Figure 8. — Relation of 



residence time to pccrtiale 

 thickness determined from 

 measurements made during 

 test fires. 



II II I _l I I I ■ 



lO'^ 2 5 lo' 2 5 12 5 10 



Particle thickness! inches) 



4, Over what range of particle sizes and bed porosities does the fuel descriptor 

 set apply? 



Values for the combustion zone and flame temperatures determined experimentally 

 were used to test equation 14 against fires in beds of different widths. This was done 

 for the fires described in "Fire Spread and Flame Shape" (Anderson 1968) . The equation 

 yielded a rate of spread that was within: 99 percent of the experimental rate of spread 

 for a 1 . 5-foot-wide ponderosa pine bed; 77 percent to 89 percent for 5.0-foot beds; 103 

 percent for a 1 . 5-foot-wide bed of western white pine; and 89 percent for 5 . 0-f oot-wide 

 beds. Rate of spread was overestimated at 136 percent of the experimental value for 

 5-foot-wide beds of lodgepole pine primarily because the flame depth measurements dif- 

 fered by a factor of three. In addition, the flame-shielded test (Rothermel and 

 Anderson 1966) was used and the value from the equation was 97 percent of the experi- 

 mental spread assuming no radiant heat from the flame. These evaluations support the 

 credibility of the equation and further work will determine whether this approach is 

 applicable to a variety of fuels. 



FIRE RESPONSE TO a AND A ^ 



In fuel beds where the particles are loosely arranged, each particle will burn in 

 a manner predominantly controlled by its own thermal properties. The energy feedback 

 to the particle from surrounding fuel elements and their pyrolysis products will influ- 

 ence the burning characteristics but may be a nearly constant value until the particles 

 are very far apart or very close together. This idea was checked by plotting research 

 data for a variety of particle sizes ranging from 1.75 X 10"^ to 4.00 inches against the 

 residence time or duration of active flaming. The results shown in figure 8 suggest that 

 in porous fuel beds, residence time in minutes is equal to 8 times the particle diameter 

 or thickness in inches. At the low end of the curve the residence time for a single 



15 



