Heat and Wind Speed

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1. Oct 28, 2014

Stevenyzs

I'm currently working on a project about wind speeds, and have come across a roadblock due to my limited knowledge... So I really hope you guys can help me out.
When you light a campfire or candle, the heat from the flames cause the air around it to rise, which leaves a "vacuum" for colder surrounding air to rush in towards the flame, causing convection currents.
I only know that the wind speed rising from the flame increases with the temperature of the fire. So here's the question, is there a known constant for this proportional relationship? Or is there an overall fluid dynamics formula for this question?

2. Oct 29, 2014

256bits

Cold air rushes in, not because the hot air rises and leaves a "vacuum", but because of the difference in density between the hot and cold air. An example you might be more familiar with is a helium balloon, a lighter object moving upwards in more dense air. In both cases, the less dense hot air and the helium balloon, rise due to the forces resulting from bouyancy. The same reason wood floats in water.
Wiki, as usual, gives an explanation.
http://en.wikipedia.org/wiki/Buoyancy

3. Oct 31, 2014

Stevenyzs

Oh, I see. Thanks for pointing it out :) Then what about fans? How do they move air? I read the explanation in wiki but I still don't understand...

4. Nov 1, 2014

oz93666

Fans 'push' air , the blades are at a slight angle so push the air through.

A burning candle creates an area around the wick where gases are less dense so there is a buoyancy effect , which creates an upward flow.

In zero gravity there's no buoyancy effect so this does not happen. a candle burns much more slowly, the wick receiving new oxygen only by diffusion, through the gaseous combustion products, which hang around the wick.

IF we lived in a world where hot gases were more dense, then the candle flame and combustion gases would travel downwards, melting the candle, and making a mess...rather awkward....

Last edited by a moderator: Nov 3, 2014
5. Nov 3, 2014

If you look up natural convection you may get an idea about the original question regarding describing the phenomena. The bottom line is that no, there isn't any simple relationship or formula, though you may be able to come up with something half-decent with a few semi-legitimate assumptions.