Cross Section of Needle Fuel Beds 



Figure 9. --Relation 

 between surface 

 area of retardant 

 and amount of 

 retardant applied 

 to a unit area of 

 fuel bed . 



Case 1 



Maximum surface 

 area 



Case 2 



Case 3 



Minimum surface 

 area 



• fuel particle 



O surface of retardant 



Com 1 



Small amount of retardant applied uniformly over each needle produces 

 maximum surface area. 

 Cam 2 



Additional retardant spread uniformly forms liquid bridges and reduces 

 surface area of the retardant. 

 Cos* 3 



Limiting case, which is not achieved, would reduce surface area of re- 

 tardant to the unit area of the fuel bed over which it is applied. 



r « K (Pv Q - Pvi) (3) 

 where: K = coefficient of diffusivity 



Pv Q = saturated vapor pressure at surface of the retardant 

 Pyj = partial pressure of the free stream water vapor. 



Vapor pressure is determined by the temperature of the gas and the degree of saturation. 

 The vapor pressure at the surface of the retardant may be taken for that of saturated air at the 

 surface temperature T . T may be obtained in two ways. As an approximation, the wet bulb 

 temperature of the air may be used. For a more exact solution, use the equation 



T _ t — — — ( p w - Pow) 



io 1 ~ k c p p < 4 > 



which was developed for estimating the surface temperature of a raindrop 9 



Johnson, John C. Physical meteorology, p. 219, illus. New York: published jointly by 

 Massachusetts Institute of Technology and John Wilson and Sons, Inc. 1954. 



17 



