Heat and Mass Transport
Solution. Rearranging Eq. (6.7) to find conductance gives
E
gv = CVS - Cva .
The evaporation rate is
From Table A.3, the saturation vapor pressure at 22" C is 2.64 kPa. From
Ch. 3, the mole fraction difference is
es(Ta)(l - h,) - 2.64k Pa(1 - 0.53)
mol
cvs - Cva =
-
= 0.012 - .
Pa
lOlk Pa
mol
The conductance is
0.00124 $
mrnol
gv =
= 10.3 - .
0.012 2
m2s
Example 6.4. Warm moist winds can quickly melt substantial depths of
snow. Heat transfer to the snow is latent as well as sensible. Compare the
latent and sensible heat fluxes to a snow drift from saturated air at 5" C,
if the boundary layer conductance is 1 rnol m-2 s-' .
Solution. From Table A.3, the saturation vapor pressure at 5" C is 0.87
Wa, and at 0" C (the surface temperature of the melting snow) it is 0.61
H a . The sensible heat flux density is
J
mol
H = 29.3 -
W
x 1-(0°C
- 5°C) = -147mol C
m 2 s
m2
The latent heat flux density is the latent heat of vaporization multiplied
by Eq. (6.7):
J
0.61k Pa - 0.87k Pa
W
h E = 44000x lm o l (
lOtkPa
) = -1142.
rnol
m 2 s
The negative signs indicate that the flux is toward the surface. The total
heat flux to the surface is 261 w/m2. The interesting thing about this
computation is that the latent heat flux is almost half of the total.
Example 6.5. A person has a sleeping bag which has a thermal conductance of 0.05 rnol m-2 s-'. The tissue conductance of the person, while
FIGURE FOR EXAMPLE 6.5.