Floating Flame Balls - Flames Explored

Flames do something odd in space: they form tiny almost-invisible balls that might reveal the secrets of combustion here on Earth.

Paul Ronney wasn't looking for flame balls. They came as a complete surprise.

It happened in 1984 when Ronney, a combustion researcher, was at the NASA Glenn Research Centre's Microgravity Drop Tower in Ohio. He pressed a button and sent a can of burning hydrogen falling down a 90 ft. shaft. For 2.2 seconds it plummeted, freely falling and weightless, with a 16mm movie camera recording the action. Ronney knew that flames did strange things in low gravity - that's why he was doing the experiment - but he wasn't prepared for what he saw in the film room later.

The flames had broken apart into tiny balls that moved around like UFOs. "I thought I had done something wrong," he recalled. Some of his colleagues didn't believe him when he described the experiment. Indeed, "it was ridiculous. No one had ever seen anything like it."

But the flame balls were real. Later experiments proved it.

"Flame balls are the weakest flames we have," says Ronney. "Compared to a birthday candle's 50 to 100 watts, a flame ball produces only 1 to 2 watts of thermal power. They burn using very little fuel. It's almost as if a hydrogen-burning flame's last line of defence as it approaches extinction is to draw itself into a simple ball."

Ronney, who is now an engineering professor at the University of Southern California, believes that flame balls will help him and others crack the unsolved mysteries of burning. Considering that combustion powers our automobiles, generates our electricity, and heats our homes, there's much about it we don't understand. "For example," he says, "a moderate amount of turbulence makes a flame burn faster, but too much turbulence extinguishes it." No one knows why.

Tiny flame balls that form in low gravity are hard to see. These were filmed in the dark by a low-light video camera onboard the space shuttle Columbia in 1997.

Flames are hard to understand because they are complicated. In an ordinary candle flame, for example, thousands of chemical reactions take place. Hydrocarbon molecules from the wick are vaporized and cracked apart by heat. They combine with oxygen to produce light, heat, carbon dioxide and water. Some of the hydrocarbon fragments form ring-shaped molecules called polycyclic aromatic hydrocarbons and, eventually, soot. Soot particles can themselves burn or simply drift away as smoke. The familiar teardrop shape of the flame is an effect caused by gravity. Hot air rises and draws fresh cool air behind it. This is called buoyancy and is what makes the flame shoot up and flicker.