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Fire whirls often happen during wildland fires. They’re usually small, but under the right conditions, big ones form. Huge whirls have been recorded climbing to heights over 3,000 feet, with wind speeds exceeding 155mph — equivalent to an F3 tornado.

Meteorologists call any such phenomenon a vortex, which is just a volume of rotating fluid. Examples include smoke rings, water going down a drain, and dust devils.

In fact, fire whirls and dust devils are very similar. Both form when a layer of cool air passes over a layer of lighter hot air, which pokes a hole in the cool layer and rises through the opening. If conditions are right, the rising air begins to spin and a whirlwind forms. The rotational trigger can be as simple as a gust of wind.

The difference between dust devils and fire whirls is mostly a matter of degrees — temperature, that is. Where the dry earth under a dust devil can be as hot as 150°F, air temperatures in a fire whirl can exceed 2,000°F. These extreme temperatures create huge columns of rising hot gas that shoot up erratically through cooler layers. Some small triggering event sets the column spinning, and a fire whirl is born. The hot gases spin like a hurricane, with greater wind speeds closer to the center, or eye. The suction pulls in extra oxygen, intensifying the flames.

The Fire Tornado dramatically illustrates the effect air has on a fire’s shape, burn rate, and fuel consumption. It’s easy to build.

caution-warning-danger-graphicThis project is to be performed by responsible adults or under their closest supervision. Keep careful watch on the fire at all times. Remove all combustible and flammable objects from the area. Keep the project away from flammable walls, surfaces, curtains, etc. Do not allow the craft sticks or anything else to ignite. This project creates smoke and fire and should be done outdoors. Keep your fire extinguisher close by.

gurstelle_290William Gurstelle is a contributing editor at MAKE. This article is excerpted from his book The Practical Pyromaniac (Chicago Review Press).

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Steps

Step #1: Prepare the crucible and turntable.

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  • Use modeling clay to create a base for the crucible in the center of the turntable, so it stays put while spinning.
  • Mount the crucible firmly in the clay, dead center.
  • Epoxy or hot-glue the craft sticks to the lip of the turntable and let dry. (I glued mine to a raised edge midway across the platter, with a diameter of about 8½".)

Step #2: Mount the screen cylinder.

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  • Roll the aluminum window screen into a 36"-high cylinder of the same diameter as your palisade of craft sticks, and cut as needed to overlap the edges by 1". Secure the seam with straight pins or wires through the mesh.
  • Carefully place the cylinder over the sticks so they hold it vertically in place. If the screen wobbles too much when it spins, try securing it to the sticks with pairs of super magnets.
  • Cut a small access flap in the screen right above the lip of the crucible.

Step #3: Light the fire.

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  • NOTE: The vortex is best viewed at night and in calm wind.
  • Working outside, with a fire extinguisher within reach: Drip 2tsp kerosene onto the cotton rag.
  • Put the rag on the turntable.
  • Secure the screen over the craft sticks.
  • Put the lighter through the flap and ignite the rag. Note the flame’s size and shape.

Step #4: Create a fire tornado!

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  • Start the turntable at its highest speed. What happens to the size and shape of the flame?
  • TIP: If your turntable’s top speed is 33rpm or 45rpm, remove the belt’s drive pulley, shave it down a little smaller, and reinstall. My modified turntable clocks at least 65rpm. (The MAKE Labs team replaced their pulley with a smaller 3D-printed one.)

Step #5: Put out the fire.

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  • Stop the turntable.
  • Put on your gloves and remove the screen.
  • Invert the large bowl and cover the crucible, smothering the flame.

Conclusion

Fire Tornado Physics

The flame produces hot gases that rise from the center of the burning materials, displacing cooler air above, which then sinks down around the edges of the cylinder.

Because the cylinder is spinning, the centrifugal motion pushes this cooler air toward the rotating screen, making the relative air pressure at the screen higher. But if the pressure is higher at the screen, it must be lower somewhere else, namely in the hot area above the flame. The lower pressure there allows hot gases from the burning rag to rise more easily, creating the flame vortex of the fire tornado.

William Gurstelle

William Gurstelle

William Gurstelle is a contributing editor of MAKE. The new and improved edition of his book Backyard Ballistics is out now.


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