FLEX-ible insight into flame behavior

FLEX-ible insight into flame behavior
Color image of a burning droplet. Credit: NASA/GRC

(PhysOrg.com) -- Whether free-burning or smoldering, uncontrolled fire can threaten life and destroy property. On Earth, a little water, maybe some chemicals, and the fire is smothered.

In , where there is no up or down, flames behave in unconventional ways. And when your entire world is the size of a five-bedroom home like the , putting out even a small quickly is a life-and-death matter.

Since March 2009, NASA's Flame Extinguishment Experiment, or FLEX, has conducted more than 200 tests to better understand the fundamentals of flames and how best to suppress fire in space. The investigation is currently ongoing aboard the space station.

"We hope to gain a better knowledge of droplet burning, improved spacecraft fire safety and ideas for more efficient utilization of on earth," Principal Investigator Forman Williams, University of California, San Diego, said. "The experiments will be used to verify that calculate droplet burning under different conditions."

When a flame burns on Earth, heated gases rise from the fire creating a buoyant flow that draws oxygen into the flame and products away from it. In space, warm gases do not rise, and drives flame behavior.

FLEX-ible insight into flame behavior
FLEX Chamber Insert Assembly Apparatus. Credit: NASA/GRC

"In space molecular diffusion draws oxygen to the flame and combustion products away from the flame at a rate 100x slower than the buoyant flow on Earth," Dan Dietrich, project scientist, NASA's Glenn Research Center, said.

Flames in space burn with a lower temperature, at a lower rate, and with less oxygen than in normal gravity. This means that materials used to extinguish the fire must be present in higher concentrations. The slow flow of air from the fans mixing the air in a spacecraft can make the flames burn even faster.

To help understand how flames behave and burn in space, FLEX researchers ignite a small drop of either heptane or . As this little sphere of fuel burns for about 20 seconds, it is engulfed by a spherically symmetric flame. The droplet shrinks until either the flame extinguishes or the fuel runs out.

"Thus far the most surprising thing we've observed is continued apparent burning of heptane droplets after extinction under certain conditions; currently, this is entirely unexplained," said Williams, who has studied combustion for more than 50 years.

From ignition to extinguishment, the entire event is recorded by cameras housed in the Glenn-designed-and-built Combustion Integrated Rack, or CIR, which is located inside the U.S. Destiny Laboratory module of the space station.

"You have both elements being represented in the FLEX experiment. Both the exploration-driven fire safety research and the more fundamental science research," Dietrich said.

As for the fundamental science of combustion, there are still many discoveries to be made, even for an experienced professor who has studied the subject since college.

"As a Princeton undergrad, I saw in a graduate course the conservation equations of combustion and realized that those equations were complex enough to occupy me for the rest of my life; they contained so much interesting physics," Williams said. "There are many currently unknown things about combustion processes waiting to be revealed by future scientific experiments."

The better we understand the physics of combustion, the better we can control it and design energy-efficient processes.

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Space image: Burning

Provided by JPL/NASA
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Nov 30, 2011

Benwl has some great vids of microgravity experiments, mainly involving fluid dynamics, but some combustion too.

Nov 30, 2011
we've observed is continued apparent burning of heptane droplets after flame extinction
How you can observe burning without flame? You can observe the evaporation of the remainder of droplet. Does it mean, that surface of droplet remains hot or something?

Nov 30, 2011
Keep in mind that plasmas can exist in three distinct modes: dark, glow and arc. Flame is a glow-mode plasma, so if you're seeing some sort of evidence for the flame after it is extinguished, would this not be the dark mode plasma remnant?

Nov 30, 2011
So this means that in case of a fire, "venting the atmosphere" like they do in the movies is a bad idea. Until all the air inside the capsule goes out, the flow will most likely cause the flames to spread quickly and destroy whatever they are burning.

Dec 06, 2011
Keep in mind that plasmas can exist in three distinct modes: dark, glow and arc. Flame is a glow-mode plasma, so if you're seeing some sort of evidence for the flame after it is extinguished, would this not be the dark mode plasma remnant?

The flames in this study are not hot enough for comparisons to plasma modes to be relevant. The heptane-air flame adiabatic flame temperature is around 2200K and the electron density is not high enough for the flame to behave in a significant way like a plasma. The researchers must have evidence of the fuel droplet continuing to shrink after the visible flame goes out at a rate faster than would be expected or some other evidence that a reaction is taking place. The reactions after flame extinction must be occurring at low temperature. This is interesting because these reactions are probably difficult to observe in gravity because buoyancy disrupts the low temperature diffusion flame.

Dec 13, 2011
Re: "The flames in this study are not hot enough for comparisons to plasma modes to be relevant"

This is a common misconception. The plasma within a fluorescent bulb is quite cool as well, and yet fluorescent lights are certainly plasmas. The mainstream models for cosmic plasmas have been a subject of debate since the time of Hannes Alfven. Many professional scientists and theorists seem completely unaware that Alfven struggled to convince mainstream astrophysicists that they were incorrectly applying MHD to their cosmic observations. I suspect that the same sort of mistake is being made here. The cosmic plasma models have been designed specifically to fit into a gravitational scientific framework. Not only do they propose that magnetic fields can be frozen-in, but they also propose that there are no E-fields in cosmic plasmas, and that the plasmas have no electrical resistance. Not one of these three assumptions bears true within the laboratory, and yet they remain consensus.

Dec 13, 2011
Thus, many problems that pertain to the study of plasmas result from the adoption of assumptions about plasma's behavior which were designed to force-fit our cosmic observations to a gravitational framework.

One need only study the work of Gerrit Verschuur to see that cold plasmas can do things of importance in space. Verschuur studies the interstellar "clouds" between stars using radio telescopes tuned to the 21-cm wavelength (which represents a subtle change in spin for hydrogen). Verschuur is clear that these "clouds" are in fact highly filamentary. Not only has Verschuur observed critical ionization velocities associated with these filaments, but he's also established dozens of correlations between WMAP hotspots and these filaments. Note that CIV's are what you get when one slams charged particles into a neutral gas.

Thus, it would seem that cold plasmas can do things of absolute importance in space. These findings suggest an investigative lead for other disciplines as well.

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