Spherical cool diffusion flames or “cool flames” are a new class of flame that recently appeared by accident in an experiment on the ISS. These flames only existed in theory before the past decade. Though they had been duplicated on Earth with much difficulty, this is the first time these ghosts of hot flames were actually seen burning gaseous fuels, which had never been achieved on terra firma or in space. They almost went undiscovered until adjusting the resolution of the video feed showed something unexpected.
As part of the Cool Flames Investigation with Gases (CFI-G) project led by fire protection engineer Peter Sunderland of the University of Maryland, attempts to light hot diffusion flames were made in zero-G for six months, but none morphed into cool diffusion flames after being snuffed out. “Cool diffusion flames require very low gas velocities because the chemistry is so slow,” Sunderland tells SYFY WIRE. “On Earth, even a candle flame generates velocities a hundred times higher because of buoyancy. We have recently seen some cool diffusion flames on Earth, but these require tricks like heated air and added ozone.”
So how much cooler are these flames than typical fire on Earth? It depends on what you call “cool.” Hot diffusion flames, like what you see when you light a candle, burn at about 3,100 degrees Fahrenheit. Cool flames are a chill 900 degrees Fahrenheit in comparison. Unlike flames that are pushed and pulled by Earth’s gravity, creating the flicker effect, flames in space stay still because next to no gravity is affecting them. They also produce no soot, meaning they could eventually power car and rocket engines with clean energy in the future.
After the hot flames were extinguished, nothing was visible on the camera communicating with Earth. The most sensitive camera that was filming was still not able to pick up on the cool flames until University of Maryland grad student Minhyeng Kim noticed that despite nothing coming up onscreen, the data showed that something was emitting heat. Hot flames had been ignited with n-butane, diluted with nitrogen, and an oxidizer of 40% nitrogen in oxygen. Sunderland had decided on that fuel for a reason.
“This particular mixture was favorable to cool diffusion flames because the butane fuel molecule is relatively large, the oxygen concentration was large, and the pressure was high,” he says. “None of our other tests had this perfect combination of conditions.”
Turned out there was a 3-second delay between hot diffusion flames going out and cool diffusion flames taking over. During that delay, no heat was being produced and no flames could be seen. Cool flames just barely became visible after that. When University of Maryland mechanical engineering student Kendyl Waddell adjusted the video feed, the phantoms of hot flames were reigniting the fuel as cool flames and starting to release heat again. The problem was that they were half the size of hot flames with a tenth of the brightness and heat.
Cool diffusion flames glow blue, and blue flames — no matter how hot they get — won’t pollute the air or clog up engines with accumulated soot. Rocket fuel could eventually be created with hydrogen and oxygen available in space, but the technology isn’t there yet. That scented candle you may light sometimes produces a minimal amount of soot (though most of it is eaten up by the flame as it burns). Upgraded compression engines could be the next thing on the road, because while diesel engines use compression, they belch out too much pollution.
“Today's gasoline engines are ignited with a spark, but their mileage is bad,” Sunderland says. “If gasoline engines could be ignited by compression, it will provide the ideal mix of good mileage and clean emissions. Many have tried, but no one has succeeded. Cool diffusion flames could be the missing link.”
Will cool flame candles be a thing? Probably not, because of the overwhelming stench of formaldehyde.