brayton Cycle vs The Otto Cycle in terms of how the 4 processes differ?
Draw a diagram.

A
  • Brayton Cycle and the Otto Cycle both have the same 4 sequences: Induction, Compression, Combustion and Exhaust.
  • INDUCTION: For the Brayton Cycle pressure is ambient and volume reduces. For Otto Cycle, Volume decreases, while pressure increases.
  • COMPRESSION: For the Brayton Cycle, pressure increases, while volume decreases. For Otto Cycle, Volume remains the same but pressure continues to increase further.
  • COMBUSTION: For the Brayton Cycle, pressure reduces slightly, but remains fairly constant, this is due to the addition of heat and burning of heat at constant pressure, volume increases slightly. In Otto Cycle, pressure also decreases as volume increases.
  • EXPANSION: For Brayton Cycle, gases resulting from combustion expand through the turbine and jet pipe back to atmosphere. Some of the energy in the expanding gases is turned into mechanical power by the turbine, remainder provides a propulsive jet.
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3
Q

Describe the Brayton Cycle vs The Otto Cycle in terms of similarities and their differences. (6)

A
  • Combustion occurs at constant pressure for Brayton Cycle, but constant volume for Otto Cycle.
  • Brayton cycle is a continuous cycle whereas the Otto cycle is intermittent.
  • Only one stroke is utilised in the production of power for the Otto Cycle, others involved in charging, compressing and exhausting of the working fluid.
  • Brayton Cycle removes the three “idle” strokes, enabling for more fuel to be burnt in a shorter time, therefore greater power output for a given size of engine.
  • No peaks or fluctuating pressures for Brayton Cycle but Otto Cycle can reach peak pressures of 1000 PSI. It is those peak pressures which make it necessary for the piston engine to use high octane fuels and cylinders of heavy construction.
  • No reciprocating parts for the Brayton Cycle.
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4
Q

What is heating at constant pressure?

A

Due to the continuous action of the turbine engine and that the combustion chamber is an open space, pressure of air does not rise, but volume does increase, this is heating at a constant pressure.

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5
Q

What are the advantages of a centrifugal flow compressor compared to an axial flow compressor?

A
  • More robust.
  • Easier to develop and manufacture.
  • Favoured for smaller engines due to its simplicity and ruggedness outweigh the disadvantages.
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6
Q

What are the advantages of an axial flow compressor compared to a centrifugal flow compressor?

A
  • Easier to maintain.
  • Consumes more air for the same frontal area.
  • More thrust for the same frontal area because consumes more air.
  • Designed to attain higher pressure ratios.
  • The ability to increase pressure ratios by addition of extra stages has led to the adoption of this compressor in most heavy aircraft.
  • Improved SFC for a given thrust.
  • Improved efficiency.
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7
Q

What is the construction and function of inlet guide vanes?

A

Vanes are stationary and the function is to direct the airflow into the rotor at the most desirable angle.

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8
Q

What is the purpose and construction of rotor blades?

Where are they located in terms of compression?

A
  • Purpose is to move air rearward through each stage.
  • Rotor blades are located at the first stage of compression.
  • They are of an aerodynamic design, constructed with a varying angle of incidence or twist.
  • Rotor blades are the rotating blades within the compressor.
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9
Q

What is the purpose and construction of stator blades?

A
  • Purpose is to receive the high-speed airflow from the airflow and act as a diffuser, changing the K.E. to potential energy in the form of pressure.
  • Shaped like an aerofoil and are stationary.
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10
Q

What does the Bypass ratio compare?

What engines utilise it?

A
  • Compares mass airflow through the bypass duct to the mass flow through the primary duct in the same time.
  • Turbo-jet and turbo-fan.
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11
Q

What is the magnitude of each flow a function of?

A
  • Size of the bypass duct.
  • Airflow velocity.
  • Primary flow intake.
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12
Q

What is the bypass ratio expressed as?

A

Ratio of the bypass flow to the core engine flow.

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13
Q

What does a 5:1 bypass ratio represent?

A

5 parts of bypass flow to 1 part of core engine flow.

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14
Q

What are the typical bypass ratios?

What is the bypass ratio of modern engines?

A
  • 2:1 and 5:1.

