Flame Tornado - A Spinning Column of Fire

14 March 2010


Flame Tornado



This is one of the most beautiful science demos out there and always makes a big impression.

If you light a fire in the centre of a turntable, and spin the turntable nothing happens, but if you spin a mesh cylinder around the fire, something rather wonderful happens.


Fire Tornado



What is going on?


 The fire heats up the air around it, which expands becoming less dense and floats upwards, which is why flames move upwards. As the hot air floats upwards cold air moves in from the sides. If you add a mesh around the fire and spin it, then the air coming in is forced to spin slowly.


FlameSpinning the mesh
Hot air rises above the fire, drawing in cold air from the sidesIf the air has to move in through a spinning mesh then it is also forced to rotate


As the air moves closer to the centre, it spins faster causing the flame to spiral in a rather beautiful way.


Flame tornado 
As the air moves in it spins faster and faster 



Why does the air spin faster?


This is the same effect exploted by an ice skater doing very fast spins or "pirouettes". They start off with their weight spread out as far as possible, and cause them to spin slowly. Although they are rotating slowly the skater's extremities are actually moving quite fast. When the skater moves all their weight into the centre it has less distance to travel, so it takes less time for it to rotate. Also all the effort they put it overcoming centrifugal force actually speeds them up.


Skater slowSkater spinning fast
When a skater starts to spin they spread their weight outThey then move their weight into the centre and speed up



Why does the flame get higher?


As the air starts to spin there are two effects which cause the flame to grow. 

The air is moving faster over the fuel so more of it evaporates, so there is more fuel in the flame, however this isn't the whole story as if you blow on the fire the flames get a bit longer but not 4-5 times longer.

But when the air is spinning it is very hard for it to move inwards because centrifugal force is effectively throwing the air outwards all the time. This slows down the rate at which air and therefore oxygen can get to the fuel, so slows down the rate the fuel can burn. This means it will take longer to burn, so it rises higher before it finishes burning. The lack of oxygen is also means that the fuel doesn't completely burn, so it also produces lots of black smoke.


Centrifugal force on flame
Centrifugal force means that it is hard for air and the fuel to mix, slowing the rate of burning



What has the experiment got to do with hurricanes?


Hurricanes work on a very similar principle, an area of the sea is heated by the sun, this heats the air above it causing it to expand and float upwards (you also get lots of water evaporating to form water vapour which also floats in air). This draws air in from the sides, and if the upwelling is large enough it draws in air from thousands of km away. Because the earth is spinning this air is also spinning so as it moves inwards it spins faster and faster until the winds can get up to over 200km/hr and you have an immensely destructive hurricane.


Convection in hurricaneHurricane Rotation
An area of hot sea causes air to heat up and float upwards drawing air in from thousands of km away.This air is spinning because the earth is spinning so as it moves inwards it spins faster and faster forming a hurricane.




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