Floating Flame Balls - Flames Explored

Flame balls, on the other hand, are simple. The balls form in low gravity where turbulence and buoyancy have little effect. Oxygen and fuel combine in a narrow zone at the surface of the ball, not hither and yon throughout the flame. Once ignited and stabilized, their size remains constant. Unlike ordinary flames, which expand greedily when they need more fuel, flame balls let the oxygen and fuel come to them. Finally, the fact that flame balls are spherical reduces their dimension to one: the radius of the flame itself. "Flame balls are to combustion scientists what fruit flies are to geneticists," says Ronney. "It's not that we want more fruit flies, or flame balls, but they provide a simple model for testing hypotheses and checking computer models."

One of many mysteries about fire is the way weak flames go out before their fuel is totally exhausted. It puzzles physicists and vexes automakers who want to build clean, efficient "lean-burning" engines that run on fuel-air mixtures with low fuel concentrations-much like a flame ball. Ronney believes that studying one (flame balls) will help us with the other (cars).

Credit: Paul Ronney. A schematic diagram of a flame ball.

Here on Earth, researchers can't study flame balls for long. A typical plunge down the drop tower lasts only 2 seconds. So, working with NASA scientist Karen Weiland and others at the Glenn Research Centre. Ronney designed the Structure of Flame Balls at Low Lewis-number (SOFBALL) experiment. It's a sealed chamber where flame balls flying onboard the space shuttle can burn for a long time.

SOFBALL orbited Earth for the first time in 1997 on shuttle Columbia, and it produced some surprises.

Computer models had predicted the flame balls would be small and either extinguish or drift into the chamber walls in a few minutes. Instead they were two to three times larger than predicted and burned for over 8 minutes until the experimental system automatically extinguished them. Furthermore, although the flames were large, they were the weakest ever seen-emitting little more than 1 watt of thermal power.

The experiment, upgraded and re-named SOFBALL-2, will soon fly again. It's slated for launch onboard space shuttle Columbia (STS-107) in late 2002 or 2003. During the mission, flame balls will be allowed to burn for 25 to 167 minutes. Instruments will monitor their temperature, brightness, heat loss, and the composition of their gaseous byproducts. Because flame balls are so sensitive to motion, the shuttle will drift during the experiments instead of using its reaction control thrusters to maintain position.