700 



600 - 



Fzgure 1 . — Heat reqmred — 



to bring 1 pound of fuel 

 to -ignition as a fimotion 

 of fuel moisture content. 



100 - 



I 1 1 : I I I i 1 I f 



4 8 12 16 20 24 28 32 36 



Fuel moisture content (percent) 



T2 = boiling temperature 



= ignition temperature 608°F. (320°C.) 



= latent heat of vaporization 

 M = moisture content, lb. /lb. (g./g.) 

 H(M)Ti = heat of desorption (16 cal./g.). 



Using this equation, we developed a curve (figure 1) to show the relation of energy 

 input to moisture content of the fuel (in percent) . 



Additional definitions are listed in the Appendix. 



• MDI.ANT HEAT 



We knew how much heat was required to raise the moist fuel to ignition and the 

 temperature at ignition. Our next step was to develop an equation that would describe 

 the change in rate of spread. Research by Thomas (1963) and Emmons (1963) indicated 

 that the heat flux from the fire to unburned fuel can be related to rate of spread. 

 Assuming that the energy required to cause ignition remains constant and that the 

 emissive power of the fire remains constant--just high enough to cause ignition when 

 the fire reaches the unburned fuel--we can write: 



' Q = o^°°E e-^dt . C2) 

 where : 



Q = heat per vertical cross section of the fuel bed to cause ignition, B.t.u./ft..^ 

 E = emissive power of the fire, B.t.u./ft.^ - hr. ' 

 a = attenuation constant 



X = distance from the fire front to the fuel particle or elemental volume in 



question, ft. 

 t = time, hr. 



This can be modified by: 



2 



