Science of Fire Matthew Trimble 12/5/12

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

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 – 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 ty/9flame.html ty/9flame.html Jouguet_condition Jouguet_condition irechemistry.htm irechemistry.htm