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Here we are talking about the flame on earth because in space it will be circular in shape but on earth it is pointed,
Let's know why,
When you burn fire, the fuel combines with oxygen to release heat and other gases. The heat energy generated in this process heats up the air around the flame and reduces it's density.
As you know hot air rises up and cold air rushes down. This hot air around the flame starts rising up and causing the cold air in the surrounding to rush down which in turn causes the flame to shoot up. Basically the buoyancy shoots the flame up.
Image credit: ebaumsworld.com
It is the hot air moving up that also causes the fire to flicker. However, in micro gravity, there is no reason for the heavier air to rush down to the place of lighter air. As a result, the flame remains a spherical blob.
So, this is the reason for the the flame to always point up.
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