INAUGURAL–DISSERTATION zur Erlangung der Doktorwürde der ...

4.4. Two-dimensional Evaporating PVP/Water Spray in Air 95
Sauter mean diameter [µm]
140
120
100
80
60
40
DQMOM
Experiment
Mean droplet diameter [µm]
100
80
60
40
20
DQMOM
Experiment
20
-100 -80 -60 -40 -20 0 20 40 60 80 100
Radial position [mm]
0
-100 -80 -60 -40 -20 0 20 40 60 80 100
Radial position [mm]
Fig. 4.47: Experimental and numerical profiles of the Sauter mean diameter (left) and
mean droplet diameter (right) of PVP/water spray in air at the cross section
of 0.16 m distance from the nozzle exit.
underpredicts the experimental results and towards the periphery of the spray some
deviation is observed particularly in profiles of Sauter mean diameter as compared to
the experiment. This can possibly be explained by the fact that the DQMOM predicts
the global droplet distribution, but there could be local discrepancies induced by the
gas phase, which is not resolved in the present study. Coupling of DQMOM with the
gas phase would eventually improve the simulation results.
Figure 4.47 displays the Sauter mean diameter (left) and mean droplet diameter
(right) at further downstream the nozzle exit, i.e., at the cross section 0.16 m.
Comparing the maxima in Fig. 4.46 and 4.47 reveals that there is an increase in the
Sauter mean diameter and mean droplet diameter, which is converse to the the water
spray where decrease in droplet size is found. At a given temperature, the evaporation
rate of water from pure water droplets is higher than from the droplets containing PVP
dissolved in water due to the non-ideality effect (see Fig. 4.25). An analysis of droplet
coalescence reveals that it occurs 1.5 times more often in PVP/water spray compared
to water spray, which also contributes to an increased droplet size in the PVP/water
spray. The elevated viscosity of PVP leading to higher viscous PVP/water droplets
compared to pure water droplets influences the droplet coalescence. The present model
is suitable to capture these effects, and a good agreement between the experiment and
simulation is found [209].
The mean droplet velocity of PVP/water spray with 112 kg/h liquid inflow rate at
0.12 m away from the nozzle is shown in Fig. 4.48. Increased liquid flow rate leads to
higher droplet velocity (compare Figs. 4.23 and 4.48), which increases the chances of