In order to better understand how flammable certain liquid fuels are in space--and how easy it is to extinguish them--scientists designed an experiment that burned small droplets of fuel inside a chamber. The chamber then had various gasses introduced to test how good or bad they were at putting out the flames. The FLame Extinguishment Experiment was called FLEX, and ran from October 2008 to May 2012.
During the first set of experiments, researchers burned 100 small spherical droplets of heptane (a fuel found in gasoline). What they found was surprising: the heptane ignited into a normal ball of flame for 20-seconds and then extinguished, at which point it continued vaporizing. In other words--at first the flame was visible, but when it burned out, the heptane continued to burn under an invisible flame. This is called a cool-flame, something that happens under controlled circumstances here on earth...except for the fact that the order is always reversed. An invisible cool-flame always leads to a hot-burning visible flame.
According to NASA: “This behavior is not explained by conventional theories of droplet combustion, and is hypothesized as a low temperature chemical reaction (cool flame) that continues after the 'hot' flame extinction. Since cool flames typically occur in premixed systems (this is a diffusion flame), are transient (this is quasi-steady) and lead to a hot-flame ignition (this is a hot-flame leading to a cool flame that extinguishes), which is a very unique observation with very significant theoretical and practical implications.”
Scientists can now use this information to better predict how fuel will burn in space. But it has implications here on earth too. Researchers on the project suspect the knowledge will be useful in re-engineering combustion engines to get better gas mileage and reduce pollution.
You can read more about the experiment here.