Focus: Fire on a Spinning Disk
From bonfires to match sticks, flames usually have simple, predictable shapes. But in the 17 January PRL a team describes a completely unexpected burning pattern: spiral shaped flames on the surface of a burning, spinning disk. Although spiral patterns have been seen in a wide range of chemical and biological systems–including the burning of premixed gases–no one had observed them amongst the complex flows of mixing gases that occur when the fuel and oxygen molecules must diffuse to find one another. The experiments may provide the first opportunity to derive spiral patterns from basic physics equations–a feat that has so far eluded researchers studying spirals in other systems.
Vedha Nayagam of the NASA Glenn Research Center in Cleveland and Forman Williams of the University of California at San Diego were studying the behavior of flames on a moving surface when they noticed the surprising pattern. Their procedure was to take a blowtorch to the underside of a suspended, horizontal plastic disk about 1.5 times the size of a CD before setting it spinning at speeds between 2 and 20 rotations per second. They found that if the disk wasn’t quite hot enough to sustain burning for more than a minute or so, the flame formed a spiral shape, something like a Nike “swoosh” curled into a circle. It moved around the disk surface in the opposite direction from the disk’s rotation, often traveling as a fixed shape, except for the tip, which “meandered” from side to side. “We were always thinking in terms of flat [circular] flames,” Williams recalls, so the spiral was “really weird–we didn’t expect that.”
Since burning requires heat, oxygen, and fuel (vaporized molecules of plastic in this case), the flame motion is determined by a delicate balance between the swirling motion of the air near the disk surface and the propagation speed of the flame front. As fuel and oxygen are depleted by the combustion at a given location, the flame dissipates, but by the time it circles back–typically in one-fifth of a second–enough plastic and oxygen molecules drift back to allow the flame to reignite. Nayagam and Williams were able to derive the shape of the moving flame front by assuming a fixed relative speed of the flame front and using their knowledge of the gas flow near the disk.
Dynamic spiral patterns have appeared in all sorts of situations, such as heart muscle electrical signals, Belousov-Zhabotinskii chemical reactions–where a colored wave front advances slowly through a medium in a petri dish–and flames where the oxygen and gaseous fuel are premixed. But none of the previous examples require diffusion of the elements involved; instead, they are so-called excitable media, where the spiral wave front propagates through a static medium as some parameter (such as temperature) becomes larger than a threshold value, pushing the process forward. Many of the spirals that Nayagam and Williams found in the scientific literature had similar properties, including a meandering tip.
Williams hopes to completely explain the spiral flame shapes with a fundamental theory, starting from basic physics principles–an achievement he says has not been accomplished with any spirals in excitable systems. He is betting that the simple geometry of the spiral flames combined with the well-documented near-disk air flow will give him an advantage.
Michael Gorman of the University of Houston doubts the work will shed light on other spiral-pattern-forming systems, but he still calls the work “really remarkable stuff.” Gorman finds it “amazing” that such well-organized patterns appear in a diffusing system and says it suggests that spirals might appear in even more complicated situations that have yet to be discovered.