The percent of total fuel weight loss accounted for by needles and branchwood in 

 the different particle size classes is shown in the following tabulation:!./ 



PondeTosa pine 

 Percent 



Douglas-fir 

 Percent 



Needles 



44 



Branchwood by 



diameters (cm.) 



0- 0.5 

 0.5-1 



1- 3 

 3-5 



11 

 37 

 8 



19 

 25 

 43 

 13 



Percentages for individual plots did not vary significantly for different loadings or 

 bulk densities. 



IvTien the above percentages were determined, we knew the percent weight loss of 

 individual particles and the proportion of the total fuel contributed by the various 

 size classes. Fuel less than 1 cm. in diameter accounted for half of the total weight 

 loss even though this size material was less than 30 percent of the total loading 

 (table 1) . Fuel greater than 3 cm. in diameter provided only about 10 percent of the 

 total weight loss in the flame front (fig. 6). No change in diameter was observed for 

 particles of this size. 



For slash and other fuels containing a similar mixture of particle sizes, our 

 findings indicate that only the fine fuel components, essentially particles less than 

 3 cm. (about 1 inch in diameter) supply the energy that characterizes propagation of 

 the spreading flame front. A generalization of this statement for all fuels likely 

 would be incorrect. Different proportions of fine fuels probably correspond to dif- 

 ferent diameter limits for the component contributing most of the energy to the propa- 

 gating flame. 



The percent weight loss of particles is compared in figure 7 with the effective 

 heating number of the mathematical model defined in equation 1.—' The effective heating 

 number is a function of particle size; it expresses the percent of a particle that is 

 heated to ignition ahead of the fire. In figure 7, the percent weight loss curve is 

 drawn through points that represent particle size and the average percent weight loss 

 in the propagating flame front for the sample particles. 



The relation between the two curves, each representing a different variable, was 

 expected and supports the concept expressed by the effective heating number. The two 

 curves are close together at the very small particle sizes because the small particles 

 are heated throughout at ignition and their entire organic mass rapidly converted to 

 heat energy. As particle size increases, up to some point, the amount of a particle 

 required for heating to ignition (e) becomes less than the amount converted to heat in 

 the flame (n) • The effective heating number only involves that part of the particle 

 that receives heat during the time when the particle surface rises to ignition tempera- 

 ture. Weight loss in the flame front, which takes place primarily after ignition, is 

 characterized by rapid heat transfer and combustion, and thus involves a larger propor- 

 tion of the particle. The percent weight loss (on an ash-free basis) should always 

 exceed the effective heating niimber for the same particle sizes. 



^The percentages are averages based on all plots. 



19 



