Scince of flame

Science of FireSajjad Hooshmandi

2016/05/23Qazvin Islamic Azad UniversityQazvin Islamic Azad University

Matthew Trimble

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 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.

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: Incandescenceand Blackbody radiation.

• Bottom: Emissions fromelectrons.

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^(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

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

• 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