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July 17, 2012

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Soundwaves: A new principle in fire suppression?

DARPA (Department of Advanced Research Projects Agency), a US military research organisation have unveiled a completely new method of fire suppression: sound.

It might sound like science fiction, but using sound acoustic fields, generated from a speaker, researchers have been able to extinguish a small cauldron fire in laboratory conditions.

They have also experimented with electrodes to try and manipulate fire with electromagnetic fields.

The whole approach is a fundamentally different way of looking at fire suppression, using the principles of physics rather than chemistry.

DARPA wanted to limit the impact of fire particularly in locations such as submarines and aircraft carriers where fires can be difficult to fight after a shipboard fire on the aircraft carrier USS George Washington in May 2008 burned for 12 hours and caused an estimated $70 million in damage.

The research was carried out by the Instant Fire Suppression (IFS) program, set up to develop a fundamental understanding of fire with the aim of transforming approaches to firefighting.

On their website, DARPA explained: “Traditional fire-suppression technologies focus largely on disrupting the chemical reactions involved in combustion. However, from a physics perspective, flames are cold plasmas. DARPA theorized that by using physics techniques rather than combustion chemistry, it might be possible to manipulate and extinguish flames. To achieve this, new research was required to understand and quantify the interaction of electromagnetic and acoustic waves with the plasma in a flame.”

After carrying out extensive research into the fundamental science of fire suppression, researchers moved into stage two where they tested their two methods, and evaluated their scalability.

In this video, researchers show how an electrode can be used to suppress a small methane fuel fire.

DARPA said: “Since the electrode is sheathed in ceramic glass, no current is established between the electrode and its surroundings. A visualization of gas flows during the suppression would show that the oscillating field induces a rapid series of jets that displace the combustion zone from the fuel source, leading to extinguishment of the fire. Put simply, the electric field creates an ionic wind that blows out the flame.”

 Another experiment showed the team extinguishing a fire with two speakers either side of a much larger fire. It works by using sound waves to disrupt the fire and cause it to burn over a larger area, but at a lower temperature.

DARPA explain again: “Two dynamics are at play in this approach. First, the acoustic field increases the air velocity. As the velocity goes up, the flame boundary layer, where combustion occurs, thins, making it easier to disrupt the flame. Second, by disturbing the pool surface, the acoustic field leads to higher fuel vaporization, which widens the flame, but also drops the overall flame temperature. Combustion is disrupted as the same amount of heat is spread over a larger area. Essentially, in this demonstration the performers used speakers to blast sound at specific frequencies that extinguish the flame.”

The team have proven that they can use physics to extinguish small fires, but they are not sure yet how scalable the technology could be.

IFS program manager Matthew Goodman is optimistic however.

He said, “We have shown that the physics of combustion still has surprises in store for us. Perhaps these results will spur new ideas and applications in combustion research.”

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