Fire On The Move

Flame is notoriously difficult to study because it is rendered chaotic by convection, a phenomenon in which hot gas rises. To eliminate convection-induced complexity, Dr. Ory Zik and Prof. Elisha Moses of the Institute's Physics of Complex Systems Department "squeezed" fire into two dimensions. In their experiment, soon to be reported in PHYSICAL REVIEW LETTERS, fire propagates in a controllable manner through a sheet of paper contained in a transparent case, leaving behind a pattern of long, finger-like projections. The scientists controlled the propagation rate and the density of the fingers by adjusting the oxygen supply.

Much to their surprise, they discovered that flame dynamics are governed by the same laws that describe more stable phenomena -- for example, penetration of a liquid into a porous material. Since these laws are relatively simple, it is a real boon for scientists to know that they can also be applied to systems as unstable as fire. The theory to explain the phenomenon was developed with the help of Dr. Zeev Olami of the Chemical Physics Department.

Interestingly, at about the same time as this study, NASA's scientists launched an experiment aboard the space shuttle to examine the way fire spreads in outer space. They were amazed to discover that their flames advanced slowly but steadily like a fiery monster with finger-like projections. The NASA researchers approached the Weizmann scientists, who provided a simple explanation to this puzzling phenomenon: The astronauts had observed exactly the same finger-like pattern as in the Institute experiment, only in three dimensions. In both cases, the key element was the absence of convection. While on Earth convection was neutralized by creating a 2-D system, in space, convection was absent because in zero-gravity hot air doesn't rise.

Thus, the Weizmann experiment provides a low-cost terrestrial alternative to studying the spread of fire aboard spacecraft. Moreover, it makes it possible to establish criteria for detecting slow-moving low-convection flame. Such fires are particularly dangerous because they may not generate enough smoke and heat to activate regular smoke detectors.

Apart from fire detection in outer space, a better understanding of flame dynamics may help detect flames that propagate through panel-enclosed surfaces. This would be crucial for airplane safety, where even a small fire can lead to catastrophe.

Dr. Olami holds the Morris and Ida Wolf Career Development Chair.

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The Weizmann Institute of Science, in Rehovot, Israel, is one of the world's foremost centers of scientific research and graduate study. Its 2,500 scientists, students, technicians, and engineers pursue basic research in the quest for knowledge and the enhancement of the human condition. New ways of fighting disease and hunger, protecting the environment, and harnessing alternative sources of energy are high priorities.