The science of combustion is the science of burning.
The study of how things burn plays a key role in everyday life
by generating new knowledge that leads to, for example, making
safer and more efficient car engines and lowering our home heating
and cooling costs. In subjects of global impact, the study of
combustion plays a role in, among other things, improving air
pollution, rocket and jet propulsion, and even global heating
(or cooling!).
On Earth,
gravity causes denser (heavier) gases or fluids to fall, and less
dense (lighter) gases or fluids to rise. If you've ever jumped
into a deep lake on a warm summer day and gone down a few feet,
you've felt the water getting much colder the further down you
go - cold water is denser than warm water*.
This stratification of the lake water is partly caused by gravity-driven
(or buoyancy-driven) fluid flow. One major difference between
buoyancy-driven fluid flows and combustion processes is that combustion
involves much larger temperature variations - occuring over a
relatively short amount of time. The temperature variations are
caused by chemical reactions which release large amounts of heat
("exothermic" reactions). For example, the temperature
of a reactive mixture can start at 25 deg. C and increase to over
2750 deg. C in less than a second! That's an increase from about
room temperature to boiling iron! Such a large temperature increase
over a short time leads to correspondingly large density differences
in the fluids or gases and hence, to the existence of strong buoyancy-driven
fluid flows - the same effect that causes the warm lake water
to rise. Buoyancy flows can modify, mask, or even dominate other
processes that mix and heat the fuel and oxidant reactants before
chemical reactions can be started.
Important as it is, buoyancy is frequently neglected in the mathematical analysis of combustion processes. Sometimes, it's left out for mathematical simplicity, or sometimes to attempt to model the characteristics of other combustion processes which do not depend on gravity. However, leaving out buoyancy in a mathematical model can make direct comparison between theory and ground-based experiments either difficult or meaningless.
The only way to eliminate buoyancy-driven flows from the math and have it make sense is to conduct your comparative experiments in an environment which eliminates buoyancy-driven flows. Where is that? You got it - in microgravity. Some experiments have been conducted on the ground in drop towers and drop tubes, but the few seconds of low-gravity are insufficient to carry out extended observations that are needed for detailed analysis.
For example, the sequence of frames on the
left shows a 3mm heptane droplet, photographed in ultraviolet
light, at the start of a 3 second descent in the NASA / Lewis
Research Center Drop Tower. Each frame (going from left to right,
then top to bottom) shows the droplet shrinking. How will it burn
itself out? Can such a flame be made to be stable? What happens
if you can control the flow of air (or other gases) around the
droplet? What happens if you change the initial size of the droplet?
What about the byproducts of burning - such as soot? Does soot
affect the combustion process itself? The only way to answer these
and other basic questions is to conduct combustion experiments
in a long-duration microgavity facility, such as Spacelab or Space
Station.
Conducting experiments like this
helps simplify the task of accurately modeling combustion processes,
leading to a better understanding of the underlying principles
of combustion. Understanding the basics will also, almost paradoxically,
help us understand how gravity affects combustion and help lead
to better solutions for everyday life.
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candle burning in 1g
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candle burning in 0g (USML-2)
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Combustion science in microgravity is led by scientists from NASA at Lewis Research Center, and their colleagues world-wide. The team at Lewis maintains in-depth combustion science information on the web.
*Actually, cold water is only denser than warm water until it begins to approach the freezing point. At approximately 4 deg. C, water begins to get less dense as the water molecules align themselves for the phase change to ice. Ice is less dense that liquid water - that's why ice floats! back up to primer
Adapted from NASA's "A Teacher's Guide With Activities", produced by the Microgravity Science and Applications Division, Office of Space Science and Applications, and NASA's Education Division, Office of Human Resources and Education.
author/curator: Bryan Walls
NASA Official: John M. Horack
April 1, 1997