Lighting fires within a contained environment shared with considerable amounts of highly flammable materials that also happens to be traveling 200 miles above Earth may not seem like the wisest pastime. Nonetheless, a group of researchers based at UC San Diego has been hard at work igniting large droplets of heptane and methane fuel in a wide variety of environments aboard the International Space Station, ranging from ones typical of Earth to those saturated with helium, carbon-dioxide, or nitrogen.
The result: "We observed something that we didn't think could exist," Forman Williams, the research team's leader and co-author of a new open-access paper describing the findings, said in a statement from UCSD.
As the researchers watched the flames seemingly extinguish themselves—within the safe confines of the station's Multiuser Droplet Combustion Apparatus—as they would on Earth, what they discovered is that their fires continued to burn, albeit invisibly, for a would-be impossible period of time. This extension is what's referred to as "cool flame." What's more, the toxic byproducts of the combustion reactions are also in turn burned away. Rather than releasing carbon monoxide and formaldehyde into the atmosphere, the cool flames stick around long enough to burn those away along with the proper fuel sources.
"After the visible hot flame radiatively extinguished around a large heptane droplet, the droplet continued to burn with a cool flame," the paper summarizes. "This phenomena was observed repeatably over a wide range of ambient conditions. These cool flames were invisible to the experiment imaging system but their behavior was inferred by the sustained quasi-steady burning after visible flame extinction."
The secret to the cool fires is in the "spherical symmetry" afforded by microgravity environments. Imagine a fire burning here on Earth with Earth gravity. As the reaction continues and byproducts are released, those byproducts are in turn pulled downwards and away from the flame itself. As such, these byproducts cease to be as available to the reaction, which ends before those released materials can themselves be burned up. In microgravity, however, the byproducts stick around the flames longer, allowing time for further chemical reactions. It's a simple enough idea.
And while the result points the way to future internal combustion engine technologies that are vastly cleaner and more efficient, at the moment it's difficult to imagine the reactions occurring outside of the microgravity environment. "Things can happen out there that can't happen here," Williams noted.
Cool flame in itself is not a phenomenon limited to space. It's uncommon on Earth, but it does happen. The differences between "hot" and "cold" varieties are fairly plain: When a cold flame ignites, it might only kick out heat hotter than its surroundings by a few tens of degrees Celsius, while a hot flame spikes the temperature by thousands. What's more, compared to hot flame, cold flame kicks out byproducts that are larger and more complex. Cold flame byproducts then tend to recombine with each other, leading to the release of less light, carbon dioxide, and, indeed, heat. As such, a cool fire might persist for far longer periods of time than its hot counterpart; cool flame might even be considered more of an extended chemical reaction that produces some heat rather than proper fire.
"The difference and exciting part of the ISS experiments is that the typical progression on Earth is that a cool flame leads to a hot flame," Daniel Dietrich, a NASA engineer and study co-author, told Motherboard. "The chemical by-products that form in the cool flame burn off in the hot flame. While they burn off, these low temperature or cool flame chemical reactions are of significance in that they are determine engine efficiency and pollutant formation."
Cold flame also happened to be a concept instrumental to the Allied victory of World War II, Dietrich notes. Likely its most applicable concept here on Earth is in "engine knock." This is where, due to a suboptimal fuel mixture, cold flame sneaks ahead of the desired hot flame progression through an engine cylinder, in essence causing an extra combustion event. Given enough time (a very small amount of time actually), this will ruin an engine.
"The Allies, specifically the British I think, understood that certain fuels resisted engine knock more than others and therefore allowed them to have higher performance aircraft engines," Dietrich said. "This was critical in achieving air superiority during the war."