June 18, 2013
Playing With Fire In Space, NASA Creates Spherical ‘Cool’ Flames
[ Watch the video "Strange Flames On The International Space Station" ]
Lee Rannals for redOrbit.com - Your Universe Online
Astronauts aboard the International Space Station (ISS) analyzed the behavior of flames in space, including how and why they form little "spheres."
Thousands of chemical reactions take place simultaneously within a flame. Hydrocrabon molecules from the wick are vaporized and cracked apart by heat, and they combine with oxygen to produce light, heat, carbon dioxide and water. The teardrop shape of the flame is an effect called buoyancy caused when hot air rises and draws fresh cool air behind it, making the flame shoot up and flicker. However, in space and without gravity to assist the buoyancy effect, flames burn differently and form little spheres.
“In space molecular diffusion draws oxygen to the flame and combustion products away from the flame at a rate 100x slower than the buoyant flow on Earth,” said Dan Dietrich, project scientist, NASA´s Glenn Research Center.
ISS astronauts record everything from ignition to extinguishment with cameras housed in the NASA Glenn-designed-and-built Combustion Integrated Rack (CIR), which is located inside the US Destiny Laboratory module of the space station. This experiment is known as the Flame Extinguishment Experiment, or "FLEX."
Flames in space help to work as mini-labs for combustion research. These flaming spheres create a much simpler system, allowing the oxygen to come to them and create a narrow zone at the surface of the sphere.
Astronauts performing the experiments aboard the ISS were trying to understand how to put out fires in microgravity when they came across something odd. They saw small droplets of heptane burning inside the FLEX combustion chamber, and as the flames burned out, droplets of fuel continued burning.
"That's right — they seemed to be burning without flames," explained Forman A. Williams, a professor of physics at the University of California, San Diego. "At first we didn't believe it ourselves."
He believes the flames didn't actually disappear, but that they transformed into "cool flames,” which burn at relatively low temperatures of 500K to 800K, compared to 1500K and 2000K typical of visible fire.
"And their chemistry is completely different. Normal flames produce soot, CO2 and water. Cool flames produce carbon monoxide and formaldehyde," Williams said.
He said the implications for these results could lead to cleaner auto ignitions here on Earth. Instead of a spark in the automobile cylinder, engineers could design a gentler, less polluting combustion process throughout the chamber.
"The chemistry of HCCI involves cool flame chemistry," says Williams. "The extra control we get from steady-state burning on the ISS will give us more accurate chemistry values for this type of research."