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Wildfires

PostPosted: April 27th, 2013, 1:11 pm
by Jessica
What is fire?
  • rapid combustion (combination of O2 with carbon, hydrogen, and other elements in a chemical reaction that produces light, heat, and flame
  • exothermic reaction (releases heat) that is the opposite of photosynthesis
  • photosynthesis: solar energy + 6CO2 + 6H2O ⟶ C6H12O6 + 6O2
  • fire: C6H12O6 + 6O2 ⟶ CO2 + 6H2O + heat

Fire Stages (all these stages are ongoing at once in different parts of the fire)
  • Pre-heating:
    • water must first be expelled from wood/fuel
    • raising the temperature of the fuel will drive off the water
    • this is accomplished through:
      • flames (kindling)
      • long periods of dryness (drought)
  • Pyrolysis:
    • Thermal degradation of the wood cellulose
    • Cellulose is stable to about 615°F (325 °C)
    • Above that, cellulose breaks down and expands
      • Gives off flammable gases & water vapor
      • Plus mineral residue, tars, etc. ⟶ ash
    • gas movement causes cracks in the wood
    • in the presence of O2 those gases ignite to form flames
  • Flaming combustion:
    • pyrolized wood burns hot and fast
    • stage of greatest energy release
    • heat transfer in several ways:
      • conduction - heat moves inward through wood by physical contact of wood molecules (inefficient because wood is a poor conductor)
      • radiation - energy released directly from flames in the form of electromagnetic (EM) radiation
      • convection - warming of the air molecules surrounding the wood
    • highly efficient and predominates in windy environments
    • wind accelerates fire spread by:
      • bringing in more oxygen
      • heating up air to the point of igniting other fuel
      • spreading material already ignited
  • Glowing combustion:
    • after the active flames die off (combustible gas is depleted)
    • "coals" stage
    • wood is slowly consumed in an oxidation reaction (lower temperature)