- Modern engines are reaching 30:1.

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15
Q

What are the advantages of a multi-spool compressor compared to a single-spool axial flow compressor? (5)

A
  • Dual and triple axial flow compressors were designed for the operational flexibility they provide to the engine.
  • Operational flexibility comes in the form of higher compression ratios, quick acceleration and better control of stall characteristics.
  • Main advantage is that when the aircraft is at altitude, the N1 compressor consumes more air, allowing the compressor to speed up.
  • This has the effect of recovering the subsequent pressure loss due to the rarified atmosphere through the N2 compressor (HP compressor).
  • N2 and N3 maintain optimum RPM.
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16
Q

What is a compressor stall defined as?

What are the symptoms/warnings of a compressor stall?

A
  • Abrupt loss of efficiency of the axial flow compressor due to the angle of attack of the compressor becoming too excessive.
    No warnings until the stall:
  • Engine sneeze.
  • Load bang.
  • Vibrations from surge.
  • High EGT.
  • Temperature, fuel flow or thrust fluctuation indications.
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17
Q

What are the 6 causes of a compressor stall?

A
  1. ) Turbulent or disrupted airflow to the engine inlet (reduces gas velocity).
  2. ) Excessive fuel flow caused by abrupt engine accelerations (reduces gas velocity due to increased combustion backpressure).
  3. ) Excessive lean mixture caused by abrupt engine decelerations (increases gas velocity due to reduced combustion backpressure).
  4. ) Damaged or contaminated compressors (increases gas velocity by reducing compression).
  5. ) Damaged turbine components, causing loss of power to the compressor and low compression (increases gas velocity by reducing compression).
  6. ) Operating above or below the designed RPM (incorrect RPM setting for phase of flight).
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18
Q

Generally, what is a compressor stall caused by?

A

An imbalance between the two vector quantities; inlet velocity and RPM.

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19
Q

What happens to the air inside the compressor during a compressor stall?

A

Compressor stalls cause air flowing through the compressor to slow down, stagnate (stop), or reverse flow depending on the stall intensity.

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20
Q

How does the sound of a compressor stall vary in audibility from light to severe?

A
  1. ) Air pulsating/fluttering type sound.
  2. ) Louder pulsating type sound.
  3. ) Sound of violent backfire.
  4. ) Explosion.
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21
Q

What happens on the compressor blades when the AoA is too high or too low?
Draw a simple vector diagram representing constant compressor RPM but with a fast and slow axial air velocity. Indicate the most critical angle of attack.

A
  • High AoA: Turbulent airflow separates from blade

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

22
Q

What is a transient stall?

What is a hung stall?

A
  • Transient stall is a mild stall condition which the cockpit gauges do not show.
  • Transient stalls are not usually harmful to the engine and often correct themselves after one or two pulsations.
  • Severe stalls are called hung stalls.
  • They can significantly decay engine performance, cause loss of power, or even damage or cause the engine to fail.
23
Q

What causes a severe compressor stall or surge?

What happens in a severe compressor stall or surge?

A
  • Fuel system malfunctioning or foreign object ingestion.
  • A reversal of airflow occurs with such force that bending stresses on the rear of compressor blades can cause them to contact the stator vanes.
  • At that point, a series of material failures can result in total disintegration of the rotor system and complete engine failure.
24
Q

What is an example of disrupted airflow to the inlet?

A
  • Aircraft with two engines mounted on the rear fuselage.
  • If the aircraft does a sharp right-hand turn, airflow to the left engine may be blocked by the aircraft fuselage.
  • Sideslip could cause low inlet air velocity and a momentary increase in effective AoA sufficient to create a compressor stall.
25
Q

What is the function of the turbine engine?
Where are the gases at their hottest temperature?
What does the turbine do to the airflow/gases in terms of energy?

A
  • Function of the turbine is to drive the compressor, accessories and the shaft of turbo-prop and turbo-shaft engines.
  • This does it by converting kinetic and heat energy from exhaust gases into mechanical work.
  • Due to the position of the turbine, immediately following combustion, gases are at their hottest at the entry point of the turbine//compressor outlet.
  • Turbine extracts energy from flowing gases by reducing the pressure.
  • This occurs because the pressure is converted to velocity by the convergent nozzles formed at the T.E. of the stator and rotor blades.
26
Q

Describe the impulse blade design?

