US defence researchers are investigating the use of new electric and acoustic techniques for extinguishing small fires in enclosed military environments.
Traditional fire-suppression technologies focus largely on disrupting the chemical reactions involved in combustion. However, a team at the Defense Advanced Research Projects Agency (DARPA) noted that, from a physics perspective, flames are cold plasmas.
The researchers therefore theorised 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.
One of the technologies explored was a novel flame-suppression system that used a handheld electrode to suppress small methane gas and liquid fuel fires. Since the electrode is sheathed in ceramic glass, no current is established between the electrode and its surroundings.
The oscillating field induces a rapid series of jets that displaces 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. This same approach was not able to suppress a small heptane pool flame.
The team also evaluated the use of acoustic fields to suppress flames.
Two dynamics are at play in this approach, according to the researchers. 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 vaporisation, 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.
Matthew Goodman, DARPA programme manager, said in a statement: ‘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.’