## 3 Results and Discussion
### Spray Characteristics
This is caused by the high velocity of the air stream emerging from the nozzle. At the edge of the traverse displacement, the velocity drops down to slightly above the bulk velocity of the gas phase. The reason for this behaviour is given by the slower bulk hot gas flow, which decelerates the air flow from the nozzle. In this context, it should be noted that the outlet bore hole diameter of the atomiser nozzle is 2 mm. The velocity \(\bar{u}_{5\,\mathrm{\SIUnitSymbolMicro m}}\) of droplets of 5 um diameter was found to be lower than \(\bar{u}_{8\,\mathrm{\SIUnitSymbolMicro m}}\) over the entire profile due to the lower inertia of the smaller droplets. Similarly, the velocity of droplets of a diameter greater than 15 um \(\bar{u}_{>15\,\mathrm{\SIUnitSymbolMicro m}}\) is presumed to feature the fastest velocity over the entire radial dimension. In contrast, \(\bar{u}_{>15\,\mathrm{\SIUnitSymbolMicro m}}\) was found to have the fastest velocity only at the centre region of the spray. It shows a strong decrease towards the edges of spray, where the lowest velocity of the three velocity profiles is measured. In this region, \(\bar{u}_{>15\,\mathrm{\SIUnitSymbolMicro m}}\) even drops below the bulk velocity of the gas flow. A possible explanation for this effect may be derived from the diameter distribution in Figure 10. These results indicate an imperfect atomisation at the outer edge of the spray, which may lead to larger droplets emerging with low initial velocity from the atomiser. A snapshot of the spray in the near nozzle region was recorded using a relatively long exposure time of approximately 0.14 ms as shown in Figure 11. Obviously, the centre of the spray can not be investigated in detail due to motion blur. In contrast to that, droplets at the outer region of the spray as well as liquid structures still connected to the atomiser nozzle are recorded sharply, which implies that they feature a considerably lower velocity in comparison to droplets within the main spray cone. Therefore, the statement of large droplets with low initial velocity is confirmed, which is also in  
Figure 10: Distribution of characteristic (**a**) droplet diameters and (**b**) droplet velocities in a radial direction at \(x=50\) mm (a spline fit is used for interpolation). accordance with the general primary atomisation process of an effervescent atomiser as reviewed by [PERSON] et al. [PERSON] _et al._ (2019). The radial position featuring the strongest velocity gradient of all velocity profiles in Figure 10b, approximately 2 mm off the centre, represents the shear layer zone between the jet flow of the atomiser and the surrounding hot gas flow. In this region, the finest spray is recorded as illustrated by the \(D_{32}\) and in particular by the \(D_{V90}\). Both characteristic diameters increase sharply at a further distance from the centre of the spray. This behaviour may be caused by the strong velocity gradient in combination with shear layer turbulence. Hence, the atomisation process in this region is dominated by the resulting high relative velocity between the droplets and the hot gas. At this point, it should be emphasised that the evaluation of this correlation between droplet size and velocity is only possible because the diagnostics is capable of detecting the size and velocity of individual droplets. In conclusion, these initial results of the test section comprise important spray data close to the atomiser. In particular, the possibility to obtain data at different positions in a radial direction in a dense spray region turns out to be a great advantage of the measurement technique. This gives a better understanding of the atomisation process or enables the tuning of the cold gas mass flow of the atomiser \(\hat{n}_{\text{cg}}\) in order to achieve desired spray characteristics. Furthermore, the recorded data in the proximity of the atomiser can be utilised as droplet starting conditions for numerical simulations of spray evaporation.