Not Just
Another Old Flame
NASA scientists have discovered
unexpected spiral-shaped flames on Earth. By studying
these peculiar flames, researchers hope to mitigate
fire hazards on spacecraft and gain new insights about
complex systems in nature.
May
12, 2000 -- 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). [more
information]
"The tall spear-shaped flame on a
candle, or the "roaring hearth" look of
bonfire-type flames, or the forced-air look of a
rocket or furnace flame are very different indeed in
the absence of gravity," Dr. Nayagam states.
"Soot production, burning rates, completeness of
combustion, exhaust products and other characteristics
all change radically in space.
"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
nicknamed the "Vomit
Comet" 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?
|
Spirals, Spirals,
everywhere....
 There
are many spiral forms in nature, both on Earth
and in space. Spirals occur in physical forms
such as DNA and the shell formation of
mollusks such as the conch and chambered
nautilus. They also occur in wind patterns,
including hurricanes and tornadoes. They are
present in air and flame forms known as
vortexes and whorls.  And
they occur in the way things fall in the
atmosphere, from leaves to aircraft. In the
human body, the spiral pattern of the
heart's bioelectric impulses causes the
chambers to beat with a spiral pulsing rhythm.
Brain waves, comprised of neuron impulses,
seem to flow along the neurons and down the
spinal cord in a spiral pattern. Some evidence
shows bioelectrical spiraling in  the
labor impulses during birth. Finally, we see
spiral forms omnipresent throughout the
visible and invisible universe, in galaxies,
accretion disks around black holes, coalescing
interstellar clouds and many other forms of
matter and energy.
|
"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."
Readers can learn more about flames
in space at the Microgravity
Combustion Research home page.
Related Links:
Rotating
Spiral Edge Flames in von Karman Swirling Flows
-- by V. Nayagam and F. A. Williams. Physical Review
Letters -- January 17, 2000 -- Volume 84, Issue 3,
pp. 479-482
Micro-fireballs
Lighting the Way to Better Engine Designs--
1997 Science@NASA headline story
Fires
in Space - basic information about
combustion in low gravity from microgravity.com
Microgravity
Research Program Office -- an overview of
NASA's microgravity research, hosted by the NASA
Marshall Space Flight Center
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