Center for Strategic Communication

Darpa is showing off a new system that can put out flames using only sound. It’s part of the U.S. defense agency’s “Instant Fire Suppression” program.

at the Defense Advanced Research Projects Agency (Darpa) have published the video alongside details of how the technique was achieved in their labs back in December 2011.

Wired U.K.

The team arranged two speakers either side of a liquid fuel flame to demonstrate how fire can be controlled by amping up an acoustic field. The sound increases air velocity, which then thins the area of the flame where combustion occurs, known as the flame boundary. Once the boundary area is thinned, the flame is easier to extinguish. At the same time, the acoustics are disturbing the pool of fuel and creating higher fuel vaporisation — this widens the flame, thinning it out so it is less concentrated and cool enough to extinguish.

Even better, the sound does not even need to be offensively loud to achieve any of this.

“We have shown that the physics of combustion still has surprises in store for us,” commented Darpa manager Matthew Goodman in a statement. “Perhaps these results will spur new ideas and applications in combustion research.”

Manipulating fire with sound is not a new trick. In the 1900s German physicist Heinrich Rubens demonstrated the technique using a length of pipe with holes punched along the top. One end was sealed off with a sound speaker attached, the other sealed off and fixed with a gas supply. After lighting the gas leaking from one of the holes and changing the sound frequency being emitted, the height of the flames could be manipulated.

Darpa, however, first announced its plans to research the viability of electromagnetism and sonic waves in fire extinguishing only in 2008, saying “despite extensive research in this area, there have been no new methods for extinguishing and/or manipulating fire in almost 50 years.”

The Instant Fire Suppression project was specifically launched to devise new ways of tackling fires in enclosed spaces, such as aircraft cockpits and ship holds, where fires are obviously devastating and incredibly difficult to control.

The premise of the research is that, since flames need a stable supply of cold plasma to persist, manipulating the flow of cold plasma could be the answer to more efficient fire extinguishing techniques.

Following two years spent researching the composition and chemistry of cold plasma, Darpa released details of its first success story in January 2012 (see the video immediately above). Using a wand-like electrode device housed in ceramic glass — that ironically looks like a kitchen fire lighter — the team successfully extinguished and bent flames from gas and fuel fires, but only on a small scale of 10 square centimetres. The electric field it emits achieved this by creating an “ionic wind” that “displaces the combustion zone from the fuel source.”

Being able to bend flames might seem like a very cool but ultimately useless method of firefighting, however the system will come in handy when fires rage out of control in enclosed spaces — the flames can be redirected to provide safe passage, if they cannot be extinguished completely. The method also prevents the fire spreading, and thus renders it localised and easier to control.

The next step for Darpa is to figure out how to reproduce these success stories on a much larger, and more practical scale.