Question

Why fire is goes always upward or direction of sky?

Answer 1

according to my thoughts combustion of oxygen takes place. so that's the reason for fire is goes always upward or direction of sky.

Answer 2

Excellent question, the answer is that fire is driven by pressure, and pressure is driven by gravity. See, flame consists mainly of superheated gasses. When you heat gasses, they expand and become less dense (since the molecules are pushing each other apart harder). 
 
Now, everything in the world is being pushed upward by Buoyancy and the gradient in air pressure. Air pressure is caused by the weight of the atmosphere being pulled down by gravity. The higher you go, there's less air above you, which means less air pressure. There's a tiny bit more air pressure at your feet than at your head, which pushes you upward, but your weight is enough to hold you down. If something is so light that the upward force of air pressure exceeds it's weight, it will float (like a helium balloon). This is also true of the gasses in a flame. The heat spreads them out so much, they become less dense than air, which means they're light enough to be pushed up by the surrounding air pressure. But since flames are gasses, they mix with the air as it rushes in to push them upward, and the oxygen in that air is enough to keep the fire going.

I hope you understand...
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