This slide describes some useful title about 1.What is fire? 2.Flame Types 3.Firelight Spectrum 4.Using Color to Determine Temperature 5.Propagation of Fire this slide released by Matthew Trimble

1. Science of Fire
Sajjad Hooshmandi
2016/05/23
Qazvin Islamic Azad UniversityQazvin Islamic Azad University
Matthew Trimble

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 dependent 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^(delta U/( k_b*T ))
• t_b= burning timescale(time the flame moves
in)
• Delta U= 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/microgravi
ty/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