From: alcore@uurth.com   
      
   On 18 May 2005, JotaCe wrote:   
      
   >I have been thinking about the way the fire behaves in microgravity. I   
   >have found little information on this in the internet, so I am mostly   
   >just wondering, and these thoughts are what I want to share with you   
   >for discussion:   
   >   
   >In absence of gravity, the flames create a sort of sphere around the   
   >core of the fire [...]   
   [snip]   
      
   Actually, no.   
      
   The traditional incandescent flame mostly doesn't happen in zero g fires.   
      
   The problem with zero g combustion is that the heat creates no   
   convection... as a result both the combustion products (heat, ash, co2,   
   water vapor, etc) all stay more or less right where they were produced.   
      
   Now most of this is actually a good thing since it retards further   
   combustion...  But because the heat stays put too, and will continue to   
   build so long as the immediate supply of either the oxidizer or fuel are   
   not exhausted, that heat can grow to *VERY* intense levels.   
      
   It's usually the oxidizer that is the limit.  And that's ordinary air.   
      
   So the moment an astronaut or cabin fan (or something else) causes   
   airflow, the fire spreads...  First by allowing more oxygen to get to the   
   hot fuel and secondly by transporting *extremely hot* exhaust gasses   
   away...  which can transfer their heat and trigger other combustion.   
      
   There will only be a visible "flame" for as long as there's fuel and   
   oxygen actively mixing... but the hazardous heat bubble can persist for a   
   very long time since air is a poor conductor of heat without convection to   
   assist it.   
      
   So the danger is this:  A fire starts and quickly chokes itself, but in   
   the process triggers an alarm...  The astronaut comes to investigate, and   
   as he approaches to have a look causes eddy currents in the air which feed   
   the fire...  and worse yet, could stumble into a pocket of superheated   
   combustion products that aren't visible but are more than hot enough to   
   set *him* on fire...   
      
   It's *not* pretty.   
      
   And to the extent that materials *do* get hot enough to glow... that's   
   heat radiation.  It spreads the heat around.  Perhaps enough so to trigger   
   other fires nearby.   
      
   If you still the air (or remove it), generally zero g fires go out on   
   their own...  very slowly, as the heat dissipates below the ignition   
   threshold.   
      
   (All bets are off though if the environment contains mixtures of   
   hypergolics... just mix and watch your fire re-ignite!)   
      
   Gene P.   
   Slidell LA   
      
   --   
   Alcore Nilth - The Mad Alchemist of Gevbeck   
   alcore@uurth.com   
      
   --- SoupGate-Win32 v1.05   
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