INAUGURAL–DISSERTATION zur Erlangung der Doktorwürde der ...

4.1. One-dimensional Evaporating Water Spray in Nitrogen 63
The variations in Sauter mean diameter with axial position of the spray for two different
liquid inflow rates of 80 kg/h and 150 kg/h are shown in Fig. 4.9. The results for
80 kg/h show an increasing Sauter mean diameter with evaporation. Inclusion of coalescence
in addition to evaporation leads to excellent agreement between computational
and experimental results. On the contrary, the computational results for 150 kg/h at
x = 0.54 m seem to be deviating far away from the experimental data. The observed
deviation is due to inconsistency in experimental data, which is evident from the fact
that the experimental flow rate does not match the prescribed value of 150 kg/h at
0.54 m. Therefore, the results from 80 kg/h will be discussed for the remaining part of
this section.
Figure 4.10 displays the profiles of the Sauter mean diameter (left) and mean droplet
diameter (right) of water spray subjected to evaporation at 293 K and 313 K temperatures
of surrounding gas as well as with and without coalescence. As expected, Sauter
mean diameter increases substantially with evaporation that causes the decrease and
eventual loss of small size droplets. Higher temperature imposes a rise in evaporation,
which considerably accelerates the rate of increase of Sauter mean diameter. A comparison
with experimental data reveals the importance of modeling the droplet coalescence,
which not only improves the simulation results but also has excellent agreement with
experiment (see left side of Fig. 4.10).
Similar to Sauter mean diameter, the mean droplet diameter is an important physical
quantity for several applications such as particle size analysis of powder sampling
in food and pharmaceutical industries [201]. Mean droplet diameter of a number density
based distribution can be computed using the Eq. (2.18). Since very small size
160
150
Experiment - 293 K
Evaporation - 293 K
Evaporation and Coalescence - 293 K
Evaporation - 313 K
Evaporation and Coalescence - 313 K
90
85
Experiment - 293 K
Evaporation - 293 K
Evaporation and Coalescence - 293 K
Evaporation - 313 K
Evaporation and Coalescence - 313 K
Sauter mean diameter [µm]
140
130
120
110
D 10
[µm]
80
75
70
65
60
55
100
0.14 0.28 0.42 0.56 0.7 0.84
Position [m]
50
0.14 0.28 0.42 0.56 0.7 0.84
Position [m]
Fig. 4.10: Profiles of Sauter mean diameter (left) and mean droplet diameter (right)
computed with and without coalescence at surrounding gas temperatures of
293 K and 313 K.