needle falls on the line. Those points above the line are residence times for the test 

 fuel beds and have porosities from 4.3 to 9.1 X 10" ^ ft.^/ft.^. Much of the other data 

 were for fuel beds with porosities between 2.45 and 6.04 X 10"^ ft.3/ft.2. it would 

 appear porosity has an important influence on the residence time and the burning rate 

 of a fuel bed and rather small changes can make major changes in a fuel bed's burning 

 characteristics . 



The tests of stationary fires in fuel beds of fixed area and loading revealed the 

 influence of porosity on burning rate and flame length. It was found that an optimum 

 porosity exists where the fuel burning rate is maximum (figure 9) . This occurs with 

 loading held constant but as loading is increased, the maximum burning rate occurs at 

 a different porosity. It is possible that as loading increases the burning rate will 

 reach a nearly constant level over a large range of porosities. Additional work is in 

 progress to fully evaluate porosity, area of combustion, and loading. The values of 

 a X for the three fuel types used in the main test series ranged from 10 for lodgepole 

 to 18.9 for western white pine with ponderosa pine at 16. These values are on the low 

 side of optimum porosity and any variation will have a significant effect on the fire's 

 behavior. Comparing these results to porosities measured in the field, we found even 

 lower porosities for the needle layer under a pure stand of ponderosa pine. .Any 

 disturbance to the forest floor litter layer increasing the spacing between needles 

 can cause an increase in burning rate and more flaming activity. It appears the 

 dimensionless group a A, may be a good indicator of potential fire spread in a fuel. 



The influence of fuel particle density seems quite straightforward and follows 

 the results presented by Pons et al . (1962). He shows that as the specific gravity of 

 the fuel decreases the rate of spread increases. The location of in the denominator 

 of equation 14 reflects this behavior. 



0.7 



o 0.5 ft.2 .125 lb./ft.2 



• 1.0 .125 Ib./ft^ 



□ 0.5 ft.^. .250 lb./ ft. ^ 



■ 1.0 ft,^. .250 lb,/ ft. 2 



0.6 



0.5 



0.1 











20 



40 



60 



80 



100 



120 



140 



160 



(Dimensionless) 



a 



— Influence of porosity j on burning rate for fires in ponderosa pine needles 

 with a a of 1^740 ft./ft.^. Each data point is average of three fires. 



16 



