Playing With Fire: Experiment Aboard the ISS Yields Surprising Results [VIDEO]

Scientists conducting experiments with fire in the International Space Station's microgravity yielded new insights into the old process that, researchers believe, could have major implications for machines that rely on combustion.

Despite being one of humanity's earliest achievements, Forman A. Williams, a professor of physics at the University of California, San Diego, explains that current understanding of fire is primitive.

"When it comes to fire," he said in a press release, "we're just getting started."

The reason, he explains, has to do with its innate complexity: an ordinary candle flame, for example, is home to thousands of chemical reactions as hydrocarbon molecules from the wick are vaporized and cracked apart by heat.

Once this happens, they combine with oxygen to create light, heat, carbon dioxide and water. Some of the hydrocarbon fragments go on to form ring-shaped molecules called polycyclic aromatic hydrocarbons and, ultimately, soot that either burns or simply drifts away as smoke.

Meanwhile, the familiar teardrop shape of the flame is the result of gravity as hot air rises and draws fresh cool air behind it. Called buoyancy, this is what makes the flame shoot up and flicker.

However, in microgravity, such as on the ISS, much of this changes.

Unlike flames on Earth, which expand hungrily when in need of more fuel, flames on the space station form spheres, or balls, that are more patient, waiting for the oxygen to come to them. Oxygen and fuel then combine in a narrow zone at the surface of the sphere, rather than throughout the flame, resulting in a much simpler process.

More recently, however, the researchers came across something even more surprising while researching how to put a flame out in microgravity.

Small droplets of heptane were burning inside their combustion chamber, and while the flames went out as planned, droplets of fuel unexpectedly continued burning.

"That's right - they seemed to be burning without flames," Williams said. "At first, we didn't believe it ourselves."

As it turned out, the flames were there, but were simply too faint to see.

Ordinary, visible fire burns at high temperatures between 1,500K and 2,000K. Heptane flame balls on the ISS, on the other hand, started out in this "hot fire" regime, but as the flame balls cooled and began to go out, a different kind of burning took over.

"Cool flames burn at the relatively low temperature of 500K to 800K," Williams said. "And their chemistry is completely different. Normal flames produce soot, CO₂ and water. Cool flames produce carbon monoxide and formaldehyde."

Similarly cool flames have been produced on Earth but always flicker out almost immediately. On the ISS, however, they are able to burn for minutes.

Going forward, Williams said, these new insights could provide for a number of advancements, including cleaner auto ignitions.

One idea auto companies have been working on for years is HCCI, or "homogeneous charge compression ignition." In the automobile cylinder, instead of a spark there would be a gentler, less polluting combustion process throughout the chamber.

"The chemistry of HCCI involves cool flame chemistry," Williams said. "The extra control we get from steady-state burning on the ISS will give us more accurate chemistry values for this type of research."