where: 



T = surface temperature of liquid °C. 10 



T = free stream ambient temperature °C. 



K = diffusivity of water vapor in air cm. 2 /sec. 



k = thermal diffusivity cm. 2 /sec. 



L = latent heat of vaporization cal./g. 

 Cp = specific heat at constant pressure cal./g- °C. 

 Pw = ambient vapor density g./cc. 

 Pow = saturated vapor density g./cc. at surface of liquid 



P = density of air g./cc. 



For condition I (90° F., 50-percent RH) equation (4) predicts T = 23.8° C. = 75° F. For 

 conditions II and III (90° F. , 20-percent RH) equation (4) predicts T = 15° C. = 59° F. Having 

 thus determined T Q , the saturated vapor pressure pv may be obtained from a steamtable. The 

 free stream partial pressure of water vapor p v ^ may be obtained by multiplying the saturated 

 vapor pressure for the ambient temperature by the relative humidity. For condition I the dif- 

 ference in vapor pressure was 3.94 mm. Hg. For conditions II and III, the difference in vapor 

 pressure = 5.56 mm. Hg. 



Air Velocity 



Air flowing over the surface of the retardant accelerates the diffusion of water vapor be- 

 tween the retardant surface and the free stream air. Johnson indicates that the correction 

 necessary is proportional to some function of the Reynolds number. The Reynolds number is 

 the product of air density, air velocity, and a significant length of the system divided by the air 

 viscosity. Air velocity was the only one of these variables changed during our tests. The dry- 

 ing rate factor r was therefore assumed to be directly proportional to the air velocity above 

 the fuel. 



Figures 8a, 8b, and 8c were first plotted on semilog paper, and a straight line fitted to 

 each of the nine groups of data. The slope of each line gave nine values of the drying rate con- 

 stant, which we shall designate r . Mathematically the modification of r assumes the follow- 

 ing form: 



10 Constants for use in equation (4) may be found in table 7.3 of Physical Meteorology , by 

 Johnson (see footnote 9). For a complete solution of the equation, a table of temperature versus 

 vapor density must be used. Such a table is available in handbooks of meteorology. The metric 

 system is used in the retardant drying section of this report because most handbook constants 

 are in this system. The remainder of the report is in the more familiar English units. 



18 



