Combustion Chambers Part 2 - Aircraft Gas Turbine Engines #09

Main ideas / concepts covered

How to remove fuel after start / during blowout

There are two ways to get rid of remaining fuel traces:

1. Fuel drain system
2. Evaporation of remaining fuel traces in the combustor/turbine/jet pipe using a special “blowout” operating mode

Fuel drain system operation

• Drain tubes connect the lowest part of each combustion chamber to the chamber below it.
• After a wet start, the fuel “finds its level” by flowing from the top of the engine downward through the chambers.
• In the bottom chamber, excess fuel exits the engine.

Evaporation of residual fuel

• Any remaining traces must be evaporated.
• The engine is motored over on a blowout cycle:
  • Engine rotation by the starter motor
  • Rotation for the time of a full start cycle
  • High-pressure fuel shut
  • Ignition system deselected
  • Compressor air flows through:
    • combustion chambers → turbine → exhaust system
• This airflow supports evaporation in the combustor area (and downstream).

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Annular combustor system (also called cannula / “can” annular)

Difference vs multiple (can/can-type) combustion chamber system

• The annular system has no individual air casings per flame tube.
• It uses multiple flame tubes inside a single inner and outer air casing, making a more compact unit.

Example described:

• One flame tube between the inner and outer air casing (typical illustration).

Advantages claimed over the multiple chamber system

• Same power output
• Annular chamber length ~75% of the compared can-annular system (as stated)
• No flame propagation problems (as claimed)
• Less air-casing area → less cooling air required
• Higher combustion efficiency → unburned fuel virtually eliminated
  • Promotes oxidation of carbon monoxide → non-toxic carbon dioxide
• Better pressure distribution on the turbine
  • More even turbine loading

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Air–fuel ratio and mixing/burning concepts

Goal

• Achieve maximum heat release by burning the fuel/air mixture correctly.

Chemically correct air–fuel ratio

• Given as 15:1 (air:fuel by weight).
• For gas turbine engines, using ~15:1 in the primary zone avoids detonation/dissociation issues because the turbine combustor lacks the pressure peaks that promote those problems in piston engines.

Burning zones / ratios

• Primary zone: fuel and air mixed and burned at ~15:1
• Secondary and tertiary air: dilute the mixture afterward
  • Overall mixture ratio varies approximately from 4.5–12? up to ~130:1
  • (Subtitles were unclear; the stated range appears to be “between … 45/12? … 130 to 1”.)

Pressure assumption vs reality

• Combustion theory often assumes constant pressure, but in reality:
  • There is small pressure loss from combustor inlet to turbine outlet.
  • Loss is attributed to turbulence/mixing requirements:
    • about 3–8% of entrance pressure during normal running.

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Self-sustaining combustion and ignition needs

• Combustion becomes self-supporting after reaching a self-sustaining speed.
• After start, the engine can accelerate without starter help once that speed is reached.

When ignition may still be required

• Flameout scenarios, such as flame extinction from ingesting large quantities of water (e.g., runway/wheels contamination during takeoff).

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Combustion stability limits (“stability envelope”)

• Stability is defined as smooth burning across:
  • a range of air/fuel ratios
  • a range of air mass flows
• A referenced graph indicates:
  • As air mass flow increases, stable burning exists only within a narrower region
  • Outside that stable region, the flame extinguishes (outside the “ignition loop”)

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Relight (restarting after flameout)

• It is harder to start combustion than to sustain it.
• If flameout occurs at high speed or high altitude, the aircraft may need to slow down and/or descend to enable relight.

Relight envelope

• A “relight envelope” graph is referenced:
  • It shows flight conditions under which relight is guaranteed (assuming the engine is fully serviceable).

Relight mechanism

• Airflow causes windmilling (compressor supplies air for combustion).
• Then:
  • Open high-pressure fuel
  • Use ignition to provide a spark
• Ignition power is controlled by selecting a relight switch.
• The relight ignition circuit electrical power is independent of the normal start circuit.

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Combustion efficiency and why it’s high

• Combustion efficiency = how effectively the combustor extracts fuel’s potential heat.
• Modern gas turbines:
  • up to about ~99% at high power
  • about ~95% at idle

Major contributor: fuel spray nozzles

• Their task is to atomize/vaporize fuel so it burns completely.
• This is described as difficult due to:
  • high compressor air velocity
  • short combustion chamber residence distance
  • low starting fuel pump pressures at engine start

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Fuel pump types and atomization problem at start

Pumps

• Plunger type or gear type
• Mounted to the high-speed gearbox driven by the high-pressure compressor shaft.

During initial start

• Pumps rotate at minimal speed.
• They cannot reach the required ~1,500–2,000 psi for a good spray pattern.

Resulting poor atomization

• A low-pressure spray forms a “bubble” pattern.
• Ignition cannot occur well with that bubble pattern.

As rpm increases

• Pump pressure rises.
• Spray pattern improves toward better atomization (subtitles mention:
  • “clu’lak / tulip” formation)
• Only when the geometry/touches the orifice (as described) does the fuel jet become atomized enough for rapid burning.

Stated tradeoff

• Small fixed orifices work well at high pressure.
• At higher fuel volumes with larger-bore nozzles, the required high pressure becomes unattainable.
• Therefore, other atomization methods are needed when pressures are low.

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Alternative fuel atomization methods (for low fuel pressure starting)

Air-spray system

• Uses a high-velocity airflow to break up fuel.
• Works with relatively low fuel pressure.
• Can use the lighter gear pump.

Duplex (variable-orifice) system

• Nozzle has an orifice of variable size.
• At low fuel pressure:
  • A pressurizing valve closes the main fuel feed
  • Fuel goes through a primary orifice (small) via the primary fuel line
  • Provides fine atomization at low pressure
• As fuel pressure rises during acceleration:
  • Pressurizing valve opens
  • Main orifice adds fuel to supplement the primary spray

Vaporizing tube method

• Fuel is sprayed into vaporizing tubes (tubes are inside the flame tube).
• Primary air enters the flame tube through:
  • the fuel feed tube opening
  • holes in the flame tube entry section
• Fuel is directed/turned 180 degrees.
• Tubes are heated by combustion, vaporizing fuel before it enters the flame tube.

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Wrap-up

• The lesson concludes with combustion chambers.

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Speakers / sources featured

• No specific individual speakers are named in the subtitles.
• Source appears to be an instructional/lecture narration for “Aircraft Gas Turbine Engines” (lesson format).