Spherical flames in space could solve the mystery of soot-free fires

Solving this burning question requires starting fires in space.

Ongoing experiments on the International Space Station could help resolve a scientific debate about why some fires burn without producing soot. Made of carbon particles created when fuel fails to burn completely, soot is a pollutant. The particles are linked to health issues, including cancer (SN: 8/4/07, p. 69), and contribute to global warming (SN Online: 3/8/11).

One technique for eliminating soot is by fiddling with the composition of the fire’s fuel and the air surrounding it. Oxygen in the air is necessary for combustion, but air also contains nitrogen, which is inert. By removing nitrogen from the air and mixing that nitrogen with the fuel instead, scientists can produce soot-free flames.

But there are two conflicting views about why the soot doesn’t form. One theory says that the clean flame is due to an altered flow of gases in the fire. The other is that, as a result of the switch-up, the temperature and makeup of the fire varies across the flame in a way that prevents soot from forming.

To test which idea is correct, scientists need control over the flow of gases in a flame. But that’s tough on Earth, since hot gases inevitably flow upward, giving candle flames their familiar elongated shape, for example. But in the space station’s microgravity, that upward flow is prevented; the gases don’t rise, producing a ball of fire that can be tweaked to adjust its flow.

“Having a nice spherical flame is just not possible on Earth,” says engineer Richard Axelbaum of Washington University in St. Louis. Axelbaum is one of the researchers working on the experiments, which are remotely controlled from the ground. “The space station allows us to carefully study the problem and better understand it.” Results from the experiments should be published in several months.