INAUGURAL–DISSERTATION zur Erlangung der Doktorwürde der ...

4.3. Single Bi-component Droplet Evaporation and Solid Layer Formation 83
solid layer formation over that of the whole particle volume, would be higher in case
of increased gas velocity (see Fig. 4.33).
Figure 4.31 shows the effect of initial gas temperature on the temporal change of
the dimensionless surface area of a mannitol/water droplet. Elevated gas temperature
leads to higher energy transfer from the gas to the droplet, and thereby, an increase in
the rate of droplet evaporation and drying. The surface area continuously decreases due
to water evaporation until the beginning of solid layer formation whereupon particle
size remains constant, which is reflected in Fig. 4.31.
The higher the gas temperature the quicker the time taken for the solid layer formation:
In case of 67 ◦ C the solid layer develops in about 2.9 s and with 100 ◦ C the solid
layer forms in 1.7 s, whereas with 160 ◦ C, the same is observed in about 0.9 s. There
is larger surface area at the time of solid layer formation with higher gas temperature,
which means that elevated gas temperature would give larger particles towards the end
of the drying process (see Fig. 4.33).
The effect of gas temperature on the development of mannitol mass fraction profiles
inside the droplet of initial radius 70 µm subjected to dry air with 0.5% R.H., flowing at
0.65 m/s with temperatures of 67, 100 and 160 ◦ C is shown in Fig. 4.32 at 0.5 s (left)
and at 0.9 s (right), respectively. Initially, the droplet interior has a homogenous
mannitol mass fraction distribution of 0.15 (not shown here) and with time, there is
development of mannitol mass fraction gradients inside the droplet, and the droplet
size reduces due to continuous water evaporation. For 100 ◦ C initial gas temperature,
the droplet radius is 62 µm at 0.5 s whereas at 0.9 s it reduces to 56 µm, which is
seen in Fig. 4.32, respectively. The increased initial gas temperature yields higher mass
fraction gradients inside the droplet mainly due to the decreased activity coefficient of
0.35
Time = 0.5 s
0.8
Mannitol mass fraction [­]
0.3
0.25
0.2
T g
= 67°C
T g
= 100°C
T g
= 160°C
Mannitol mass fraction [­]
0.7
0.6
0.5
0.4
0.3
Time = 0.9 s
T g
= 67°C
T g
= 100°C
T g
= 160°C
0.15
0 10 20 30 40 50 60 70
Radial position inside the droplet [µm]
0.2
0 10 20 30 40 50 60 70
Radial position inside the droplet [µm]
Fig. 4.32: Effect of gas temperature on the temporal development of mannitol mass
fraction inside the droplet at 0.5 s (left) and 0.9 s (right).