INAUGURAL–DISSERTATION zur Erlangung der Doktorwürde der ...
INAUGURAL–DISSERTATION zur Erlangung der Doktorwürde der ...
INAUGURAL–DISSERTATION zur Erlangung der Doktorwürde der ...
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96 4. Results and Discussion<br />
23<br />
21<br />
DQMOM<br />
Experiment<br />
Mean droplet velocity [m/s]<br />
19<br />
17<br />
15<br />
13<br />
11<br />
9<br />
7<br />
-75 -60 -45 -30 -15 0 15 30 45 60 75<br />
Radial position [mm]<br />
Fig. 4.48: Experimental and numerical profiles of the mean droplet velocity of<br />
PVP/water spray in air at the cross section of 0.12 m distance from the<br />
nozzle exit.<br />
collision. The smaller size droplets that lie closer to centerline of the spray are moving<br />
at higher velocity than the larger size droplets, which is in quite contrast with that of<br />
water spray (see Fig. 4.23) where both the larger and smaller size droplets move with<br />
higher velocity. This may be because initially the gas around the spray is stagnant<br />
and the droplets decelerate by aerodynamic drag. The surrounding gas acquires the<br />
momentum lost by the droplets, and this creates a flow field in which gas is continually<br />
entrained into the spray. As the entrained gas enters the spray, it drags small liquid<br />
drops at the outer regions of the spray inward, and the momentum lost by the droplets<br />
at the periphery of the spray is larger than the ones that lie closer to the axis of<br />
symmetry, which explains the smaller velocity of larger droplets [227, 228].<br />
Further downstream of the nozzle exit, i.e., at the cross section of 0.16 m away<br />
from the nozzle exit, the retardation of the droplet velocity in large size droplet region<br />
is observed (see Fig. 4.49) similar to water spray as this effect is dependent on initial<br />
liquid flow rate, where low liquid flow rate leads to larger droplets, which take more<br />
time to follow the streamlines of the gas than the smaller size droplets [209, 225].<br />
The simulation results are in good agreement with the experiment, particularly in<br />
smaller size droplets region whereas towards spray edge there is deviation, which can<br />
be attributed to the post-processing of the experimental data and the non-resolved gas<br />
phase.<br />
Concerning the differences in evaporation characteristics for water and PVP/water<br />
droplet evaporation in air, it is found that for a given liquid flow rate and axial po-