The forms of flames on Earth are familiar to everyone. We all know what a burning match, candle, fireplace or blowtorch look like -- or a burning building, or rocket ignition blast. The presence of gravity and the effects of air or gas movement, plus the type of fuel and oxidant, determine everything from a flame's shape and temperature to burn rate, burn pattern, soot production and deposition and how fast it will or won't be extinguished.
"But in the microgravity of space, we are not dealing with just another old familiar flame," says Dr. Vedha Nayagam of NASA's National Center for Microgravity Research on Fluids and Combustion at the Glenn Research Center (GRC) in Cleveland, OH, where the nature of combustion in space is being studied intently by teams of scientists.
Above : On Earth, gravity-driven buoyant convection causes a candle flame to be teardrop-shaped (A) and carries soot to the flame's tip, making it yellow. In microgravity, where convective flows are absent, the flame is spherical, soot-free, and blue (B).
"The absence of gravity's effects on convection aboard the Space Shuttle, a space station or other space vehicle makes flames behave in ways that can be either beneficial -- as a test bed for research -- or very dangerous in the case of a fire in materials, chemicals or electrical devices. It is vital to know what makes flames start and stop in low gravity, and how flames behave while burning. The safety of NASA's space crews and vehicles can depend on our knowledge of combustion in space."
Watching the Flame Go 'Round
Recently, Dr. Nayagam and Dr. Forman Williams of the University of California at San Diego, a co-investigator in NASA/GRC's microgravity combustion science program, came upon some startling discoveries about flames on Earth that could help scientists understand how flames behave in microgravity.
Nayagam and Williams ignited a plastic disk a little bigger than a CD with a blowtorch and then spun it slowly (2 to 20 revolutions per second) in still air. They expected to see flames burning as a horizontal disk. Instead, the flame burned in a flat spiral pattern, with the spiral moving in the direction opposite to the disk's spin. As the flames lessened their tips exhibited a strange meandering motion from side to side.
Right: Flames on top of a disk slowly spinning in a clockwise direction burn in a spiral headed counterclockwise. Vedha Nayagam and Forman Williams are studying this phenomenon, which occurs both on Earth and in microgravity, in the hopes of fully explaining the pattern with basic physics principles.
Starting a fire at the center of a still disk is like dropping a stone in a quiet pond, says Nayagam. It produces a flame front that moves outward in a circle, fading as the fuel (the disk) is consumed. If you spin the disk, then the circular disk flames become spiral flames under some conditions.
"Under slow spin conditions ... just before circular flames extinguish, [the flames] break symmetry -- and spirals appear in the center hole of the flames and propagate outwards in a spiral instead of in a circular wave front," he explained.
"Spiral burning could be common in the slow, swirling flows that we can establish in a microgravity environment -- but these results were very unexpected in normal Earth gravity," added Dr. Williams. "We plan to explore further what causes the spiral flame pattern, and what causes the tips to follow a [chaotic] meandering path."
Left : At NASA's Johnson Space Center, there is a microgravity research aircraft used to fly parabolas to investigate the effects of "zero" gravity. The KC-135, typically used by the USAF for aerial refueling, is the military version of the venerable Boeing 707airliner.
Nayagam says it's an advantage to be able to generate these flames in the lab under normal gravity, where it is easier and less expensive to study them than on the Space Shuttle. The investigators plan to conduct further tests with spiral flames on board the Johnson Space Flight Center's KC-135, which can create brief microgravity conditions in parabolic flight.
Why Set A Spinning Disk On Fire?
"We need to discover how and why flames propagate in microgravity, and under what conditions flame propagation changes. Hopefully the studies will also explain turbulent combustion, as the swirling flow is vital to understanding the phenomenon called fire whorl," says Dr. Nayagam.
"Understanding these surprising phenomenon may enable scientists to predict flame extinction and to help mitigate fire risks on Earth and in microgravity," states Dr. Nayagam. "The initial and on-going basic reason for NASA's combustion studies is to learn about spacecraft fire safety. We need to answer questions such as: what is the worst condition for fire in a microgravity environment, and under what conditions a fire will increase its burn rate or be extinguished. Our goals include learning under what conditions materials in a spacecraft will or won't support fire."
"The bottom line," Dr. Nayagam says, "is that this simple system of flames on a spinning disk under variable controlled conditions illustrates more complex systems on Earth, in spacecraft, and in the human body."
Source : Nasa Science Journal - 2000