Fire station in space - experiments onboard the ISS. Credit: NASA
It is known as the Flame Extinguishment Experiment, or FLEX for short.
The work on improving fire-fighting techniques in space is being done on the Destiny module of the International Space Station. While important for human space travelers, the experiments on the ISS are also providing a better understanding of fuel combustion here on Earth.
Forman Williams, a professor of mechanical and aerospace engineering at the Jacobs School of Engineering at the University of California, San Diego, has been working on fire research and fire safety with NASA since the 1970s.
The experiments take place in a chamber onboard the ISS Destiny module – part of a piece of equipment called the Combustion Integrated Rack. The hardware is roughly the size of a 5.5-foot bookcase and weighs close to 560 lbs.
The rack itself is crammed with sensors and equipped with video cameras that record FLEX experiments. A device called the Multiuser Droplet Combustion Apparatus is used to generate and ignite droplets from different fuels in different atmospheric conditions.
Flame behavior
Flames in space can burn at a lower temperature, at a lower rate and with less oxygen than in normal gravity. This means that materials used to extinguish fire must be present in higher concentrations. The slow flow of air from the fans mixing air in a spacecraft can make flames burn even faster.
To help understand how flames behave and burn in space, inquisitive FLEX researchers have ignited a small drop of either heptane or methanol. As this little sphere of fuel burned for about 20 seconds, it was engulfed by a spherically symmetric flame. The droplet shrank until either the flame extinguished or the fuel ran out.
ISS is equipped with carbon-dioxide (CO2) fire extinguishers, so FLEX researchers are focused on how fuel droplets burn in the presence of different amounts of CO2. Also, ambient air can become completely fire safe when there is not enough oxygen for fuels to ignite. This threshold is called the limiting oxygen index.
As an output from their ISS FLEX work, Williams and colleagues pinpointed this index for methanol and heptane on the space station.
Cleaner fuels
Williams is now working on a new series of experiments, called FLEX-2, which aims to recreate conditions that are closer to what actually happens in a combustion engine.
Findings could lead to new designs for cleaner fuels that have a smaller carbon footprint and emit fewer pollutants, among other applications.
There are some intriguing test results from earlier FLEX studies.
For example, when the flame around a fuel droplet extinguishes, that droplet should stop shrinking because combustion has essentially stopped. But in about a dozen instances during the FLEX experiments, heptane droplets kept shrinking at the same rate as when the flame was still burning.
Williams, who has studied combustion for the past 50 years, said he has never seen anything like it.
Burning interest
William has harbored a long-time burning interest in combustion, dating back to his undergraduate days at Princeton. He was taking a graduate-level course when his professor wrote out on the blackboard the conservation equations of combustion. “When I realized how complicated they were, I said to myself that there is enough there to last me a lifetime,” Williams explained.
“Research leads to a better understanding of fire behavior,” Willams said in a UCSD press statement. “And better understanding ultimately leads to better safety designs.”
Early combustion in space work included several experiments that ran on Spacelab, a science module flown in the cargo bay of U.S. space shuttles.
Common occurrence in microgravity
The holy grail of combustion science is a flame around a fuel droplet that looks like a perfectly symmetrical sphere. That is very hard to achieve here on Earth. It is however a common occurrence in microgravity. Spherical symmetry makes it easier to observe droplets’ behavior and to craft the calculations that explain it, Williams said.
While the original FLEX experiments looked at fuels with only one component, FLEX-2 will run tests on fuels with two components, more similar to fuels used in real-life conditions, which usually have multiple components. While FLEX examined the behavior of single fuel droplets, the new round of tests will also look at the interaction of two fuel droplets.
The FLEX experiments are run by remote control from NASA’s John Glenn Research Center in Cleveland, Ohio. Williams and colleagues at Princeton, UC Davis, the University of Connecticut and Cornell analyze the FLEX results at their home institutions.
By Leonard David