The different fuel types produced a slight difference in the average temperature 

 of the combustion zone, as follows: 



Fuel type 



Ponderosa pine 

 Western white pine 

 Lodgepole pine 



Average temperature 



1591° 



F. 



(867° 



C 



) 



1434° 



F. 



(779° 



C 



) 



1492° 



F. 



(811° 



C 



) 



These combustion zone temperatures are not unusual if moisture is removed from the fuel 

 before the fire front or combustion zone reaches it. The irradiance of the fuel could 

 change with moisture content changes because emissivity varied or the absorptivity of 

 the intervening gases varied. The heat rate sensors for the combustion zone were 

 placed too close to the surface of the fuel and were not sufficiently collimated. As a 

 result, poor repeatability was obtained. However, these measurements did indicate a 

 rather constant heat flux from the combustion zone. We found no decrease in heat flux 

 with increases in fuel moisture content; this tended to support the thermocouple data 

 but due to the lack of repeatability no reliable estimate of combustion zone emissivity 

 could be made. The equation was evaluated for rate of spread by using a value of 1.0 

 for the combustion zone. 



The thermocouples located above the fuel bed showed that the average flame 

 temperature decreased as fuel moisture content increased: 



Fuel type Temperature range Rate of change 



("F . ) ("F. /percent MC) 



Ponderosa pine 1680 - 1254 24 



Western white pine 1531 - 1229 24 



Lodgepole pine 1548 - 1135 42 



Also, the burning rate, flame length, and flame depth were found to decrease in this 

 same manner, as shown in table 3. These three parameters along with flame temperature 

 govern the emissive power of the flame. The values obtained from the heat rate sensors 

 do show a decrease with moisture increase but not as rapidly as the above variables of 

 burning rate, flame length and depth. The emissive power values derived from the heat 

 rate sensors at various distances from the flame agreed very closely with the values 

 determined after the sensors had entered the flame (figure 3) . The average emissive 

 power was obtained from these values and compared with the blackbody emissive power 

 determined by the thermocouple readings to calculate the emissivity of each test set 

 (figure 4) . Ponderosa pine fuel beds produced taller and thicker flames than the other 

 fuels which generated flames with smaller dimensions; the calculated emissivities are 

 substantiated by these physical characteristics of the flames. 



By using the data of the emissive powers we can estimate the total radiant heat flux 

 that has an impact on the fuel ahead of the fire. The values of heat flux can be cal- 

 culated according to equation 7 and summed for comparison with the total heat flux found 

 to be necessary to produce the measured rate of spread (see table 2) . The results of 

 this analysis are shown in figure 5 with the summed radiant heat flux as a percentage of 

 the total heat flux and plotted against fuel moisture content. The radiant heat fluxes 

 were determined by using a combustion zone emissivity of 1.00 and flame emissivities of 

 0.28 for ponderosa pine fires and 0.16 for lodgepole and western white pine fires. 



This analysis shows that radiant heat can account for 40 percent or less of the 

 total heat flux necessary for the fire spread observed in these fuel beds. The rest 

 of the heat flux must come from some other heat transfer mechanism. Conduction appears 

 to be insignificant; a brief analysis indicated only 1/50 of the necessary energy 

 would be available through this mechanism. This means convective heat transfer must be 

 the primary source through some horizontal transport mechanism. 



11 



