Technique 



A unit cell from the upper layer was instrumented to obtain the temperature profile 

 within the cell members. Two fuel sizes were used (0.6 cm. (1/4 inch) and 1.3 cm. (1/2 

 inch)) in square cross section. Both fuel arrays had a packing ratio of 0.08. The 

 packing ratio, 3, is the ratio of the occupied volume to the total volume of the unit 

 cell. From the geometry of the bed, 



3 = t/l. (8) 



The horizontal layers lie one upon the other and are alternately shifted, so that 

 identical horizontal members are spaced 4t on centers below each other. The greatest 

 separation between fuel crib members is in the horizontal direction, the direction of 

 fire spread. From equation (8) the spacing, I dimension, is 8 cm. for t = 0.6 cm. and 

 16 cm. for t = 1.3 cm. 



Equation (8) is developed from the unit cell shown in figure 2 and applies ex- 

 clusively to that arrangement. Intuitively, the rate of spread is dependent on both 

 t and £, but a simple dependency cannot be derived by substituting 3p or (t/£)p into 

 equation (1) for p^, since I is also dependent on 3 in a less direct manner. 



The instrumented cribs were burned under controlled (no-wind) conditions at the 

 Northern Forest Fire Laboratory (Rothermel and Anderson 1966) . A uniform fire front 

 across the full width of the bed was obtained by igniting alcohol-soaked excelsior in 

 the crib spaces at one end. 



Temperature fields within the members of the unit cell were obtained from 5 mil. 

 chromel-alumel thermocouples within the longitudinal and transverse fuel members 

 (figs. 3 and 4). (The view shown above the fuel array in figure 2 illustrates how 

 the transverse member was sectioned to implant the array of thermocouples. Resorcinol 

 glue was used to hold the parts together.) The leads immediately adjacent to the 

 junction of the thermocouple are parallel to the axis of the transverse member for at 



8 



