Today on MSL-1
we highlight some of the combustion science research being done in space.
Combustion Science is the science of burning, and it touches our everyday
lives in many ways -- it is used to propel our cars, to provide heat for
our homes, and to enable industries to create necessary products like food
and clothing. Yet the complexity of the combustion process under varying
conditions often leaves scientists with more questions than answers about
the phenomena associated with burning. To answer these questions, scientists
must look at combustion in its most basic form, and the best place to do
that is in a microgravity environment.
Four combustion experiments, each focusing on a different aspect of burning, will be conducted aboard MSL­1. All four experiments focus on understanding the basic processes of combustion:
Three of these were conducted for the first time in space aboard the initial flight of MSL-1/STS-83. In the Combustion Module-1 (CM-1), Dr. Paul Ronney, of the University of Southern California, will investigate the "Structure of Flame Balls at Low Lewis Number" (SOFBALL); and Dr. Gerard Faeth, of the University of Michigan, will examine "Laminar Soot Processes."
In a separate facility, specifically designed for this experiment, Dr. Forman Williams, of the University of California, San Diego, and his co-investigator, Dr. Fred Dryer, of Princeton University, will perform the "Droplet Combustion Experiment."
A team of scientists are also conducting an investigation in the Microgravity Glovebox titled "Fiber-Supported Droplet Combustion-2" (FSDC-2).
A
rare combustion phenomenon is the focus of SOFBALL. In 1984, Ronney discovered
- by accident - an unusual flame structure during a drop tower experiment.
Expecting to see an expanding spherical flame front upon the ignition of
a gaseous fuel, Ronney instead saw the flame break apart into individual
balls that burned separately, without shrinking or dividing. These flame
balls had never before been observed, although the phenomenon had been predicted
(unknown to Ronney at the time) by a Russian physicist in 1944. That prediction
was dismissed because it was believed that if such flame balls could exist,
they would be so unstable that they would extinguish before they could ever
be seen. However, under the right conditions, the flame balls can exist
and can be observed. With SOFBALL, Ronney continues his exploration of those
conditions and the phenomena they produce. On the first flight of MSL-1,
in two separate experiment
runs, stable flameballs were produced in space for the first time.
The results of the experiment may have practical applications as well. One important application is in the area of spacecraft fire safety. The dilute mixtures of hydrogen and air are not combustible on the ground, and therefore haven't necessarily been considered a fire hazard in the past. However, SOFBALL makes it quite clear that these mixtures are a safety concern in microgravity. Because there are numerous sources of hydrogen aboard a spacecraft, it is important to learn more about how such hydrogen-air mixtures would burn in order to determine the best means of fire prevention or extermination. Another application involves hydrogen-burning engines, which may be used in the future for cars and other devices.
The other experiment housed by CM-1, "Laminar Soot
Processes," will investigate a different aspect of combustion, centering
around how, why, and where soot, a solid byproduct of the combustion of
hydrocarbon fuels, is generated. Inside the module, a laminar jet flame
will be produced by igniting a stream of gaseous hydrocarbon fuel coming
out of a nozzle. A laminar flame is steady, like an unflickering candle
flame, as opposed to turbulent, like a campfire, which flickers and has
unpredictable flow patterns. Laminar flows, which are best produced in
microgravity due to the lack of buoyancy forces, are more easily modeled
by scientists, making them the preferred means of testing theories of combustion.
In this case, Faeth is looking at theories concerning the production of
soot.
Soot is usually an undesirable product of combustion for several reasons. One reason is that soot is a visible pollutant; one sees soot in the black fumes emitted by diesel trucks, processing plants, and chimneys. (It should be noted that recent changes in Environmental Protection Agency (EPA) regulations call for significant reductions in amounts of such particulate materials in the atmosphere.) In addition, soot production is tied to the emission of carbon monoxide - a toxic material - and PAHs (polyaromatic hydrocarbons), many of which are carcinogenic. Another of soot's undesirable qualities is that the thermal radiation, or heat emission, of soot particles is often responsible for the spreading of fires. Soot also hampers efforts to fight fires because its presence can obscure their sources, making it more difficult to extinguish them. However, soot production is useful to the carbon black industry, which is a large industry that uses soot in such products as tires, black plastic, and dry-cell batteries. In addition, many furnace applications rely heavily on heat radiation from soot to transfer heat from flames to boiler tubes in order to produce steam from water. For all of these reasons, understanding the production of soot is a goal that is important to researchers. Once understood, the process could be manipulated to control both visible and invisible pollution from combustion technologies like diesel engines and aircraft gas turbines, to enhance fire-fighting abilities, and to produce soot with qualities that are beneficial to industry.
The Droplet Combustion
Experiment (DCE) will take advantage of the fact that liquid drops tend
to form perfect spheres in a micro-gravity environment. Theories of droplet
combustion are, for mathematical simplicity, best applied to perfect spheres,
which are difficult to form on Earth. Therefore, to test these theories
with any accuracy, the microgravity environment is necessary. Ultimately,
the objective of the experiment is to increase understanding of droplet
combustion processes, which include the transfer of heat and mass within
and around the droplet, the flow of liquid within the droplet, and the
chemical kinetics of the combustion. The knowledge gained from this understanding
could be used to improve fire safety and to obtain cleaner and more efficient
power production from liquid fuels.
In practical applications involving droplet combustion, fuel droplets are free in the atmosphere, making research of free-floating droplets more practical than other methods. To get the best information that they can, Williams and Dryer are aiming for the ideal testing situation - free-floating spherical droplets.
Although droplet combustion experiments have previously been conducted in drop towers and aboard reduced-gravity aircraft, Williams is looking forward to the advantages of the space shuttle. "Those experiments are pretty well restricted to droplets about a millimeter in diameter or smaller because of the time available. The larger the droplet diameter is, the longer the burning time. . . . [The longer time available] is why the space shuttle experiments are attractive, because chemistry such as that involved in pollutant production can change at longer times." The knowledge gained in this extended experiment will lay the groundwork for future theories and future combustion experiments.
we study combustion
in space! 
we study combustion
in space!
June 19, 1997
Author: Dr.
John Horack*
Curator:Bryan Walls
NASA Official: John M. Horack
*Text Adapted from the Microgravity Science Newsletter - Winter 1996