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Q & A: Types of Flames III

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Most recent answer: 10/22/2007
Q:
Why do we have luminous and non-luminous flames?
- PangJiingHuey (age 13)
Anglican High, Singapore
A:
Pang -

Most flames work by combining molecules with carbon (C) in them with oxygen (O2) from the air to make carbon dioxide (CO2) that's released back into the air.

Luminous flames are the yellow ones. Although they can get at some oxygen, they can't get as much as they need to turn all of the carbon that's being burnt up into CO2. This is why this kind of flame produces soot - since it can't release all of the carbon as CO2, some of it gets released as the black stuff in smoke (soot).

Non-luminous flames are the ones that burn blue. These flames have access to as much oxygen as they could possibly use, so they can burn very efficiently. All of the carbon that gets used can be turned into CO2, so there's actually no soot.

Because luminous flames don't burn as efficiently as non-luminous ones, they don't produce as much energy. This means that the non-luminous flames have a lot more energy than luminous ones, and their flames are actually hotter. This is why the luminous ones look yellow and the non-luminous ones look blue. Hotter flames burn blue and (relatively) cooler ones burn yellow.

So, getting back to your question, the reason that we have both types of flames is simply that not all situations are the same. Either the flame can get enough oxygen or it can't, so it's either non-luminous or luminous.

-Tamara

(published on 10/22/2007)

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