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Hot flame - cold plasma

02 August 2012

Fire in enclosed military environments such as ship holds, aircraft cockpits and ground vehicles is a major cause of material destruction and jeopardises the lives of military personnel. 

For nearly 50 years, despite the severity of the threat from fire, no new methods for extinguishing or manipulating fire have been developed. Back in 2008, the US Defense Advanced Research Projects Agency (DARPA) launched its Instant Fire Suppression (IFS) programme to develop a fundamental understanding of fire with the aim of transforming approaches to fire fighting.

Traditional fire-suppression technologies focus largely on disrupting the chemical reactions involved in combustion. However, from a physics perspective, flames are cold plasmas. DARPA 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. 

The IFS programme was executed in two phases. In Phase I, participants studied the fundamental science behind flame suppression and control, exploring a range of approaches before finally focusing on electromagnetics and acoustics. In Phase II, participants determined the mechanisms behind electric and acoustic suppression and evaluated the scalability of these approaches for defence applications.  

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. A visualisation 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, extinguishing the fire. Put simply, the electric field creates an ionic wind that blows out the flame. This same approach, however, was not able to suppress a small heptane pool flame.

Acoustic fields were also evaluated as a means of suppressing flames. In a video accessible from DARPA's YouTube pages, a flame is show being extinguished by an acoustic field generated by loudspeakers on either side of the pool of fuel. Two dynamics are at play here. 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. Essentially, in this demonstration loudspeakers were used to blast sound at specific frequencies that extinguish the flame.

IFS Phase II was completed in December 2011, and participants succeeded in demonstrating the ability to suppress, extinguish and manipulate small flames locally using electric and acoustic suppression techniques. However, it was not clear from the research how to scale these approaches to the levels required for defence applications.

DARPA programme manager, Matthew Goodman says these demonstrations have shown that the physics of combustion still has surprises in store for us, and hopes that the results will spur new ideas and applications in combustion research. Data collected by the IFS programme could potentially be applied in areas that are the complete opposite of fire extinguishing - namely, increasing the efficiency of combustion.

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