Science of Fire

1. Science of Fire Matthew Trimble 12/5/12

2. 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

3. Flame Types • Premixed: oxygen and fuel are already added together • Diffusion: oxygen is added to fuel during the burning

4. Premixed

5. Diffusion

6. 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.

7. 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.

8. Top/Middle: Incandescence and Blackbody radiation. • Bottom: Emissions from electrons.

9. 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.

10. 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)

11. 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

12. Zero Gravity Candlelight

13. Propagation of Fire • After burning, the fire has to move to continue burning. • Deflagration: subsonic propagation (flames) • Detonation: supersonic propagation (explosion)

14. 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)

15. 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

16. 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.

17. Deflagration: Burning Log

18. 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.

19. 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

20. 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).

21. Detonation: 500 tons of TNT

22. References • http://quest.nasa.gov/space/teachers/microgravity/9flame.html • http://en.wikipedia.org/wiki/Detonation • http://www.doctorfire.com/flametmp.html • http://en.wikipedia.org/wiki/Chapman-Jouguet_condition • http://en.wikipedia.org/wiki/ZND_theory • http://en.wikipedia.org/wiki/Deflagration • http://chemistry.about.com/od/chemistryfaqs/f/firechemistry.htm