Piece of cake, according to yesterdays Guardian supplement, a candle burning in freefall (I know I was being misleading in the title, but hey, I'm only human) would have a spherical, blue, sootless flame.

Now for the science bit.
This would be a best guess effort so feel free to add any comments below.
Since in zero-g, hot air cannot rise above the cold air (no convection in freefall), the hot air gets hotter, giving the flame its blue colour. Since there is no up or down, the flame spread in all directions not just rising above cooler air which gives rise to the tall flames here on Earth. The higher temeratures would also burn the fuel more efficiently - meaning no soot.

ps. to clear up some of the confusion - I meant some atmosphere that happens to be in zero-g like a space shuttle lab

Tsk, tsk.

Remember your high school physics?

Air, in a sense, moves randomly, due to the fact that atoms are always moving. This principle is called diffusion. Since diffusion is present in most substances, this means that a continual supply of oxidizer will be present. Fires in zero-G burn longer, due to fewer oxygen needs, and diffusion keeps them supplied with oxygen.

For more information, I would suggest looking at the May '98 issue of Scientific American.

Sirius is very close to right. What happens is that you end up with a very small, dim, round flame on a wick. It sometimes gets just enough oxygen through the methods mentionned by Lockheart, but often it just chokes out.

Interestingly enough, though diffusion does eventually move all the air around, it's slow enough that shuttles have fans to prevent carbon-dioxide pockets from forming around astronauts.
A candle in freefall, eh? Lets browse some theory together...

Inside a gravitational field...

1) A flame is merely an extremely hot volume of air (essentially around/inside where the combustion takes place). When you have something on fire, the heat from the 'burning' material heats the surrounding air causing it to emit light. This light is the 'flame' that you see. The heat also causes a net increase in particle speed (in fact it is this that causes the light emission...); ie the gas will attempt to expand and in doing so lowers it's density. In a gravitational field, the force on a given volume is less than that of it's surrounding gas so the gas will tend to rise above it as well as directly outwards. This gives the flame it's characteristic appearance... (note the small part of the flame *under* the match head due to downward velocities temporarily outweighing upward net force)

_                       |
 -_                    / \
   -_        |        |   |
     -_     / \       |   _\
       -_  |   |     /   / |
         -/    \     \  /  /
          |-___ |     \_|_/
          \ |_| |   |\  |  /|
           \___/    | -_|_- |

2) The flame is allowed to burn because underneath it, the (unburned) air is *rising* to fill the gap that the (burned) air left. The candle actually evaporates it's fuel before burning it, so the flame doesn't start at the base of the wick (the fuel inside the flame is evaporated and the (melted) fuel inside the candle wax is taken up the wick by capillary action and eventually evaporates into the flame itself).

3) Finally, the candle wick is pulled downward by gravity, causing it to bend and where it nears the edges of the flame (the hottest point) the wick itself is combusted and is destroyed.

In zero-g/freefall/space

1) With the gravity taken away, the flame loses it's appearance. All forces acting upon the flame are now equal so the flame becomes symmetrical, ie a sphere (the sphere will 'wobble' due to fluctuations in the air, mind).

2) In this case, the huge flame noted with the candle example cannot exist - unused, oxygen rich air needs a route to the burning material and it cannot obtain this route with a flame as large as in a gravitational field, therefore because the equality of forces acting upon the gas particles forces a spherical flame, the flame *must* shrink. The reduced flame size means there will be less emitted energy, hence the flame is cooler. Because the flame is cooler, and the necessity of melting the wax to sustain a flame, the flame itself exists far lower down on the wick. Note that in this case you can't overmelt the wax because instead of simply dripping down the sides, your wax will wobble off into space in a wobbly blob and create a small mess.

3) In a gravity free environment, the wick is no longer forced over to the side and so remains straight. As the flame is lower down on the wick, the wick itself will be burned sooner (at the top of the flame) and thus will be shorter. It's also worth noting that the flame's equality means it will be burning the bottom of the wick as well as the top. Cross your fingers and hope it doesn't sever itself ;).

 -_                    |  
   -_                  |   
     -_                |    
       -_              |    
         -_ ___       /|\  
           /___\      \|/ 
           \|_|/   |\  |  /|
            \_/    | -_|_- |

In conclusion

You're average candle probably won't stand up to the rigours of space - it will need a slow burning wick, a low melting point wax and probably won't be too interesting if it burned at all. I'd guess if you did burn one it was specially chosen/designed for space or you'd have problems.

I have seen pictures of candles burning in space, and the flame was completely blue. This means that the temperature of the flame is actually higher than on earth, contrary to what bucko says. The amount of wax being burned, and hence the amount of energy released is of course lower, as the supply of oxygen to the flame depends on diffusion rather than convection, as convection is driven by gravity acting on a system with hot and cold gasses. But because convection also doesn't bring in any fresh, cool air to the flame it can reach higher temperatures. This leaves us with a hot but very small flame, as a small flame would also maximise the surface-to-volume ratio of the flame, which is advantageous because the surface determines how much oxygen can enter the flame and the volume determines how much oxygen is needed for the combustion taking place. Also, a smaller flame decreases the distance oxygen has to penetrate into the flame. In the absence of macroscopic mixing of fuel and oxygen, all these factors suddenly become relevant.

Also, as the length of the wick underneath the flame is an interplay of forces, in an environment with gravity the flame creeps up the wick because hot gasses rise. Now this is counterbalanced by the necessary supply of fuel to the flame, which will stop if the flame doesn't melt enough wax. The flame will dwindle, first consuming all the fuel higher up in the wick, gradually creeping down the wick, until it starts melting enough wax to feed itself again. This tendency of the flame to "go up" of course wouldn't play any role in an environment without gravity, so the flame would in all probability touch the surface of the candle itself.

Now there is one difference between the "bottom" (the end extending into the candle) of the wick and the "top" (far end) of the wick: the bottom of the wick is connected to the reservoir of molten wax. As the flame also burns farther along the wick, this means that there is molten wax present in the wick at the "bottom" end of the flame. This will burn before the material of the wick burns. Now at the far end the flame has drained the wick of fuel entirely, and thus the wick itself will burn. This has nothing to do with the edge of the flame being the hottest part. On the contrary, I would suppose that the edge is the coolest, also evidenced by the blue color of the inner and the yellow color of the outer parts of the flame of a candle burning on earth.

Now this serves to show that such a simple experiment actually is very complex matter, with an interplay of many, many forces, and I wouldn't be surprised to hear that the reason they actually did this experiment was to settle some very heated discussions.

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