Why does Fire Burn?

qchu on why fire burns?

Combustion is in some sense the opposite of photosynthesis, an endothermic reaction which takes in light, water, and carbon dioxide and produces hydrocarbons

I have never thought of fire this way before but it makes absolute sense.

Here is Feynman’s take on fire. Always trust Feynman to explain complicated things in simpler terms.


Two videos I’d like to share (well, one video, and one playlist)… The first is a ‘flashover simulator’. This is a training tool for firefighters to help them recognize the signs of a ‘flashover’. A flashover occurs when the radiant heat of the fire is sufficient to cause pyrolysis and ignition of more or less every surface in the room simultaneously (the floor, furniture, etc). I’m mostly sharing this because sitting in one of these is where it first clicked for me that fire burns gasses, not solids. Watching the fire snake lazily through the smoke overhead, until it felt like I was sitting on the bottom pool, looking up at the surface. https://www.youtube.com/watch?v=-Z4xxObrTlI&t=40s

The second is Bill Hammack (The Engineer Guy) delivering Faraday’s lecture series on The Chemical History of a Candle. This is a truly excellent lecture series, despite its age (they did choose to keep most of the original language, which can take a little getting used to) https://www.youtube.com/playlist?list=PL0INsTTU1k2UCpOfRuMDR-wlvWkLan1_r

The effects of gravity on fire

The characteristic shape of a flame on Earth depends on gravity. As a fire heats up the surrounding air, natural convection occurs: the hot air (which contains, among other things, hot soot) rises, while cool air (which contains oxygen) falls, sustaining the fire and giving flames their characteristic shape. In low gravity, such as on a space station, this no longer occurs; instead, fires are only fed by the diffusion of oxygen, and so burn more slowly and with a spherical shape (since now combustion is only happening at the interface of the fire with the parts of the air containing oxygen; inside the sphere there is presumably no more oxygen to burn)

…and then it gets more mathematical😢. Still It is a good read to understand the basics of fire. The Hacker News Discussion has some really good conversations about fire.