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Gas Turbines > Combustion > Flashcards

Combustion Flashcards

1
Q

What is the purpose of the combustor?

A
  • To release heat at the correct air/fuel ratio
  • Accelerate airflow into a smooth, uniformly heated gas stream while producing a minimum pressure drop.
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2
Q

What do swirl vanes do?

A

Slows the axial velocity of the air by imparting a radial motion for the required air/fuel mixing time

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3
Q

What percentage of the combustor airflow are Primary airflow and Secondary airflow?

A

Primary: 20-25%
Secondary: 75-80%

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4
Q

What is the purpose of primary airflow?

A

Used in the combustion
- 10-15% enter the snout, 10% enter the primary zone through the combustion walls.

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5
Q

What are the 3 purposes of secondary airflow?

A
  • Centers and shapes the flame
  • Provides a cooling air blanket for the liner
  • Dilutes the mixture to a suitable temperature for turbines.
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6
Q

In modern combustion chambers, what are the operating efficiencies at sea level and altitude?

A

Sea Level: 100%
Altitude: 98%

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7
Q

What is combustion stability?

A
  • When the combustor is able to support combustion at a wide range of operating conditions
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8
Q

As the mass airflow is increased, what happens to the range of the rich and lean operating limits?

A

Reduced

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9
Q

What are ignition limits?

A

Similar to combustion stability limits but they lie well within the Stable Region.

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10
Q

Why are ignition limits situated well within the Stable region?

A

Because it is harder to establish combustion when ‘Cold’.
- Operations outside the ignition limits can lead to Flame Out

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11
Q

What is flame out, what are the two types of flame out?

A

When the flame is extinguished
- Rich Flame Out: Too much fuel is sprayed
- Lean Flame Out: Too little fuel is sprayed

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12
Q

How does Rich Flame Out happen?

A
  • Occurs with very fast acceleration
  • Over-rich mixture increases combustor pressure to the point where compressor airflow stagnates and extinguishes the flame.
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13
Q

How does Lean Flame Out happen?

A
  • Occurs at low fuel pressure, low engine speeds or high altitude
  • Sets up weak mixture that is easily blown out
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14
Q

What conditions can trigger a Flame Out?

A
  • Turbulent weather
  • High Altitude
  • Slow Acceleration during maneuvers
  • High speed maneuvers
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15
Q

What percentage does non-pollutants make up from engine emmisions?

A

81%
- Water vapor and inert gases

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16
Q

What percentage does pollutants make up in engine emmisions?

A

0.04% of emissions
- Smoke
- Unburned Hydrocarbons
- Carbon monoxide
- Oxides of Nitrogen (NOx)

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17
Q

How is the majority of pollutants formed?

A

Incomplete combustion and inefficiencies of combustion

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18
Q

How is NOx formed as a pollutant?

A

High flame temperatures converting ozone into nitric acid.

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19
Q

What are the 4 types of combustion chambers?

A
  • Multiple Can
  • Annular (Basket type)
  • Can-Annular
  • Reverse flow annular
20
Q

What is a Multiple can combustion chamber?

A

Individual combustion cans arranged radially around the engine (Older)

21
Q

What are the advantages and disadvantages of a Multiple Can combustion chamber?

A

Advantages:
- Individual cans can be replaced without disturbing other cans
Disadvantages:
- Uneven turbine temperatures
- Extreme temperatures in the turbine if a burner can fail, leading to turbine failure.

22
Q

What is a Annular (basket type) combustion chamber?

A

Consists of a single flame tube which is contained in the inner/outer casing.

23
Q

What are the advantages and disadvantages of a Annular type combustion chamber?

A

Advantage:
-For the same output, length is 75% less than a Can-Annular system (Weight and Cost saving)
- Elimination of flame propagation problems found in Can type chambers.
Disadvantage:
- Maintenance requires engine to be removed from the aircraft.

24
Q

What is a Can-Annular combustion chamber?

A

Consists of a flame tube fitted inside a common outer casing with INTERCONNECTOR tubes for flame propagation.

25
Q

What are the advantages of a Can-Annular combustion chamber?

A

Advantages:
- Allows for on-wing maintenance of the hot section
- Shorter than the Can Type arrangement and has more uniform heat distribution.

26
Q

What is a Reverse Flow Annular combustion chamber and what are its advantages?

