science of fire

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Science of Fire Matthew Trimble 12/5/12

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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. - PowerPoint PPT Presentation

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Page 1: Science of Fire

Science of FireMatthew Trimble12/5/12

Page 2: Science of Fire

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

Page 3: Science of Fire

Flame Types

• Premixed: oxygen and fuel are already added together

• Diffusion: oxygen is added to fuel during the burning

Page 4: Science of Fire

Premixed

Page 5: Science of Fire

Diffusion

Page 6: Science of Fire

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.

Page 7: Science of Fire

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.

Page 8: Science of Fire

• Top/Middle: Incandescenceand Blackbody radiation.

• Bottom: Emissions fromelectrons.

Page 9: Science of Fire

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.

Page 10: Science of Fire

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)

Page 11: Science of Fire

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

Page 12: Science of Fire

Zero Gravity Candlelight

Page 13: Science of Fire

Propagation of Fire

• After burning, the fire has to move to continue burning.

• Deflagration: subsonic propagation (flames)• Detonation: supersonic propagation

(explosion)

Page 14: Science of Fire

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)

Page 15: Science of Fire

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

Page 16: Science of Fire

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.

Page 17: Science of Fire

Deflagration: Burning Log

Page 18: Science of Fire

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.

Page 19: Science of Fire

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

Page 20: Science of Fire

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).

Page 21: Science of Fire

Detonation: 500 tons of TNT

Page 22: Science of 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