Most people probably imagine that astronauts rarely light flames in the cramped quarters of the International Space Station - but fires are quite common up there, albeit for sound scientific reasons.
In the “microgravity” of the ISS, flames behave very differently from fire on Earth - burning in glowing orbs, without hot air leaping upwards to form the shapes we think of as flames.
"In microgravity, flames burn differently - they form little spheres," says Dr. Forman Williams, a professor of physics at UC San Diego.
While performing an ISS experiment with flames, Williams and his colleagues saw something astonishing as droplets of heptane (a fuel) were burning inside a combustion chamber.
The flames went out, but the droplets continued to burn up.
"They seemed to be burning without flames," says Williams. "At first we didn't believe it ourselves."
Williams believes the flames are there, just too faint to see. "These are cool flames," he explains.
Ordinary, visible fire burns at a high temperature between 1500K and 2000K. Heptane flame balls on the ISS started out in this "hot fire" regime. Williams believes that 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," says Williams. "And their chemistry is completely different. Normal flames produce soot, CO2 and water. Cool flames produce carbon monoxide and formaldehyde."
Similar cool flames have been produced on Earth, but they flicker out almost immediately. On the ISS, however, cool flames can burn for long minutes.
"There are practical implications of these results," notes Williams. "For instance, they could lead to cleaner auto ignitions."
Car companies have worked on for years is HCCI--short for "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," says Williams. "The extra control we get from steady-state burning on th