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UCSD spots ‘cool flame’ on space station

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Experiments that UC San Diego crafted for the International Space Station have revealed the existence of a type of cool-burning flame that poses a previously unknown hazard to the orbiting outpost.

The research also had an upside, pointing to possible ways to create more efficient engines for automobiles, says Forman Williams, the Jacobs School of Engineering scientist who led the global team of scholars involved in the project.

In on-going combustion studies, a space station astronaut ignited large droplets of heptane fuel, which created flames that later appeared to go out, Williams says. But highly sensitive sensors used in the experiment showed that the heptane continued to burn inside a control chamber. The “cool flames” cannot be seen with the naked eye.

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Williams has worked with NASA for almost 40 years on fire safety, one of the greatest concerns in the space community. In 1997, a fire broke out on the Soviet space station Mir and nearly overwhelmed the crew. He discussed the new findings with U-T San Diego.

Q: Does the discovery represent a hazard on space station? NASA is diligent about preventing things from burning inside station.

A: Yes, it is a potential combustion hazard in spacecraft that was not previously known. NASA will now plan for that, to make sure their spacecraft are safe.

Q: The words cool and flame don’t seem to go together. Are you simply referring to a type of fuel that burns at a lower temperature than others?

A: No, the same fuel can burn in two different ways, one with a flame temperature around 3,000 degrees Fahrenheit and the other with a flame temperature around 1,500 degrees -- cool by comparison, but still too hot to touch.

Q: Why is the “cool flame” invisible?

A: The cameras aren’t sensitive enough to detect it. There is a faint blue color that should be visible in a darkened room. Also, radiometers detect radiant energy emission in the ISS experiments. It just isn’t strong enough to be seen with the equipment that we currently have on ISS. We are designing better cameras in the hopes of being able to see it in future experiments.

Q: Would it have been possible to make this discovery in a laboratory on Earth?

A: No, not so far as we know. Cool flames on earth have been known since 1934, when they were first discovered, but they all are transient and quickly lead to hot flames. They cannot be supported by burning droplets on earth. The absence of buoyancy in ISS allows the longer times that are needed for burning droplets to support cool flames. In ISS, extinction of the hot flame leads to the cool flame, which has never been seen on earth, where it is always the other way around -- the cool flame leads to the hot flame.

Q: Why do you believe that this discovery that could lead to cars that burn fuel more efficiently, and with less impact on the environment?

A: It is a bit complicated.

Today’s automobile engines have combustion in their cylinders that burns in hot flames, which produce oxides of nitrogen (NOx) and (in diesels) soot. There is no production of NOx or soot in cool flames. Therefore, if the engines could be designed to burn as cool flames, these pollutants would not be generated.

For a number of years engine manufacturers have been studying combustion concepts to achieve cool-flame combustion in engines (suc as homogeneous-charge compression-ignition (HCCI), spark-assisted compression-ignition (SACI), partially-premixed compression-ignition (PPCI) and others). That has been difficult to do (except where it is not wanted, such as in knock in spark-ignition engines, which involves cool-flame chemistry), and so no such cars are available. One reason is that the chemistry of cool flames is still not nearly as well understood as is the chemistry of hot flames. Our hope is to provide better knowledge of cool-flame chemistry by our experiments on ISS, where we can study how they burn for longer times.

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