In the case of the conduction/diffusion model, Fig. 11 shows the
contour plot of the mass concentration profile of the solute x2
corresponding to the temperature profile of Fig. 7. The diffusion
coefficient of KNO3 (2) in H2O (1) was set to D12 = 1.82
109m2/s
[30]. The difference in the mass diffusion and heat conduction
coefficient results in a very different appearance of the two
profiles. The evaporation of the solvent in the outer layer and the
slow diffusion lead to high concentrations of the solute in this
layer. The solute concentration reaches, thereby, values of x2 = 1.8
x2,0. These peaks of concentration dissipate due to diffusion for
later times. After t
ffi
370 ms, the solute concentration inside the
droplet is x2 = 0.553 kg anhydrate/kg solution.
The saturation level calculated inside a droplet for the
conduction/diffusion model shows a quite different appearance
to the uniform model. Fig. 12 is a plot of the saturation level Sðt; ^rÞ
for two different droplet sizes d0. Areas of constant saturation level
are marked with the contour lines.