A flame is not a flame is not a flame. They might look similar, but flames have different properties and energy releases, or heat. 

Researchers from Washington University in St. Louis collaborated with the University of Maryland and the University of California San Diego to develop a new class of fire called spherical cool diffusion flames, or “cool flames.” They did it by using the special qualities of a unique lab: the microgravity in space aboard the International Space Station.

“Being able to perform experiments in microgravity allowed us to observe a flame that just would not exist on Earth. This type of discovery is what makes the Space Station so invaluable to scientific exploration,” Richard Axelbaum, professor of environmental engineering science at Washington University, said in a news release.

The discovery could lead to cleaner combustion engine developments. 

Flaming out

Cool flames were first observed aboard the ISS in 2012 during an experiment that burned a droplet of fuel. They appeared only momentarily before the fuel was depleted. The brief sighting fueled research on a combustion phenomenon that previously had only been theoretically predicted.

The new cool flame discovery occurred aboard the ISS during experimentation from January through June this year. After work on more than 150 hot diffusion flames during that time, a slight change in conditions happened on June 23. In three of the experiments, the hot flames extinguished and disappeared from camera view, but heat was still being produced. 

Even the most sensitive cameras at the researchers’ disposal were not able to pick up images of what was happening with the residual heat. A mechanical engineering student enhanced the video feed to find surprising behavior: After the hot flame extinguished and all reactions ended, the residual heat in the porous burner re-ignited the flowing fuel to become a spherical cool diffusion flame. This is a completely new class of fire.

The flames are fueled by the light, gaseous n-butane, which is found in most cigarette lighters. They burn at 600-1000 kelvins (620-1340 degrees Fahrenheit), which is much cooler than the typical hot flame temperature of 1100-2200 K (1520-3500 F). They have several favorable characteristics that previously haven’t been documented as occurring at the same time.

“They have known, controlled, and steady flow rates. They are self-sustaining without heated reactants or exotic oxidizers. Their reaction zones are thick (on the order of 6 mm), which facilitates measurements and simulations,” Peter Sunderland, University of Maryland professor, said in a news release.

Next steps

The cool flame discovery happened just days ago but already is generating a lot of attention among combustion researchers. Many researchers anticipated that cool flames would be easy to produce in microgravity, but they are learning the process is still difficult. Currently, little is known about combustion chemistry at these low temperatures.

The researchers plan to continue running experiments to gain a full understanding of cool flame chemistry, how and why it occurs, and its applications. Already, they believe cool flames will enable the creation of cleaner and more efficient combustion engines and burners. Cool flame research also could help to improve fire safety.