INAUGURAL–DISSERTATION zur Erlangung der Doktorwürde der ...

86 4. Results and Discussion
1
0.8
R.H. = 1%
Simulation [ T g
= 60°C ]
Experiment [ T g
= 60°C ]
Simulation [ T g
= 95°C ]
Experiment [ T g
= 95°C ]
1
0.8
R.H. = 30%
Simulation [ T g
= 60°C ]
Experiment [ T g
= 60°C ]
Simulation [ T g
= 95°C ]
Experiment [ T g
= 95°C ]
(d/330 µm) 2 [-]
0.6
0.4
(d/360 µm) 2 [-]
0.6
0.4
0.2
0.2
0
0 10 20 30 40 50 60 70 80
Time [s]
0
0 50 100 150 200 250
Time [s]
Fig. 4.35: Effect of gas temperatures of 60 ◦ C and 95 ◦ C and relative humidity of
1% R.H. (left) and 30% R.H. (right) on the droplet surface area.
30% R.H. the same observed in about 205 s, see right part of Fig. 4.35. The profiles of
the normalized droplet surface, (d/d 0 ) 2 , shown in the Fig. 4.35, reveal that the droplet
evaporation rate prior to solid layer formation in the present case deviates from the
linear decrease with time as would be expected from the classical d 2 law, where a
constant evaporation constant is assumed.
These experiments are carried out with different initial droplet radius for every
experiment, and Tab. 4.4 gives the initial droplet radii (R 0 ) and particle size at the
time of solid layer formation (t s ) in every experiment and its corresponding computed
value from simulation.
The comparison between rapid mixing model (RMM) and the present model is given
in Fig. 4.36, which shows the time evolution of mannitol/water droplet surface area for
initial droplet radius of 70 µm at 20 ◦ C temperature and subjected to hot air of 160 ◦ C
with 0.5% R.H. and flowing at 0.65 m/s. Even though there is little difference between
RMM and the present approach during the initial time period, however, in the later
Tab. 4.4: Experiment vs simulation
T g R.H. R 0 Particle radius at t s [µm]
[ ◦ C] [%] [µm] Simulation Experiment
60 1.0 330 145.1 147.9
30.0 360 155.2 155.1
95 1.0 215 95.0 109.4
30.0 280 122.5 121.4