What you NeedThis 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.
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.
As the air moves closer to the centre it spins faster and faster causing the flame to spin in a rather beautiful way.
Why does the air spin faster?It is the same effect as an ice skater doing very fast spins. 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.
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 completesly burn so it produces lots of black smoke
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 vapor 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.
What to doWhat may happenWhy does it happen?Related ContentCommentsFascinating experiment! I draw them in Inkscape Many thanks and I run Linux as well - doesn't everyone? johnmuir, Fri, 16th Apr 2010 What is the fuel source for your flame? Due, Sat, 15th May 2010 Centrifugal force? Really? If you're actually going to explain things, don't throw false notions into the mix. Inertia exists, centrifugal forces do not. They are only a manifestation of inertia in the local reference frame, as such claiming that "they" are doing this or that is highly misleading. Nothing is acting on the mass to throw it out or its orbit. Rodj, Sat, 19th Jun 2010 The flame is fueled using fire/BBQ lighting gel, but any hydrocarbon based fuel that burns with a yellow flame will work well daveshorts, Fri, 30th Jul 2010
im doing a fire tornADO FOR MY SCIENCE PROJECT BUT I NEED TO KNOW WHAT SIZE TURNTABLE TO USE.... rara, Fri, 5th Nov 2010 What kind of mesh cylinder are you using? Dan, Sun, 5th Dec 2010 I believe you meant to say centripetal force as opposed to centrifugal force. Harsh Singh, Tue, 22nd Feb 2011 |
