There are a number of chemicals, compounds and techniques that can put out a fire, but DARPA is researching new methods to put out flames. One of them is sound.
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According to a recent statement from the agency, DARPA is researching the "interaction of electromagnetic and acoustic waves with the plasma in a flame." The team came up with a couple new methods as part of its research for the Instant Fire Suppression program, which began in 2008.
One of them involves using acoustic fields. Here's how it works:
In the video below, a flame is extinguished by an acoustic field generated by speakers on either side of the pool of fuel. 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.
See it snuff out the fire for yourself:
Another technique DARPA developed uses a "handheld electrode" to create an "ionic wind that blows out the flame." Watch that device work here:
The purpose of this research is to develop a method of instant fire suppression to protect a variety of military environments. According to DARPA's statement, it has been nearly 50 years since new methods had been introduced for putting out or manipulating flames.
Even though these new findings are interesting, DARPA said it is currently "not clear [...] how to effectively scale these approaches to the levels required for defense applications."