Fire whirl and stretching a vortex

Background

A fire whirlwind, otherwise known as a “fire devil”, “fire whirl” or “fire tornado,” occurs naturally in wild fires (see photos from National Geographic). They occur when a vortex forms around a fire plume. The hot air from the fire plume stretches the vortex vertically narrowing it and intensifying it. In nature the vortex can form due to the ambient wind being deflected around a fire by local topography. In the lab there are a couple of ways to form a fire whirl. The Phaeno Science Center uses an array of air jets blowing tangentially in a circle around the flame to create the vortex. At a smaller scale one can just rotate the flame.

Equipment:

  • Nonflammable Turntable (Lazy Susan)
  • Wire Mesh Trash Can (the finer the mesh, the better)
  • 1 Glass Pyrex Bowl (or any nonflammable container)
  • Cotton Balls
  • Rubbing Alcohol (or any fuel source)
  • Duct Tape
  • Lighter
  • Damp Towel (or any form of extinguisher)
  • fire

Procedure

  1. Using the duct tape, affix the trash can to the center of the turntable.
  2. Place cotton balls, 10-15, into the glass bowl and pour rubbing alcohol over the cotton balls. Pour enough alcohol to lightly dampen the cotton balls, careful not to pour too much; do not completely saturate the cotton balls.
  3. Using the duct tape again, affix the glass bowl inside the trash can, careful to center it in the bottom.
  4. Use the lighter to ignite the cotton balls. Observe the flame.
  5. Begin to rotate the turntable and observe the effect on the flame. Vary the speed (without being reckless) and note how the flame stretches with higher rotational speeds.
  6. Put out the fire using the damp towel.

CAUTION:

  • If not attached well, the trash can will slide off of the turntable when rotated.
  • Also, if not attached, the glass bowl will slide around the inside of the trash can.
  • Be careful when removing the glass container, it becomes very hot during the experiment.

no-whirlwhirl

Flame height (left) before spinning and (right) when spinning

Discussion

This is a good ‘wow that’s cool’ demonstration for any fluid mechanics class. It is particularly useful when discussing vortex dynamics and vortex stretching. Essentially, once the vortex forms around the fire the buoyancy generated by the fire stretches the vortex vertically. This makes the flame and vortex narrower and, by conservation of angular momentum, increases the intensity of vortex. this process is self reinforcing and the flame length can then dramatically increase. While this basic mechanics is fairly well understood, the actual prediction of when such fire whirls will occur in nature is very much an open question.

Thanks to Alex, Meredith, and Ali for putting together this write up and demonstration. Videos to follow soon.

An index of all the demonstrations posted on this blog can be found here. Don’t forget to follow @nbkaye on twitter for updates to this blog. If you have a demonstration that you use in class that you would like to share on this blog please email me (nbkaye@clemson.edu). I also welcome comments (through the comments section or via email) on improving the demonstrations.

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