Flames in space promise better engines on Earth

The discovery was made during a series of experiments on the International Space Station by a team led by Forman Williams, a professor of mechanical and aerospace engineering at the University of California, San Diego. Researchers detailed their findings in Microgravity Science and Technology.

‘We observed something that we didn’t think could exist,’ Williams said in a statement.

A better understanding of the cool flames’ chemistry could help improve internal combustion engines in cars, for example by developing homogenous-charge compression ignition that could potentially lead to engines that burn fuel at cooler temperatures, emitting fewer pollutants while still being efficient.

During the experiments, researchers ignited large droplets of heptane fuel. Initially, the flames looked as if they’d been extinguished - as they would have on Earth - but sensors showed that the heptane was still burning, although the resulting cool flames were invisible to the eye.

The cool flames occurred in a wide range of environments, including air similar to the Earth’s atmosphere and atmospheres diluted with nitrogen, carbon dioxide and helium. The resulting combustion reaction creates toxic products, such as carbon monoxide and formaldehyde, which in turn burn off.

Researchers believe that the cool flames are the result of elementary chemical reactions that do not have the time to develop around burning fuel droplets on Earth, where they can only exist for a very short period of time.

The difference between Earth and the space station is buoyancy. When droplets of fuel burn on Earth, buoyancy limits the amount of time gases can hang around in the high temperature zone around the droplets so there isn’t enough time for the droplets’ chemistry to support the cool flames. In micro-gravity, however, there is no buoyancy, which helps provide enough time for the gases to stay around the droplets and for that chemistry to develop.

The challenge for future applications is to get the right mix of fuels to generate this cool flame combustion here on Earth. To investigate this question, NASA is planning a new series of experiments dubbed called Cool Flame Investigation, starting next winter and continuing for about a year.

The research team, including scientists from UC San Diego, the University of Connecticut, NASA, Princeton, the University of South Carolina, UC Davis, and Cornell, conducted their experiments by remote control from NASA’s John Glenn Research Center in Cleveland.