INAUGURAL–DISSERTATION zur Erlangung der Doktorwürde der ...

4.3. Single Bi-component Droplet Evaporation and Solid Layer Formation 81
Tab. 4.3: Experimental drying conditions
Drying condition
Values
Initial solute mass fraction 0.075, 0.05 and 0.15
Initial droplet radius 70 µm
Initial droplet temperature
Gas temperature
Gas velocity
20 and 70 ◦ C
60, 67, 95, 100, 160 and 210 ◦ C
0.05, 0.65 and 10 m/s
Relative humidity (R.H.) 0.5 1.0, 2.0 and 30%
likhov [222]. The critical temperature and pressure of PVP and mannitol are taken
from Daubert and Danner [217]. The vapor diffusion coefficient through the solid layer
of PVP or mannitol, D s , and solid thermal conductivity, k s are not available in literature,
therefore, they are computed similar to the work of Nesic and Vodnik [151]. The
physical and thermal properties in the film are estimated at the reference composition
using the 1/3 rule [223]. The PVP/water and mannitol/water solution physical and
thermal properties are computed with the standard rules of mixing. The variation
of saturation solubility of PVP in water and mannitol in water with temperature is
taken from measurements [218, 219], and it is shown in Fig. 4.29. The solid layer at
the droplet surface is presumed to develop when the PVP mass fraction at the droplet
surface reaches 20% above its saturation solubility limit, and in the case of mannitol,
it is assumed that the crust and solid layer formation begins when the mannitol mass
fraction reaches 0.9, which is much higher than the saturation solubility, in order to
avoid re-dissolution of solid layer with increased temperature as it shows large variation
of solubility with temperature, see Fig. 4.29.
The numerical results presented refer to a droplet of initial radius 70 µm at 20 ◦ C
containing 0.15 PVP or mannitol initial mass fraction subjected to air with 0.5% relative
humidity (R.H.) flowing at 0.65 m/s with 100 ◦ C initial gas temperature [172].
The various drying conditions for numerical simulations taken from the experimental
study of Littringer et al. [21] and Sedelmayer et al. [224], are listed in Tab. 4.3 and
numerical results are compared with available experimental data [21, 224].
Figure 4.30 shows the change in mannitol/water droplet mass and temperature
with time for the above conditions and for increased initial gas velocity (U g = 10 m/s).
Initially, there is no significant increase in droplet temperature, and droplet mass reduces
due to continuous water evaporation. After an initial heating period, the droplet
temperature rises very quickly indicating the formation of solid layer whereupon the
rate of evaporation is reduced due to added resistance coming from solid layer, which is
reflected in the droplet mass profile. The higher gas flow rate increases convection and