Eli5: How come fire flames go upwards and why do the flames always “dance” around?
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They go up because hot gas is lighter than air, and therefore rises. For comparison, on a space station, flame doesn’t go up, it forms a sphere.
They “dance” because the burning is irregular, plus the moving particles cause turbulence. For comparison, a gas flame is very regular and barely “dances” at all.
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Heat rises.
In some circumstances (in the case of the London Tube fire), it actually LAID DOWN. The angle of the escalators at kings Cross formed superheated gas AHEAD of the flames, fuelling the flames as it went up the Escalator, accelerating as it went. A jet of super heated flame hit the ticker hall.
Nobody thought it possible, until they built a scale replica of the Escalator and ticket hall and set it alight, and watched in horror as the flames mimicked what eye witnesses at the station described.., Firefighters had never seen the phenomenon before.
[https://youtu.be/HQfqt0LDANQ?t=592](https://youtu.be/HQfqt0LDANQ?t=592) And [https://youtu.be/PBbJXvJYeq0?t=2345](https://youtu.be/PBbJXvJYeq0?t=2345)
Density and gravity are your two reasons for rising.
As a gasses from the fire heat up, their molecules move further apart. This makes the hot gas less dense. If its less dense, the denser gasses above these hot fire-gasses, sink below them. So, the fire gasses get higher and higher. A good analogy is a hot air balloon. The gasses within the balloon are heated up whenever the pilot wants to rise. Denser, cooler gasses outside of the balloon, sink below it and therefore the balloon goes up.
Gravity…. Without it, the gasses that are cooler than the fire gasses would not sink beneath them. That’s why your in space flame is round.
The dancing around is basically turbulence from cool gasses interacting with hot gasses and then trading places at a high rate of knots. That is basic, I’m no scientist and I’m super happy to be corrected and learn new stuff.
Source – I’m am ex fire behaviour instructor from UK fire service.
Fire is way lighter than the air around it, so it floats up like a balloon. It keeps floating up even when it cools down enough to be invisible, like how lava turns into dark rocks when it cools down. Fire is more like a stream that turns invisible the higher it goes than a thing that just sits there on wood or whatever. That’s why you feel heat if you wave you hand over a fire without touching it.
Other air rushes into the place the fire left, like how after you get out of a pool there isn’t a hole in the water after you get out. Air works like water and fills in the hole from the fire as it floats up. Fire dances around because of the wind of the air rushing down around it: it flutters like a flag in the wind.
The light you see is the shape at which the hydrocarbon gas makes contact with the surrounding oxygen.
The hydrocarbon gas is created when fuel is evaporated from the heat of the light aka the chemical reaction.
This hot gas rises up. The outermost layer of gas hits oxygen first, then the next layer and so on making a cone.
Without any wind or irregular supply of fuel, this would be a perfect cone. Without gravity, it’s a perfect sphere.
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