A

Where the airflow path is an ‘S’ shape
- Allows for longitudinally compact engine
- Provides pre-heating of compressor air.

27
Q

What is the combustion liner and what is it made of?

A

The perforated inner section where the fuel burning is controlled. Subject to high temperatures and prone to cracking.
- Made of stainless steel

28
Q

Is stop drilling acceptable to stop crack propagation on the liner?

A

No

29
Q

Is minor cracking acceptable on the combustion liner?

A

Yes, it relieves stress in the steel and crack propagation ceases.

30
Q

When marking, what markers can be used?

A

Approved markers.
- Never use graphite, lead, grease, or wax pencils due to carbon and lead becoming infused into the metal at high temperatures.

31
Q

Does liquid jet fuel burn?

A

No, only burns when vaporized or atomized so the fuel can mix with the air and form a combustible mixture.

32
Q

What are the two different types of fuel nozzles?

A
  • Atomizing (splits liquid into small molecules)
  • Vaporizing (turns liquid into vapors)
33
Q

What are the 4 types of atomizing nozzles?

A
  • Simplex
  • Duplex
  • Spill type
  • Air blast type
34
Q

How many spray orifices and spray patterns does a Simplex fuel nozzle have?

A

1 spray pattern and 1 spray orifice.

35
Q

In a Simplex fuel nozzle, what can be used to reduce the velocity of the fuel and improve atomization?

A

Flutes that impart a swirling motion on the fuel.

36
Q

What can be added to a simplex nozzle to help with engine starting?

A

A second, smaller nozzle can be incorporated called the ‘Primer Nozzle’ or the ‘Starting Nozzle’ to spray a very fine mist when starting, then it switches off.

37
Q

How many flow rates and spray patterns does a Duplex fuel nozzle have?

A

2 different rates and flow patterns.
- One for low airflow and one for high airflow.

38
Q

What are the two different types of Duplex fuel nozzles?

A
  • Single Line Duplex
  • Dual Line Duplex
39
Q

How does a Single Line Duplex nozzle work?

A

Receives fuel from 1 inlet port and uses a flow divider to distribute fuel through 2 orifices.
- Centre Orifice (Primary/Pilot Fuel) sprays at wide angle for startup.
- Outer Orifice (Secondary/Main Fuel) opens at a preset fuel pressure to spray at a higher volume and pressure, narrowing fuel pattern to prevent fuel from contacting combustion chamber at high power.

40
Q

How does the Dual Line Duplex nozzle work?

A

Two Inlet Ports: Primary Fuel Inlet and Secondary Fuel Inlet.
- Flow of fuel to the 2 inlets is controlled by the Pressurization and Dump Valve which contains a single flow divider for all fuel nozzles.

41
Q

In terms of fuel dividers, how is the Dual Line Duplex fuel nozzle different from the Single Line Duplex nozzle?

A
  • A Dual Line Duplex has a single fuel divider for all fuel nozzles (A.K.A Pressurization and Dump Valve)
  • A Single Line Duplex has a flow divider for each nozzle.
42
Q

What does the Fuel Pressurization and Dump Valve do?

A

Acts as a central fuel divider for the Dual Line Duplex nozzle.
- The pressurization component controls fuel flow during starting and ground idle.
- The dump valve is used to drain the manifold after combustion and prevents left-over fuel from boiling and leaving contaminants to clog fuel passageways.

43
Q

What is the Spill Type nozzle?

A
  • A type of Simplex nozzle which has a passage from the swirl chamber for fuel spilling away?
44
Q

What is the advantage of a Spill Type nozzle?

A

Allows the fuel supply to the nozzle to remain at a high pressure at all stages of the engine operation as excess fuel is spilled away prior to passing the atomizing nozzle.

45
Q

What is an Air Blast nozzle and what is the primary use?

A

Uses a high velocity airflow to enhance atomization
- Most effective during starting operations.

46
Q

How does the Vaporizing nozzle work?

A

Premixes the air and fuel using combustor heat to vaporize the fuel before entering combustion.

47
Q

What is a disadvantage of the Vaporizing nozzle?

A

Does not provide an effective spray pattern during engine starting.
- A small set of atomizing nozzles are used during starting to overcome this.

Gas Turbines (5 decks)

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