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An Old Theory Goes up in Cool Flames
August 27, 2012

Before you blow out the candle on your next cake or make a batch of S'mores over a campfire, look closely at the flame. There may be some very cool secrets inside those flames waiting to be revealed. While fire has warmed us and cooked our food for thousands of years, it still holds mysteries that can surprise us and change our understanding of how the world works.

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"There are many currently unknown things about the combustion process waiting to be revealed by future scientific experiments," said Forman Williams, University of California, San Diego. Williams has studied combustion for more than 50 years.

After decades of flame studies that have produced well understood theoretical models and numerical simulations, recent flame investigations on the International Space Station produced some unexpected results. For the first time, scientists observed large, about 3 mm, droplets of heptane fuel that had dual modes of combustion and extinction. The fire went out twice; once with a visible flame, once without. While the initial burn had a traditional hot flame, the second-stage vaporization was sustained by what is known as cool-flame chemical heat release.

This is the first time scientists observed low-temperature, soot-free cool flames in steadily burning pure fuels. A cool flame is one that burns at about 600 degrees Celsius. To understand how cool this is, consider that a typical candle is about two-and-a-half times hotter, burning at around 1,400 degrees Celsius.

This two-stage burning was not seen in smaller heptane droplets that were less than 2.4 mm in size.

"Thus far the most surprising thing we've observed is the continued apparent burning of heptane droplets after flame extinction under certain conditions. Currently, this is entirely unexplained for heptane," said Williams, FLEX Principal Investigator.

While burning the heptane droplets in the Flame Extinguishing Experiment, or FLEX, investigation, the first stage had a visible flame that eventually went out. Once the visible flame disappeared, the heptane droplet continued rapid quasi-steady vaporization without any visible flame, according to the researchers. This ended abruptly at a point called second-stage extinction. At this point, a smaller droplet was left behind that either experienced normal time-dependent evaporation or sometimes grew slightly through condensation of vapor in the cloud that formed upon extinction.

This result, which was not anticipated when the study was designed, came during the FLEX investigation on the space station using the Multi-User Droplet Combustion Apparatus in the Combustion Integrated Rack, or CIR. More recent FLEX experiments reveal similar two-stage burning phenomena with n-octane and decane fuels.

For the investigation, over one hundred heptane droplets were burned in varying conditions. Some of the videos showing this phenomenon are available on the Glenn Research Center, Cleveland, Ohio, FLEX Website.

The new findings are in press and available online in Combustion and Flame, the journal of the Combustion Institute. This new discovery will help scientists and engineers modify numerical models and better predict the behavior of flames, fuel and combustion. It also has many long-term implications both in space and on Earth.

"There are all kinds of implications for practical applications. For internal combustion engines, it can lead to pollution reduction and better gas mileage," said Vedha Nayagam, FLEX co-investigator, National Center for Space Exploration Research, Cleveland, Ohio. "You could use this cool flame burning to partially oxidize the fuel, like in the HCCI engine."

The Homogeneous Charge Compression Ignition, or HCCI, engine combines diesel ignition with spark-ignition and can be used in any type of diesel engine, both stationary and for transportation. By merging these two technologies, engines could have the efficiency of diesel engines, while also providing reduced particulate and NOx emissions. Basically, this could eliminate the need to burn diesel-fuel sprays, which are notorious for pollutant production, according to the researchers.

Fire safety in space can also benefit from this new discovery. Since the flame continues to produce combustion after the hot-flame goes out, fire suppression techniques will need to consider the possibility that the pure-fuel flame is not really out, even with no visible flame. It could behave like a solid that smolders and reignites into open flame under the right conditions, leading to the need for space-specific suppression techniques.

Thanks to the FLEX investigation in the reduced gravity environment of the space station, we have new insight into the mysteries of flames and fuel. Whether it's a candle, a campfire, or some other blaze, the combustion process may be waiting for the right investigation to pry loose more secrets. Microgravity research may prove to be the tool that helps force free those secrets.

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A vapor cloud forms around the heptane fuel droplet
A vapor cloud forms around the heptane fuel droplet following its dual-stage extinguishment in the FLEX investigation.
Image Credit: 
NASA
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Deployment of the heptane droplet
Deployment of the heptane droplet during the FLEX investigation.
Image Credit: 
NASA
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ignition of the heptane droplet
Ignition of the heptane droplet during the FLEX investigation.
Image Credit: 
NASA
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A burning heptane droplet
A burning heptane droplet during the FLEX investigation on the International Space Station.
Image Credit: 
NASA
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Page Last Updated: February 3rd, 2014
Page Editor: Kelly Heidman