Science of Fire
Matthew Trimble
12/5/12
What is fire?
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Rapid oxidation (loss of electrons)
Very exothermic combustion reaction
Combustion: Fuel + O2 = CO2 + H2O + Heat
Gives off heat and light
Sometimes considered a plasma, but not all of
the flame is ionized gas
Flame Types
• Premixed: oxygen and fuel are already added
together
• Diffusion: oxygen is added to fuel during the
burning
Premixed
Diffusion
Firelight Spectrum
• Primarily dependant on either premixing of
oxygen or diffusion rate, depending on type of
flame
• These determine rate of combustion, which
determines overall temperature and reaction
paths molecules take.
• Composition of fuel (wood, paper, propane)
determines how much energy can be given
off.
Other Contributors
• Blackbody Radiation from gas and fuel
particles
• Incandescence from small soot particles gives
off a continuous spectrum.
• The complete combustion of gas in a region
produces a blue flame from single wavelength
radiation from electron transitions in
molecules.
• Top/Middle: Incandescence
and Blackbody radiation.
• Bottom: Emissions from
electrons.
Using Color to Determine Temperature
• The many factors in the flame spectrum make
experimentally gathering data much more
convenient than theoretically describing it.
• Assumption: most of the light is emitted from
Carbon-based molecules.
Color/Temperature Table
• Red
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Just visible: 525 °C (980 °F)
Dull: 700 °C (1,300 °F)
Cherry, dull: 800 °C (1,500 °F)
Cherry, full: 900 °C (1,700 °F)
Cherry, clear: 1,000 °C (1,800 °F)
• Orange
– Deep: 1,100 °C (2,000 °F)
– Clear: 1,200 °C (2,200 °F)
• White
– Whitish: 1,300 °C (2,400 °F)
– Bright: 1,400 °C (2,600 °F)
– Dazzling: 1,500 °C (2,700 °F)
Gravity Effects
• Convection doesn’t occur in low gravity
• More soot becomes completely oxidized,
lowering incandescence
• Spectrum becomes dominated by emission
lines.
• Diffusion flames become blue and spherical
Zero Gravity Candlelight
Propagation of Fire
• After burning, the fire has to move to continue
burning.
• Deflagration: subsonic propagation (flames)
• Detonation: supersonic propagation
(explosion)
Deflagration
• t_d approx. = d^2/k, where
• t_d = Thermal diffusion timescale (transfer of
heat)
• d= thin transitional region in which burning
occurs
• k= thermal diffusivity (how fast heat moves
relative to its heat capacity)
Deflagration
• t_b~ e^(deltaU/(k_b*T))
• t_b= burning timescale(time the flame moves
in)
• deltaU= activation barrier for reaction
• k_b= Boltzmann’s constant
• T= flame temperature
Deflagration
• In typical fires, t_b=t_d.
• This means d (the distance the fire travels) =
(k*t_d)^1/2 = (k*t_b)^1/2
• And the speed of the flame front: v = d/t_b =
(k/t_b)^1/2
• Note: this is an approximation assuming a
laminar flame; real fire contains turbulence.
Deflagration: Burning Log
Detonation
• An exothermic front accelerates through a
medium, driving a shock front directly ahead
of it.
• Pressures of flame front up to 4x greater than
a deflagration.
• This is why explosives are more destructive
than just burning.
Detonation
• Chapman-Jouguet theory- models detonation
as a propagating shock wave that also releases
heat.
• Their approximation: reactions and diffusive
transport of burning confined to infinitely thin
region
Detonation
• Zel’dovich, von Neumann, and Doering (ZND)
theory- more detailed modeling of detonation
developed in WW2.
• Their approximation: detonation is an
infinitely thin shock wave followed by a zone
of subsonic, exothermal chemical reaction
(fire).
Detonation: 500 tons of TNT
References
• http://quest.nasa.gov/space/teachers/microgravi
ty/9flame.html
• http://en.wikipedia.org/wiki/Detonation
• http://www.doctorfire.com/flametmp.html
• http://en.wikipedia.org/wiki/ChapmanJouguet_condition
• http://en.wikipedia.org/wiki/ZND_theory
• http://en.wikipedia.org/wiki/Deflagration
• http://chemistry.about.com/od/chemistryfaqs/f/f
irechemistry.htm