Why is a Candle flame round and blue at zero gravity?

​​Hot air is less dense than cold air. When you light a candle, the air near the wick gets hotter and ( being less dense) gets pushed up by heavier and denser cold air(being heaver cold air is pulled down to earth). Flame of a candle is actually burning wax vapors. When the hot air rises, wax vapors also rises(because they are also hot!). This gives the flame its familiar rising shape.
Hot air (including unburnt wax vapors and products of combustion) rising up from candle is replaced by relatively cooler, oxygen rich and denser air. The cool air remains fresh as long as it is below the wick. There the clean oxygen is used up in ‘complete’ combustion of wax vapors and therefore the bottom part is bluish in color.

 

(Blue color signifies complete combustion. Meaning the wax vapors get completely burnt)

In space, The gravity experienced is almost negligible. Hence the air inside does not get pulled up or down and therefore remains stationary(w.r.t. to an astronaut sitting inside). When the candle is lighted, the air near the wick heats up and since there is no cold air below the candle(because no gravity) it doesn’t rise. The hot air simply diffuses in the surrounding air.

No air rising means no wax vapors rising and hence the flame is spherical.
Diffusion of hot air containing products of combustion causes oxygen rich air to participate in combustion and thus the blue color.

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