CONVECTIVE HEAT 



As mentioned earlier, it was necessary to determine a convective heat transfer 

 coefficient. In addition, both the Reynolds and the Nusselt numbers had to be computed 

 Using the procedures outlined previously, a gas velocity and Reynolds number were 

 calculated for each test set. If solid fuel fires respond in a manner similar to 

 liquid pool fires, then nearly equal values of Reynolds numbers should occur for fires 

 of similar dimensions. Figure 6 shows how the solid fuel fires grouped near the curve 

 for gasoline pool fires. The same forces seem to apply to the various fires resulting 

 in turbulent, transitional, and laminar regimes. Fires in ponderosa pine beds with 

 less than 9 percent moisture content were turbulent and in the transitional regime for 

 moisture contents above 9 percent. All fires in western white pine and lodgepole pine 

 were in the transitional or laminar regimes. 



Values for the gas velocity in each test set were determined and were used with 

 the diameter of the fuel particle to determine the Reynolds number. The Nusselt number 

 for cylinders was used to calculate the convective heat transfer coefficient. This 

 combined with the temperature difference between the combustion zone and the ambient 

 air provided a measure of the convective heat transfer component. 



In all tests the calculated convective heat flux exceeded the net heat flux which 

 is obtained by subtracting the radiant heat flux from the total heat flux values in 

 table 2. This means that a horizontal transfer efficiency coefficient does exist. 

 This horizontal transfer coefficient was calculated for each fuel: 



Ponderosa pine: n 

 Western white pine: n 

 Lodgepole pine: n 



= (0.90 - 0.014 MC*) 

 = (1.00 - 0.028 MC) 

 = (1.00 - 0.036 MC) 



■ ■ *MC is the fuel moisture content in percent. 



These coefficients infer less heat transfer in ponderosa fuel of low moisture contents 

 and a more rapid decrease in heat transfer for western white pine and lodgepole pine 

 fuels because their burning regimes are transitional or laminar. 



Incorporating the horizontal transfer coefficients into equation 7 the new descrip 

 tion of fire spread by heat transfer becomes : 



[oee^c''^o^'° ^12dx) + aepT no^'" Fi2dx) + nh^T^-TJ] (14) 



f ig 



where : 



T^.= combustion zone temperature 



Ti - ambient air temperature. 



Values for each component of the equation were obtained for the experimental data. The 

 resulting rates of spread are compared to the experimental values in figure 7. The 

 equation was found to describe rate of spread quite accurately but does point out the 

 following areas where additional research is needed: 



1. Estimation prior to a fire of how 



2. Is there a physical limitation to 

 the fuel bed? 



3. What is the relation between fuel 

 transfer? 



much flame will be generated. 



fire spread caused by maximum heat flux into 



bed characteristics and convective heat 



13 



