fire spread accurately. Factors that affect flame size and burning rate include: 

 (1) physical properties of the fuel bed and of the fuel particles; and (2) chemical 

 properties of the fuel elements (e.g., crude fat content, silica-free ash content, 

 and the amount of cellulose and/or hemicellulose in the fuel. Investigations are being 

 conducted at the Northern Forest Fire Laboratory to determine how much the chemical 

 properties of the fuel influence flammability- -discussion of such investigations is 

 outside the scope of this paper. 



The physical properties of fuels are also under study at the Laboratory and include 

 fuel particle size, thermal properties, fuel bed porosity and continuity, and several 

 other factors. These are of importance to the spread equation since some enter the 

 fuel descriptor set, aX/p^Q^^ . Frequently it has been observed that the residence time 

 of flame at a given point in a fuel bed is related to diameter of the fuel particle. 

 A review of fire research literature and unpublished tests here at the Laboratory 

 indicate that residence time is a function of particle diameter (Fons et al . 1962; 

 Byram et al . 1966; McCarter and Broido 1965; Wooliscroft and Law 1967). 



TEST PROCEDURES 



The measurements necessary to evaluate the rate of spread according to equation 7 

 were determined from the unknowns in the equation. These are the emissivities of the 

 combustion and flame zones, the temperature of each zone, and the flame shape mean 

 configuration factor for each zone. By comparing the calculated rate of spread to the 

 observed rate we obtain a means of determining the relative importance of the role of 

 radiant heat transfer in the propagation of fire. This does not give us any information 

 about the generation of radiant energy. However, we gain some insight into the 

 generation phase through measurements of weight loss, flame residence time, and radiant 

 heat fluxes to the fuel. 



Many of the methods mentioned above except for two have been described in previous 

 publications (Anderson and Rothermel 1965; Anderson 1968) . Two measurements not used 

 in previous work are: determination of the configuration factor; and the radiant heat 

 flux passing through a fuel element. Configuration factor determinations were made 

 using the techniques described in "Flame Shape and Fire Spread" (Anderson 1968) . A 

 sequence of at least 10 photographs (1 second exposure at f:ll on Plus X film) in an 

 overlay was used to determine average flame shape and length. 



Radiant heat fluxes from the flame and the combustion zone were measured with 

 Gardon-t)'pe heat rate sensors. These sensors were water cooled and shielded with 

 sapphire windows so that only radiant heat was measured. The recorded millivolt signal 

 from these sensors was corrected for transmissivity , percent radiation below the 

 wavelength cutoff of the window, configuration factor, and view area enclosed. Besides 

 indicating the heat flux passing through a particular point, the resulting values 

 could be compared with the combustion and flame zone temperature measurements for 

 calculation of the emissivity of each zone. 



The heat rate sensors in the fuel bed were placed with the sensing element 0.5 

 inch below the surface of the fuel bed. Each sensor faced the oncoming fire front and 

 was positioned to view all of the fire in the fuel bed. Thermocouples were located at 

 the surface of the fuel bed and 0.5 inch below it to provide data on temperatures as 

 the fire front approached and passed. The peak values represented the fire front's 

 arrival and were used to calculate the blackbody emissive power of the combustion 

 zone's fire front. The emissivity of the zone was determined by dividing the heat 

 rate sensor value of heat flux by the calculated blackbody value. 



The heat flux of the flame was measured in nearly the same manner as were the heat 

 rate sensors in the fuel bed except the sensors were positioned so that one edge of 

 their view angle coincided with the surface of the fuel bed. The sensor then viewed 

 the space above the bed. Thermocouples j 5 mil chromel/alumel , were placed 2, 11, and 



6 



