science of fire matthew trimble 12/5/12. what is fire? rapid oxidation (loss of electrons) very...
TRANSCRIPT
What is fire?
• 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
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.
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
– 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
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.
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).
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/Chapman-Jouguet_c
ondition• http://en.wikipedia.org/wiki/ZND_theory• http://en.wikipedia.org/wiki/Deflagration• http://chemistry.about.com/od/chemistryfaqs/f/
firechemistry.htm