New Space and Tech

Why NASA Is Lighting ‘Cool Flames’ Inside the ISS

By James Careless
July 20, 2023
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Why NASA Is Lighting ‘Cool Flames’ Inside the ISS
NASA astronaut Shane Kimbrough completing the Multi-user Droplet Combustion Apparatus reconfiguration to the cool flames investigation setup.
Image credit: ISS National Laboratory

The inside of the International Space Station (ISS) isn’t somewhere where you would expect astronauts to be deliberately lighting fires. But they are — specifically to generate ‘cool flames’ that burn at temperatures much lower than traditional hot flames. The reason? Cool flames could enable far more efficient and less-polluting internal combustion engines here on Earth, if the science can be worked out.

This project is being conducted by NASA’s ISS National Laboratory, with assistance from the University of Maryland, Washington University in St. Louis, and the University of California San Diego. They’ve decided to use the ISS over a terrestrial lab because it is far easier to create and maintain cool flames in microgravity than it is on Earth — in a one-G environment, cool flames snuff out very quickly due to gravity-based buoyancy.

However, if the scientists can crack maintaining cool flames, they could be used to produce far more efficient engines that waste less energy during the combustion process — an upgrade that could have serious implications for transit-related greenhouse gas emissions.

“Consider the example of a highway vehicle fueled by gasoline or diesel,” University of Maryland fire protection engineer Peter Sunderland told SpaceRef. “One-third of the fuel energy goes into the radiator to keep the engine cool. The resulting thermal efficiency is about 35 percent. Now imagine running much more air into this engine [by using cool flames]. The combustion would be much cooler and perhaps no radiator would be needed. The resulting thermal efficiency could increase to 60 percent.”

The fact that cool flames disappear quickly in gravity explains why the ISS is an ideal place to study them. To learn how they function, these ‘cool diffusion flames’ (their official title, aka CDFs) “require long residence times, which is difficult when buoyancy is present,” Sunderland said. “It is much easier to get these times in long-duration microgravity. CDFs are easier to obtain in the high pressures of engines, but this is a difficult place to observe them.”

Thanks to the ISS’ ongoing research into CDFs, the researchers have identified conditions that support their persistence over time. “The next step will be to run detailed simulations of these flames and test the available chemistry models,” said Sunderland. “Improvements in these models could then be used to design highway vehicle engines with up to 60 percent thermal efficiency.”

Image captured during cool diffusion flame research. Image credit: ISS National Laboratory.

Similar improvements could also be made in aircraft and ship engines, both of which contribute to greenhouse gas emissions as well. This same research could even help scientists “learn more about the transition from smoldering to flaming in wildland fires,” Sunderland added.

The good news: The ISS’ research into cool flames is part of a larger effort that is already showing results in Earth-based applications.

“It has already been demonstrated [as feasible] for some internal combustion engines,” Sunderland told SpaceRef. “However, these have become unstable for different speed and throttle settings, or when the fuel is changed. More research is needed to overcome these hurdles.”

James Careless

James Careless is an award-winning satellite communications writer. He has covered the industry since the 1990s.