Unusual “cool flames” documented aboard International Space Station

A team of researchers, including engineers at UC San Diego, documented an entirely new class of fire aboard the International Space Station. The spherical cool diffusion flames, or simply "cool flames," could ignite development of the combustion engine of the future. Read the full release from the University of Maryland.

Cool flames are aptly named: They burn at extremely low temperatures and emit a near-invisible blue glow. (For comparison, a natural gas burner at high heat on a conventional stove top can burn at around 3100 degrees Fahrenheit/2000 Kelvin; a typical cool flame hovers around 900 degrees Fahrenheit/800 Kelvin.) They are also somewhat mysterious: As recently as 10 years ago, cool flames had only been theoretically predicted. First observed during an experiment aboard the International Space Station (ISS) in 2012, the cool flames appeared only briefly before the liquid fuel was depleted. The sighting catalyzed an emerging, rapidly growing field in combustion research.

Forman Williams, a professor emeritus of mechanical and aerospace engineering at UC San Diego, has been working on fire research and fire safety with NASA since the 1970s. For this most recent experiment and discovery, he collaborated with researchers from the University of Maryland and Washington University in St. Louis. The team launched to the ISS in late 2020 an experiment designed to generate controlled cool flames that burn, steadily, for at least two minutes. 

In a first for microgravity flame research, they achieved this feat—with the help of astronauts aboard the ISS testing a variety of gas fuels—on June 23, 2021. 

"This may help improve our knowledge of cool-flame chemistry, which is involved in the design of better internal combustion engines," said Williams. "I didn't expect to find these stable cool flames because the stabilization mechanism for them in droplet combustion is absent, but apparently there is a related stabilization mechanism for these gaseous fuels."

Williams has been involved in these experiments to better understand fire here on Earth by pushing the limits in space for decades, including leading experiments to improve fire-fighting techniques in space and better understand combustion on Earth, and leading the design of the experiments that led to the first observation of this new type of cool burning flames.