The simple flame is an extraordinarily complex phenomenon, one that involves thousands of chemical reactions. But fire also plays off the effects of gravity. So what happens when you take that gravity away? As a recent experiment aboard the ISS has shown, you get a very strange flame, indeed.
We've talked about flames in space before, but it's worth a quick review to understand the recent experiment and the unexpected observation.
It may seem counter-intuitive, but a flame’s characteristic teardrop shape is actually the result of gravity. The effect is called buoyancy, and it happens when hot air rises and draws fresh cool air behind it. So it’s gravity that essentially makes the flame shoot up and flicker.
When you take away that gravity — or microgravity, as is the case aboard the International Space Station (ISS) — flames tend to burn a bit differently. They form little spheres.
And fascinatingly, unlike a flame bound by gravity, a flame placed in a microgravity environment lets the oxygen come to it. As NASA researchers observed in a recent ISS experiment called FLEX (FLame Extinguishing eXperiment), oxygen and fuel combine in a narrow zone at the surface of the sphere. The process is much simpler and considerably less chaotic.
Strangely, the NASA scientists, which included Dr. Forman Williams of UC San Diego, noticed that small droplets of heptane continued to burn even after the flames went out — they appeared to be burning without flames.
The researchers later realized that the flames were not in fact extinguished, but were instead just too faint to see. Called cool flames, they burn a bit different than the flame balls. Cool flames burn at the relatively low temperature of 500K to 800K. Regular fire, on the other hand, burns between 1,500K and 2,000K.
What’s more, the chemistry is completely different. Normal flames produce soot, CO2 and water, whereas cool flames produce carbon monoxide and formaldehyde.
Interestingly, a similar process may be replicated here on Earth, which could result in cleaner auto ignitions.
All images: NASA.