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During start phase of a jet engine, igniters in the combustion chamber create sparks to initiate combustion. As far I know, when the engine reaches self sustaining speed, igniters get turned off (this might be different (takes longer) in an Airliner due to FAA regulations during take off).

My question is how do flames in the combustion chamber (or canisters) get sustained (like a candle) without igniters support?

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Another answer to add to caseys:

  1. Flow speed near the fuel injectors is very slow, on the order of 30 m/s. This is caused by a widening of the flow path between compressor exit and combustor called a diffusor or diffuser. See here for more.
  2. Radiation from the burning fuel-air mixture in the middle part of the combustor heats up the not-yet-burning mixture near the fuel injectors, which helps the fuel droplets to evaporate (this is a necessary step before combustion can start) and causes the mixture to be heated above its autoignition temperature. Hence, this radiated heat will provide the fuel-air mixture with the needed activation energy for combustion.

Some combustors have small notches in their wall which create local turbulence and help to keep some of the burning fuel-air-mixture back, which helps to start ignition in the following flow. These are called flame holders.

If all of the burning gas is washed out from the combustion chamber without igniting the following gas, the ignition process will stop. This is called a flame-out. The heat from the walls of the combustion chamber will not be enough to re-ignite the fuel-air mixture, because they are cooled by the airflow from the compressor and will rapidly cool down once the flame is blown out.

Note that this effect is used to extinguish burning oil or gas wells with dynamite.

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How does a candles flame sustain itself after you have lit it? It stays lit by continually burning fuel. The jet engine works the same way. Once the fire is lit (by the ignitors) it is constantly burning. Air is constantly being supplied by the compressor and fuel is constantly being supplied by pumps.

This is fundamentally different to a traditional combustion engine you might be trying to reconcile this idea against. In a 4-stroke internal combustion engine, for example, you have intake, compression, power and exhaust happening separately and you need spark plugs (or glow plugs) to ignite the fuel each cycle. In a jet engine, however, instead of 4 independent cycles you have a constant flow of air through the engine. Air comes in the front, is compressed by the N2 stage and fed continually into the hot section where the fire is constantly burning and then exhausts through the back of the engine.

Look at it like a propane barbecue grill or a gas stovetop -- you only need the ignitor to start the fire. Once the fire is burning it stays that way until you turn off the flow of gas.

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    $\begingroup$ I think the asker is wondering why the 500mph airflow through the jet engine doesn't simply blow out the flame -- if you blow hard enough on a candle or a BBQ grill, you'll blow out the flame... why doesn't the same happen in a jet engine? I suspect that the answer involves the compression stage where the airflow is slowed as it's compressed before fuel is injected and the mixture is ignited in the combustion chamber which constrains the burning gases to allow combustion before exiting through the turbine? $\endgroup$ – Johnny Jan 24 '15 at 1:56
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    $\begingroup$ @Johnny Thanks for elaboration. Yes, I exactly wonder why high speed air from compressor does not blow out the flame. I just wonder if the temperature of the compressed air has a role for ignition? Or compressed air in 30000 feet has enough temperature to ignite the fuel? $\endgroup$ – albin Jan 24 '15 at 2:22
  • $\begingroup$ I have the same question as the OP, and yeah, this doesn't answer the question for the same reason as in the above comments. $\endgroup$ – Mehrdad Jan 24 '15 at 11:22
  • $\begingroup$ Minor point - glow plugs are only used to help start a diesel engine (when it is below operating temperature). After that it is self-sustaining, albeit for different reasons than for a jet engine. A diesel/air mixture ignites spontaneously simply by result of the heat of compression. $\endgroup$ – J... Jan 24 '15 at 13:11
  • $\begingroup$ Further reading here $\endgroup$ – vasin1987 Jan 24 '15 at 13:17
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To add to casey's post, the flame holder keeps the flame from getting blown out. Its a part of the combustion chamber that allows the high pressure air from the final compression stage to swirl at a low speed to keep the flame front contained.

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  • $\begingroup$ As far as I know the flame holder is placed right after gas turbine, not in the combustion chamber. $\endgroup$ – albin Jan 24 '15 at 2:24
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    $\begingroup$ @albin: The flame holders behind the turbine are for the afterburner and much bigger than the ones in the combustion chamber. $\endgroup$ – Peter Kämpf Jan 24 '15 at 7:29

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