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Lets say there is a fire on a zero-g space station (no efforts have been made to give gravity to the structure and it's far enough away from any planet/sun that gravity is negligible). It is contained and does not breach any of the structure itself (so ignore the vacuum elements here). For simplicity, lets assume it's a candle sitting in the middle of a contained room (no air is entering and air circulation is minimal). Assume 'air' has the standard earth like composition.

2 part question:

  1. Heat rises on Earth as the hot air is significantly less dense than cool air. A flickering flame flickers upwards for the same reason. However in a zero-g environemnt, there really is no up. So what does a zero-g flame visually look like?

  2. Heat rising also fuels the fire by dragging in more oxygen as the hot air rises giving room for new oxygen rich air to move in behind it. Without this cycle, to fuel itself and without some other mechanism to cycle the air, how long would a fire feasibly burn for? Would it consume all the oxygen in a room before burning out, or would it simply consume the oxygen in the immediate area around the flame and burn itself out before the oxygen in the room is fully consumed?

And an extention to #2 - would it be possible that the fire would sit in a state where it's ready to ignite the second enough oxygen was present (or very slowly burn as the remaining oxygen in the room diffuses)?

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  • $\begingroup$ If the air was absolutely still it might put itself out, but any air currents would avoid that, and the flame would probably create its own convection cells around itself and stay burning. $\endgroup$ – Oldcat Nov 21 '14 at 19:29
  • $\begingroup$ @Oldcat Doesn't convection require gravity though? $\endgroup$ – Ajedi32 Nov 21 '14 at 22:21
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    $\begingroup$ From WIKI on Convection: In a zero-gravity environment, [with] no buoyancy forces, and thus no natural convection possible, flames smother in their own waste gases. However, flames may be maintained with any type of forced convection (breeze); or (in high oxygen environments in "still" gas environments) entirely from the convection that occurs as heat-induced expansion of gases allows for ventilation of the flame, as waste gases move outward and cool, and fresh high-oxygen gas moves in to take up the low pressure zones created when flame-exhaust water condenses. $\endgroup$ – Oldcat Nov 21 '14 at 22:29
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Without gravity to constrain the flames and give us the "lighter"/"heavier" aspects of "hot air is lighter than cold air", there are no convection currents (at least not the "upward" ones we're familiar with) to give the flame its familiar flame shape; the result is that the flame burns in a sphere. These literal fireballs have even been observed to exist without visible fuel or ignition source, and have confounded experts in combustion. Here's a really cool video of exactly this.

A side-effect of this fact is that with the flames burning in literally all directions, rather than being pulled down toward its fuel, it burns slower in micro-gravity than it does under "normal" gravity. You do end up with a low-pressure zone as the hot air expands, which pulls in more oxygen to keep the flames going (strictly speaking, "convection currents", just not "upward" as we're used to), so it may actually burn far more efficiently as it slowly consumes its fuel (this last bit is my own speculation on my completely non-expert review of what I could find on the subject).

To answer the second part of your question: Yes, in exactly the same way the same situation can happen here on Earth. Basically you have to have something hot enough to combust, but held in a state where there's not enough oxygen to actually do so; the moment you add that oxygen, however, you get what firefighters call a backdraft and/or flashover. This type of situation could easily exist in e.g. a sealed (or mostly-sealed) storage room after a slow-burning fireball goes unnoticed and consumes the oxygen in the room, but lingers as a hot ember until someone opens the door and lets the heated air rush out, replaced by the cooler, oxygen-rich air the poor sap has just turned loose. (There are other ways to get the same effect, this is just one example that occurs to me off the top of my head. Read up on backdrafts and flashovers to learn more and adapt the situation to your needs.)

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    $\begingroup$ Of course the round flame assumes that at the place of the flame there's no significant air flow from the space station's ventilation system (or anything else with a fan, like a computer). $\endgroup$ – celtschk Nov 22 '14 at 20:31
  • $\begingroup$ @celtschk True, air currents will affect the shape -- and size, since they'll provide a better supply of oxygen -- of the fireball. $\endgroup$ – Kromey Nov 23 '14 at 5:56
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First, this is what fire looks like in micro-gravity: (what we often mistakenly call zero-g)

Secondly, air will still be sucked towards the flame via the pressure difference created by the fire consuming the local oxygen, but the flame will not be as hot without the effects provided by gravity. (Note how pale the flame is in the above video.)

Disclaimer: I am not a scientist, and I may have grossly misused several of the above terms.

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  • $\begingroup$ You got "micro-gravity" right, and that's half the battle. Though I'm not a scientist, either. :-) $\endgroup$ – HDE 226868 Nov 21 '14 at 21:16

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