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Resource Group Mod 15 2024 > 15.5 Combustion > Flashcards

15.5 Combustion Flashcards

1
Q

What is a major part of the energy produced from combustion, used for?

A

To drive the turbines.

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2
Q

What requirements must the combustion chamber meet, in order to provide safe and efficient combustion? (3)

A
  • It must permit safe ignition of the fuel-air mixture both on ground and in flight.
  • Give stable combustion in all operating conditions of the engine.
  • Combustion must also be complete (No unburnt fuel should leave the combustion chamber).
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3
Q

What are the important requirements for the combustion chamber, in terms of temperature and pressure?

A
  • To give equal temperature distribution at the combustion chamber outlet. This is important for the first turbine stage.
  • Combustion should happen with minimum pressure loss in the combustion chamber to increase the engine efficiency.
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4
Q

What are the structural requirements for the combustion chamber? (3)

A
  • Must be as small and light as possible to save weight.
  • It must have a dependable cooling system.
  • Have a high operating lifetime.
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5
Q

What are the two main components of the combustion chamber?

A
  • Casing (inner and outer).
  • Flame tube, inside the casing.
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6
Q

What is the purpose of the combustion chamber casing?

A
  • The outer shield of the combustion section.
  • It takes the air pressure loads and protects engine parts from the hot combustion gases.
  • Gives support of the flame tube and other components (fuel nozzles, igniter plugs).
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7
Q

What is the purpose of the flame tube?

A

Controls and guides the flame.

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8
Q

In basic steps, how does the combustion process occur?

A

Compressed air enters the combustion chamber. Fuel nozzles/injectors spray the fuel into the centre of this airflow.

The fuel vaporises and mixes with the airflow. The fuel-air mixture is then electrically ignited by an igniter plug.

After the combustion has started, the ignition will be switched off. The combustion cycle continues because fuel and air are continuously supplied to the combustion chamber. (self-sustaining).

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9
Q

What are the two primary conditions required for correct and safe combustion?

A
  • The correct air:fuel ratio.
  • Air velocity must be slow enough to ensure combustion is completed fully in the chamber.
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10
Q

What is the air:fuel ratio required for complete combustion?

A

15 parts air: 1 part fuel.

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11
Q

Airflow from the compressor is split into two types, which are…?

A
  • Primary airflow.
  • Secondary airflow.
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12
Q

What is the purpose of the primary air?

A

For combustion.

(About 20-30% of the airflow from compressor).

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13
Q

What is the purpose of secondary air?

A
  • Mainly used for cooling.
  • Keeps flame away from flame tube walls.
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14
Q

Why is the forward part of the combustion chamber, and its entry duct, shaped like a diffuser?

A

To help slow the velocity of the airflow into the combustion chamber.

(This stops the flame from blowing out and helps complete combustion take place).

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15
Q

Prior to the diffuser, what is the approximate airflow velocity that exits the compressor?

A

150m/s.

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16
Q

What velocity does the diffuser slow the airflow down to?

A

25m/s.

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17
Q

What airflow velocity does the combustion chamber require?

A

2 - 15m/s.

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18
Q

Following the diffuser, how is an airflow velocity of 2-15m/s achieved?

A

Use of a restrictor and swirl vanes.

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19
Q

Other than reducing airflow velocity, what other benefit do the restrictor and swirl vanes provide?

A

Generate a vortex that helps the fuel and air to mix.

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20
Q

What are the 3 types of combustion chamber?

A
  • Can.
  • Can-Annular.
  • Annular. (Most common on modern GTEs).
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21
Q

What is the main design of the Can-type combustion chamber?

A

Multiple individual cans (chambers) are used, where each can has its own; Air supply duct, fuel nozzle, flame tube, and casing.

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22
Q

The cans are connected to eachother via ‘Interconnector tubes’. What are the 2 main purposes of these?

A
  • Permit ignition from one can to another. (Only 2 of the cans have igniter plugs).
  • Equalise the pressure between cans.
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23
Q

On what engines are can-type combustion chambers usually used?

A
  • Older engine designs, with centrifugal compressors.
24
Q

What are the advantages of can-type?

A
  • Simple design.
  • Good stuctural strength.
  • Can be replaced individually.
25
Q

Disadvantages of can-type?

A
  • Heavy.
  • Require a lot of space.
  • Complicated air ducts from the compressor cause aerodynamic losses.
  • Ignition from can to can is difficult.
26
Q

What is the main design of can-annular?