A
  • Total pressure drop across each stage occurs in the fixed nozzle guide vanes due to the convergent shape, increases gas velocity and reduces the pressure.
  • Gas is directed to the turbine blades, which experience an impulse force, caused by the impact of the gas on the blades.
27
Q

Describe the reaction blade design?

A
  • Fixed nozzle guide vanes are designed to alter the gas flow direction without changing the pressure.
  • The converging blade is turned by a reaction force resulting from the expansion and acceleration of the gas.
28
Q

Describe the impulse-reaction blade design?

What is the most efficient ratio?

A
  • Consists of rotor blades with compound curvature.
  • Twist in the blade is used to distribute the workload evenly along the blade length by keeping the exit pressure and velocity uniform from base to tip.
  • Accomplished by extracting different amounts of kinetic energy at various blade stations.
  • Most efficient is 50% reaction and 50% impulse, with the reaction being greatest at the tip and impulse being greatest at the root.
29
Q

Describe the airflow through a propelling nozzle?
Include the speed at the exit and under what conditions, the term choked and how additional thrust is obtained from the propelling nozzle?
(6)

A
  • Exhaust gases pass to the atmosphere through the propelling nozzle, and because of their convergent duct, it increases the gas velocity.
  • In a turbo-jet engine, the exit velocity of the exhaust gases is subsonic at low thrust-conditions only.
  • During most operating conditions, the exit velocity reaches the speed of sound, propelling nozzle is said to be ‘choked’, meaning no further increase in velocity can be achieved, until temperature increases.
  • As the upstream total pressure increases above the value at which the propelling nozzle is ‘choked’, the static pressure at the exit increases above atmospheric pressure.
  • This pressure difference across the propelling nozzle gives ‘pressure thrust’ and is effective over the whole nozzle exit area.
  • This is additional thrust obtained due to the momentum change of the gas stream.
30
Q

How does a convergent-divergent duct work?

Why are they required?

A
  • Necessary as with the convergent nozzle, a waste of energy occurs since the gases leaving the exit do not expand rapidly enough to immediately achieve outside air pressure.
  • Some high-pressure ratio engines can, with advantage, use a C-D nozzle to recover some of the wasted energy.
  • This nozzle utilises the pressure energy to obtain a further increase in gas velocity, thus, increase in thrust.
31
Q

List the requirements of oils and describe why they are required?
What type of oil is used in aircraft?

A
  1. ) Low volatility: Prevent evaporation at high altitudes.
  2. ) Anti-foaming qualities: Provide positive lubrication at all operating conditions.
  3. ) Low lacquer and coke deposits: To keep solid particle formation to a minimum.
  4. ) High flashpoint: The temperature at which, when heated, flammable vapours will ignite if there is a flame source.
  5. ) Low pourpoint: The lowest temperature at which the fluid will flow.
  6. ) Film strength: Excellence qualities of cohesion and adhesion, a characteristic of the oil molecules allowing them to stick together during compression loads and to stick to the surface during centrifugal loads.
  7. ) Wide temperature range: Between -60ºF to + 400ºF.
  8. ) High viscosity index: An indication of how well the oil keeps its viscosity when it is heated to its operating temperature.
    - Synthetic anti-freeze oils are used.
32
Q

What is the purpose of magnetic plugs and chip detectors?

A

These are fitted into the return side of the oil system to collect any ferrous debris that the returning oil may be carrying.

33
Q

How does a magnetic plug work and where are they fitted?

A
  • Is a permanent magnet inserted into the oil flow to collect debris.
  • Can be readily moved and inspected, providing an early warning of impending failures.
34
Q

How does a chip detector work?

A

Magnetic plugs that incorporate an electronic device, transmits a signal to the cockpit in the event of significant collection or significant size of debris on the plug.

35
Q

What is the FMU? How does it work?

A
  • An engine-driven accessory which can operate by:
  • Mechanical forces.
  • Hydraulic forces.
  • Pneumatic forces.
  • Electrical forces in various combos such as hydropneumatic, hydro-mechanical or electro-hydromechanical.
36
Q

What devices are designed into the FMU to limit the engine from exceeding set parameters?
How is this achieved?
How is control of FMU achieved?

