In microgravity, flames burn spherically (Credits: NASA).

In the weightless environment of space, fire burns differently because the convective forces are not present. Flames burn in a sphere, at lower temperature, and require less oxygen compared to those in the gravity of Earth. The Flame Extinguishment Experiment-2 (FLEX-2) is a continuation of the FLEX program, which studies the special burning characteristics of fireballs on board the International Space Station.

The FLEX-2 project is conducted in the Combustion Integrated Rack within the American Laboratory Destiny onboard the ISS, which is equipped with sensors and a video camera that records the experiments.  Inside the rack, is the Multi-User Droplet Combustion Apparatus (MDCA), which can be filled with various mixtures of oxygen, inert gases, and combustible fluids. Small drops of fuels such as methanol and heptane are injected, ignited, and burned for approximately 20 seconds.  These experiments are recorded and measurements are taken for further analysis back on the ground.

While the first FLEX project investigated the physics of flame extinction, which is important for fire safety on a spacecraft, FLEX 2 is focusing on improving the understanding of the combustion of liquid-fuels.  Forman Williams is a combustion researcher at the University of California, San Diego and principal investigator for the FLEX program.  Williams and his colleagues have already discovered an interesting result. “We were burning these heptane droplets out there on station, and we saw the hot flame extinguish, but the droplet kept decreasing in size. It was just like if it was burning, but we could not see any flame — it was almost like an invisible flame was causing these heptane droplets to burn steadily,” Williams explains. “We didn’t even believe it for a year.” The existence of cool flames has been known for some time; however, they have not been fully understood.  Improving the knowledge of cool flames will aid the development of efficient, low emission engines which have been troubled by this problem.

Cool flames were discovered in the 1810’s by Sir Humphry Davy when certain fuel and air mixtures would not ignite a match or burn his finger.  The first recording of the spectra of such phenomena was recorded in 1929 and was the origin of the name cool flame.  Cool flames can result in conventional flames without an ignition source through changing conditions; this is not just a laboratory condition this phenomenon is responsible for engine knocking in poorly tuned engines.

Space provides a unique environment for the study of flame behaviour.  In space you remove gravity and convection from the experiments and can study elements that could not be studied on Earth. Although microgravity conditions can be achieved through Drop Towers on Earth, these facilities are not suitable for long-duration flame experiments. Fire experiments are not only important for improving the fire safety aboard current and future spacecrafts. It can also have practical benefits here on Earth.

“If we hadn’t done these experiments in station, this phenomenon [in space] would not be known today, so we were really excited about that,” Williams says.

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