Method of Analysis 



Fuel cribs were used in this investigation because of the relative ease of con- 

 struction and instrumentation, and their symmetry relative to the fire front. The 

 thermal gradient within the particle members of the cell must be obtained to determine 

 the heat absorbed up to ignition by the unit cell. 



The unit cell reproduces the crib by consecutively repeating the cell in all 

 three cartesian coordinates. Each horizontal layer of cells was alternately shifted 

 with respect to adjacent layers to obtain a more uniform array (fig. 2) . 



Ignition takes place when a significant portion of the cell undergoes visual 

 flame attachment. At ignition, isotherms are constructed from the observed gradient. 

 Through symmetry, these isotherms are assumed to form curvilinear surfaces that are 

 parallel to the front of the spreading fire. 



The heat absorbed between adjacent isotherms is calculated and summed over the 

 area of the isothermal profile to determine the total heat absorbed. The total heat 

 absorbed by the unit cell up to ignition is i?V u , where J? is the nonuniform heat absorbed 

 per unit volume and V is the total volume of the unit cell including the unoccupied 

 space. The equivalence of this product to the heat absorbed by the members of the 

 unit cell is: 



2V = £ApQ + tBpQ (2) 



4 



