## 3 Results and Discussion
### Spray Characteristics  
An ambitious goal of numerous spray experiments is the detection of the spray characteristics like droplet diameter distribution or velocity profiles as close as possible to the atomiser. This is in particular important for an insight into the primary atomisation process or if the starting conditions of the droplets are sought for numerical simulations. The first measurement position of this study was placed 50 mm downstream of the atomiser because secondary atomisation is almost completed at this position. On the other hand, an acceptable droplet concentration is present which facilitates post-processing. The results at this position reveal spray characteristics typical for the twin-fluid atomiser, which will be discussed in detail in the following. The present diagnostics offer the possibility to vary the position of the measurement volume across the cross section of the pipe by simply moving the camera in the direction of the optical axis as depicted in Figure 9. In this context, the small depth of field is a significant advantage of the present microscopic imaging setup, since a slim and thus well-defined measurement volume is achieved. Finally, a traversing of \(\pm\)5 mm in a radial direction around the centre of the pipe was employed covering 11 measurement positions. At each position, 3000 double frame images are taken with a repetition rate of 10 Hz. The resulting number of evaluated droplets varied from a few thousands at the edge to approximately 120,000 at the centre of the spray. A typical result is plotted in Figure 10a in terms of characteristic droplet diameters. Obviously, the profiles are not perfectly symmetric. The reason for this phenomenon is assumed to result from some measurement positions featuring low statistical significance. First of all, it is found that the statistical significance is reduced with increased distance from the centre of the spray due to a lower concentration of droplets. Moreover, the high concentration of droplets at the centre of the spray causes a stronger background noise if the inner spray cone is situated between camera and measurement position. Hence, the outer positions in positive radial direction in Figure 10a are characterised by a lower signal-to-noise ratio, and therefore a lower quantity of droplets is evaluated. Speaking in numbers, at the extreme positions of +5 mm and -5 mm, approximately 4000 and approximately 19,000 droplets are detected, respectively. Nevertheless, the overall quantity of evaluated droplets is sufficient for a further analysis of the spray characteristics. In general, the atomiser is capable of producing a very fine spray. The Sauter Mean Diameter \(D_{32}\) stays at a constant level just above 10 um over a range of \(\pm\)2 mm in a radial direction. In this core  
Figure 9: Schematic of traversing the camera and hence the measurement volume in a radial direction of the pipe. zone large droplets, represented by the diameter at 90% volumetric quantile \(D_{V90}\), have a diameter in the range of 20 to 25 um. While the arithmetic mean Diameter \(D_{10}\) yields no strong variation in a radial direction, a clear inhomogeneity is revealed by the profiles of \(D_{32}\) and \(D_{V90}\). Both characteristic diameters increase sharply at a distance of 2-3 mm from the centre of the spray, which gives evidence to an imperfect atomisation process at the outer edge of the spray cone. In addition to the information about droplet size, the velocity has been determined for each detected droplet. The result is summarised in Figure 10 by means of three average velocity profiles plotted over the radial direction of the pipe flow. The first profile \(\bar{u}_{8\,\mathrm{\SIUnitSymbolMicro m}}\) represents the average velocity of droplets of 8 um diameter, which is calculated for a diameter range of 8 \(\pm\) 0.5 um in order to achieve statistical significance. At the centre of the spray, a very high velocity is detected, which is far above the bulk velocity of the gas phase \(\bar{u}_{\mathrm{bg}}\) = 96 m s\({}^{-1}\).