A
  • Pressure control.
  • Flow control.
  • Pressure ratio control.
  • Acceleration.
  • Speed control.
  • Achieved through a governor.
  • In modern aircraft, control of FMU is achieved through a computer, not directly from the pilot.
37
Q

What is the final component of a fuel system?
What are their alternative names?
What is their task?

A
  • Fuel nozzle.
  • Also called injectors or distributors.
  • Task of atomising or vaporising the fuel to ensure rapid burning.
38
Q

What are the components of a turbojet fuel system?

A
  • HP Fuel pump.
  • Fuel flow regulator.
  • LP shaft governor.
  • HP shutoff cock.
  • Temp control actuator.
  • HP Compressor delivery pressure limiter.
  • EGT amplifier
39
Q

Describe the simplex type fuel nozzle?

A
  • Used on early jet engines.

- Consists of a chamber, inducing a swirl into the fuel, and a fixed-area atomising orifice.

40
Q

Describe the duplex type fuel nozzle?

A
  • Have a primary and main flow tube which have two independent orifices, one much smaller than the other.
  • Smaller orifice handles the lower flows.
  • Larger orifice handles the higher flows as fuel pressure increases.
41
Q

What is the principle of operation of afterburning?

When is AB primarily used and why?

A
  • Introduction and burning of more fuel in the exhaust gases between the turbine and propelling nozzle.
  • Increased temp of exhaust gases results in increased jet velocity and therefore more thrust.
  • Mainly used to improve takeoff and climb performance of the aircraft.
42
Q

How is additional energy provided with regards to AB?

A

Products of combustion, which were cooled prior to the entry of the turbine are utilised.

43
Q

How is it possible to achieve combustion for a second time with regard to AB?

A
  • Because only 30-40% of the air was burned entering the combustion chamber.
  • Plenty of oxygen left to burn for further expansion.
44
Q

How are the spray bars arranged in an afterburner? Why?

How is cooling provided?

A
  • Because the temperature of the AB flame is roughly 1700ºC, spray bars are arranged so that the flame is concentrated along the wall of the pipe.
  • Results in a portion of unburned air used in combustion, remainder flows along the walls of the jet pipe providing a cooling barrier.
45
Q

What is the power output of the gas turbine proportional to?

What key factors affect this?

A
  • Directly proportional to mass airflow therefore air density.
  • Increase in air temperature or altitude will cause a reduction in power produced/available (reduction in mass airflow).
46
Q

Describe the principle of operation of water injection?

A
  • Water or methanol can be added to the engine airflow.
  • This increase air density, through cooling, therefore restoring its output or increasing it.
  • Water can spray in the compressor inlet or combustor inlet.
47
Q

What happens if water is completely consumed during take-off with regard to water injection?
What about if not all water was completely consumed for the scenario?

A
  • Pure water is used to cool the mass airflow if all water is consumed during take-off.
  • For water that is not all used up during the take-off roll, methanol is added for its anti-freeze properties so water does not freeze at high altitudes. (mainly turboprops and helicopters).
48
Q

What are the methods of injecting coolant into the airflow? Which is preferred and why?

A

Two methods:

  • Have coolant sprayed directly into the compressor inlet.
  • Injection of coolant into the combustion chamber inlet is more suitable for axial flow compressor.
  • LATTER is preferred as more even distribution can be obtained and more coolant can be injected.
49
Q

What happens when water/methanol mixture is sprayed into the compressor inlet? What about if it was only water?

A
  • The temperature of the compressor inlet air reduces, thus density and thrust are increased.
  • If only water was injected, it would reduce the TIT, but with the addition of methanol, TIT is restored by burning the methanol, thus power is restored without having to adjust fuel flow.
50
Q

How does the injection of coolant into the combustion chamber inlet provide additional thrust?

A
  • Increases mass flow through the turbine, relative to the compressor.
  • Pressure and temp drop across the turbine is reduced, resulting in an increased jet pipe pressure, giving additional thrust.
  • Reduction in TIT due to water injection enables the fuel system to schedule an increase of fuel flow.

Systems Part 4 (6 decks)

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