Strange Flames on the ISS   
       
   June 18, 2013:  Fire, it is often said, is mankind's oldest chemistry   
   experiment.   
       
   For thousands of years, people have been mixing the oxygen-rich air of Earth   
   with an almost endless variety of fuels to produce hot luminous flame. There's   
   an arc of learning about combustion that stretches from the earliest campfires   
   of primitive humans to the most advanced automobiles racing down the   
   superhighways of the 21st century. Engineers study burning to produce better   
   internal combustion engines; chemists peer into flames looking for exotic   
   reactions; chefs experiment with fire to cook better food.   
       
   You would think there's not much more to learn. Dr. Forman A. Williams, a   
   professor of physics at UC San Diego, would disagree. "When it comes to fire,"   
   he says, "we're just getting started."   
       
   http://www.youtube.com/watch?v=BxxqCLxxY3M   
       
   A new ScienceCast video explores the surprising behavior of "cool flames" on   
   the International Space Station. Play it   
       
   Flames are hard to understand because they are complicated. In an ordinary   
   candle flame, thousands of chemical reactions take place. Hydrocarbon   
   molecules from the wick are vaporized and cracked apart by heat. They combine   
   with oxygen to produce light, heat, CO2 and water. Some of the hydrocarbon   
   fragments form ring-shaped molecules called polycyclic aromatic hydrocarbons   
   and, eventually, soot. Soot particles can themselves burn or simply drift away   
   as smoke. The familiar teardrop shape of the flame is an effect caused by   
   gravity. Hot air rises and draws fresh cool air behind it. This is called   
   buoyancy and is what makes the flame shoot up and flicker.   
       
   But what happens when you light a candle, say, on the International Space   
   Station (ISS)?   
       
   "In microgravity, flames burn differently-they form little spheres," says   
   Williams.   
       
   Flaming spheres on the ISS turn out to be wonderful mini-labs for combustion   
   research. Unlike flames on Earth, which expand greedily when they need more   
   fuel, flame balls let the oxygen come to them. Oxygen and fuel combine in a   
   narrow zone at the surface of the sphere, not hither and yon throughout the   
   flame. It's a much simpler system.   
       
   Recently, Williams and colleagues were doing an ISS experiment called "FLEX"   
   to learn how to put out fires in microgravity when they came across something   
   odd. Small droplets of heptane were burning inside the FLEX combustion   
   chamber.  As planned, the flames went out, but unexpectedly the droplets of   
   fuel continued burning.   
       
   "That's right-they seemed to be burning without flames," says Williams. "At   
   first we didn't believe it ourselves."   
       
   http://issresearchproject.grc.nasa.gov/Investigations/FLEX-2/   
       
   A color image of a burning fuel droplet. MoreIn fact, Williams believes the   
   flames are there, just too faint to see.  "These are cool flames," he explains.   
       
   Ordinary, visible fire burns at a high temperature between 1500K and 2000K.   
   Heptane flame balls on the ISS started out in this "hot fire" regime.  But as   
   the flame balls cooled and began to go out, a different kind of burning took   
   over.   
       
   "Cool flames burn at the relatively low temperature of 500K to 800K," says   
   Williams.  "And their chemistry is completely different. Normal flames produce   
   soot, CO2 and water.  Cool flames produce carbon monoxide and formaldehyde."   
       
   Similar cool flames have been produced on Earth, but they flicker out almost   
   immediately.  On the ISS, however, cool flames can burn for long minutes.   
       
   "There are practical implications of these results," notes Williams. "For   
   instance, they could lead to cleaner auto ignitions."   
       
   One of the ideas that auto companies have worked on for years is HCCI--short   
   for "homogeneous charge compression ignition."  In the automobile cylinder   
   instead of a spark there would be a gentler, less polluting combustion process   
   throughout the chamber.   
       
   "The chemistry of HCCI involves cool flame chemistry," says Williams. "The   
   extra control we get from steady-state burning on the ISS will give us more   
   accurate chemistry values for this type of research."   
       
   Just getting started, indeed.   
       
   Credits:   
       
   Author: Dr. Tony Phillips | Production editor: Dr. Tony Phillips | Credit:   
   Science@NASA   
       
   More information:   
       
   Flame Extinguishment Experiment -- FLEX home page   
       
   More information about FLEX -- from the Glenn Research Center   
       
       
   Regards,   
       
   Roger   
      
   --- D'Bridge 3.94   
    * Origin: NCS BBS - Houma, LoUiSiAna (1:3828/7)