Prometheus would be so proud. As part of a NASA experiment, humans have brought fire to the International Space Station (ISS) to see what happens to flames in vanishingly low gravity. The experiment, called Flame Extinguishment-2 (FLEX-2), aims to improve our knowledge of how various liquid fuels burn and what they produce so that we can create cleaner, more efficient combustion engines.
Installed on the space station in 2009, FLEX-2 takes advantage of the unique conditions in space to simplify studies of combustion. In microgravity, liquid fuel can form almost perfectly round droplets. When these spheres ignite, the flame burns in a ball, giving scientists cleaner geometry for running models and calculations.
Achieving this level of simplicity, though, was no mean feat, says C. Thomas Avedisian at Cornell University, who is a co-investigator on the FLEX-2 team. “I would argue that this is the most difficult combustion configuration to create for liquid fuel,” he says. “This experiment took decades to perfect, going back to the mid-80s.”
In the latest test run, seen in the video above, the FLEX-2 chamber—about the size of a breadbox on the inside—is filled with a pressurized mix of oxygen and nitrogen designed to simulate the air at Earth’s surface. Needles dispense a 3-millimeter droplet that is half isooctane and half heptane. This chemical brew serves as a simpler stand-in for gasoline, says Avedisian. The two liquids generally burn in similar fashion, but gasoline can contain so many different compounds that its behavior is harder to model.
Two wire loops conduct current to heat the drop until it ignites, sparking a glowing ball of blue flame that burns at around 2000 Kelvin. Don’t be deceived—the burning sphere isn’t suddenly transported to a starry sky. The chamber lights go out to make the flame easier to see, but that also makes specks on the images, caused by tiny imperfections in the video sensors, more apparent. The ball of flame then begins to oscillate as combustion dies out, making it seem to pulse through the chamber like a jellyfish swimming. Eventually, the ball radiates away so much heat that the scorching-hot flame is snuffed out.
Avedisian and his team have run several tests like this, mixing up the fuel types and drop sizes to check for various effects. They are able to control the initial setup in real time via a video feed routed to the lab in Cornell, then watch as the automated test runs its course. The lab team also runs similar experiments on the ground looking at droplets closer in size to the micro-scale variety created as fuel is injected inside a car engine. To simulate low gravity on Earth, the Cornell team drops their droplets—they send the burning orbs through a 25-foot free-fall chamber and film them on the way down.
The droplets formed in the space experiments let the team see the combustion physics at larger scales and compare the results to the tests done on Earth. One somewhat puzzling discovery is that the jellyfish-style pulses only happen when the droplet is big enough—about 3 millimeters or larger—and they don’t happen all the time. “The flame oscillations are really not well understood,” says Avedisian.
Ultimately, studying the levitating fireballs might reveal ways to make fuels burn cleaner. “What we think is that there is a low-temperature, or 'cool flame', combustion zone—the droplet is still burning even though we can’t see the flame,” says Avedisian. In this zone, the fire is only burning at about 600 to 800 Kelvin.
“Engine manufacturers have been studying ways to reduce pollution that involve the use of cool-flame chemistry, and that chemistry is not as well understood as hot-flame chemistry,” adds FLEX-2 principal investigator Forman A. Williams at the University of California, San Diego. “By studying the cool flames that we found in the ISS experiments, we may be able to obtain better understanding of that chemistry, which then could be helpful to the engine manufacturers in their designs.”