July 16, 2012

DARPA Manipulates and extinguish small flames using electric and acoustic suppression

DARPA performers demonstrate techniques to manipulate and extinguish small flames locally using electric and acoustic suppression; results could benefit combustion research.

Fire in enclosed military environments such as ship holds, aircraft cockpits and ground vehicles is a major cause of material destruction and jeopardizes the lives of warfighters. For example, 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. For nearly 50 years, despite the severity of the threat from fire, no new methods for extinguishing or manipulating fire were developed. In 2008, DARPA launched the Instant Fire Suppression (IFS) program to develop a fundamental understanding of fire with the aim of transforming approaches to firefighting.

New Scientist - By using specific frequencies, a fire is killed in a two-pronged attack. First, sound increases the air speed, thinning the layer where combustion occurs and thus making it easier to disrupt the flame. But the acoustics also disturb the surface of the fuel which increases vaporisation, widening the flame and cooling its overall temperature.

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.

The IFS program was executed in two phases. In Phase I, performers studied the fundamental science behind flame suppression and control, exploring a range of approaches before down-selecting to electromagnetics and acoustics. In Phase II, performers determined the mechanisms behind electric and acoustic suppression and evaluated the scalability of these approaches for defense 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. In the video below, performers sweep the electrode over the ignited burner array and progressively extinguish the 10-cm2 gas flame. 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. This same approach was not able to suppress a small heptane pool flame.

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