The Standing Invitation

Posts Tagged ‘Combustion

Burning Curiosity

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Something about writing this thesis makes me think of setting things on fire. This prompted me to wonder exactly what a flame is.

Although a flame appears to be a stable, defined structure, we know that this is an illusion. Particles enter at the bottom and leave at the top, becoming visible for only a part of that journey; the region of space in which they’re visible we call a ‘flame’. It’s rather like a queue: it has a shape, a duration and a certain characteristic behaviour, but nothing about it is permanent. (Remember how no part of our bodies is the same after 20 years…?)

So a flame is really a time-averaged aggregate of microscopic events. But what light-emitting events lead to the thing we call a flame?

Candle wax is made of long-chain hydrocarbons that have a low melting point. Heat turns wax from a white solid to a clear liquid, and then to a gas. Heat rises, and the gas molecules are carried upwards. When they are hot enough, they react with oxygen to form carbon dioxide and water vapour, like this:

 

But this is too simple. The process of combustion is incredibly complex, with countless steps and short-lived intermediates – something a bit more like this.

Many of these reactions give off heat. The heat excites electrons in nearby molecules, and these electrons relax back to their original positions by emitting light. The colour of the light given off depends on how hot the molecule was. It’s how astronomers measure how hot the stars are, and what they’re made of.

The hottest part of a candle flame is the very bottom, where oxygen is plentiful and there is a high density of heat-generating combustion reactions occurring per second, driving up the temperature. The molecules here burn at about 1400 °C and give off a hot blue colour.

These very efficient reactions near the bottom effectively starve the rest of the flame of oxygen. Oxygen still gets in through the sides, but not as efficiently. The result is incomplete combustion: the wax is converted into particles of soot carried upwards by the current of air generated by the heat. These are still hot enough to glow, but the temperature is much lower, and so this region of the flame is a cool yellow.

The flame is only the part of the process we can see. It is misleading to see that soot particles emit light within the flame; better to say that the flame-space is defined as that region within which the particles are hot enough to glow. The tapering shape of the flame comes directly from the low availability of oxygen. When you trap a flame under a glass, the flame extends before going out, because the lifetime of a glowing particle is longer in the absence of oxygen.

This is demonstrated in a lovely picture from NASA of a flame in microgravity. Because there is no ‘up’ for the air currents to go, the flame burns in all directions. This is a much more efficient use of space: oxygen can get in from all directions, so the fire burns strongly, with no soot to give it a yellow colour.

 

REFERENCES

http://en.wikipedia.org/wiki/Candle

Also, see a fascinating and rather whimsical discussion on the ‘philosophy of candles’ by the mighty Michael Faraday here.

 

 

 

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Written by The S I

November 8, 2011 at 11:59 pm