A

A set of flame tubes are installed around the combustion chamber, which share a common outer and inner casing.

27
Q

Advantages of can-annular?

A
  • Simpler, smaller and lighter than can-type.
  • Good structural strength.
  • Doesn’t neeed complicated air supply ducts.
28
Q

Disadvantages of can-annular?

A
  • High aerodynamic losses.
  • Very difficult ignition from one flame tube to another.
29
Q

Annular type is most commonly used on modern GTEs. What is the main design?

A

A single annular flame tube, supported by an inner and outer casing.

30
Q

What’s important about the inner and outer casing of an annular flame tubes, in terms of load bearing?

A

These casings are a part of the engine load-bearing structure.

31
Q

What are the inner and outer casings known as?

A

‘Diffuser case’, or ‘compressor rear frame’.

32
Q

Advantages of Annular type?

A
  • Smaller and lighter for the same given airflow .
  • Large combustion area gives better efficiency.
  • Combustion is complete, smokeless, and the flame spreads easily.
  • Better thermal load distribution for the turbine.
33
Q

Disadvantages of Annular type?

A
  • Expensive.
  • Very difficult removal, but have a long service life to counteract this.
34
Q

What 3 main sections make up an annular type combustion chamber?

A
  • Combustion case.
  • Flame tube.
  • External components.
35
Q

What does the combustion case on an annular type chamber do?

A
  • Carries structural loads between compressor case and tubine case.
  • Houses the flame tube.
36
Q

What elements does the combustion case consist of?

A
  • Outer wall.
  • Inner wall.
  • A diffuser.
37
Q

What is a key feature of the outer wall of the combustion case, with regards to bleeding air?

A

It has openings to supply bleed air to pneumatic system, from the last compressor stage.

38
Q

What is the purpose of the flame tube, in annular designs?

A
  • Stabilises the flame during combustion.
39
Q

What are the 3 major parts of an annular flame tube?

A
  • Dome.
  • Inner Liner.
  • Outer Liner.
40
Q

What does the dome section of a flame tube provide?

A

Openings for the fuel nozzles, surrounded by swirl vanes.

41
Q

What feature of the flame tube allows for thermal expansion?

A

They are fixed at one end, and moveable at the opposite end.

42
Q

What are classed as the external components?

A
  • Fuel nozzles.
  • Igniter plugs.
  • Mounting flanges for bleed air tubes.
43
Q

What is film cooling?

A

Where the secondary airflow forms a cooling film on the inner surface of the liners.

44
Q

What’s used on the inner surface of the flame tube to protect it from the heat?

A

Ceramic coatings.

45
Q

What is the definition of a ‘rich mixture’?

A

Too much fuel in the primary air, which leads to unburned fuel entering the exhaust section.

46
Q

What ratio of air:fuel mixture allows for safe combustion?

A

Between 7.5:1 and 24:1.

(air:fuel).

47
Q

What supports the formation of Carbon Monoxide (CO) build-up?

A

Excessive fuel.

48
Q

What supports the formation of Nitrous Oxide emissions?

A

The gas being in contact with the hot zone for a longer period of time.

49
Q

What effect does a high combustion temperature have on the types of emissions?

A
  • Nitrous Oxide emissions increase.
  • CO and hydrocarbon emissions decrease.
50
Q

What effect does a low combustion temperature have on the types of emissions?

A
  • Nitrous Oxide emissions decrease.
  • CO and hydrocarbon emissions increase.
51
Q

What is used to reduce the effects of increasing emissions from either high or low combustion temperatures?

A

Dual Dome combustion chamber.

52
Q

What are the 2 stages of a dual dome chamber?

A
  • Pilot stage.
  • Main stage.
53
Q

What is the pilot stage of the dual dome chamber?

A

The combustion area that is always operating.

54
Q

What is the main stage of the dual dome chamber?

A

Only operates in high power conditions.

55
Q

What aspect of the dual dome chamber helps reduce CO and hydrocarbon emissions?

A

It has better control over air:fuel ratio, so CO and hydrocarbon emissions are reduced.

56
Q

What aspect of the dual dome chamber helps reduce Nitrous Oxide emissions?

A

It’s shorter than a standard combustion chamber, so gases spend less time in the hot zone, reducing Nitrogen Oxide emissions.

Resource Group Mod 15 2024 (13 decks)

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