It's precisely because of gravity that they go upwards. The gravity is pulling on both fire and the air. The air, being cooler and thus denser, puts pressure on the lighter flame when it is displaced(Due to Newton's Thirld law).
That pressure is stronger on the bottom than the top, and pushes the flame upwards. That's buoyancy; it's the same thing that makes boats float.
(Buoyancy forceis an upward force exerted by a fluid(liquid or, gas) that opposes the weight of an immersed object.)
When the gravity is absent( as in space), the cooler, denser air doesn't flow downwards. It continues to put pressure on the flame from all directions, which means it comes out as a sphere rather than going upwards. This is what a flame looks like on a space station:
- answered 5 years ago
- Nitish kumar