## 4 Conclusions  
In this study, a microscopic imaging technique has been developed for recording quantitative spray data in challenging operating conditions. Challenges arise in particular from the high temperatures of the gas phase, which may cause Schlieren effects in the optical path and an astigmatism phenomenon. Both effects did strongly compromise the image quality. As a countermeasure, the optical setup was equipped by a water-cooled window and an astigmatism correction device. By means of an advanced post-processing routine, it is possible to detect droplets as small as 4 um and droplet velocities up to 250 m s\({}^{-1}\) at operating conditions of the gaseous phase of up to 873 K and 0.6 MPa.  
Moreover, the traversing of the camera in the direction of the optical axis in a dense spray region turns out to be a great asset. This gives the opportunity to detect inhomogeneous diameter and velocity profiles across the spray cone at a distance of 50 mm from the atomiser. Hence, valuable droplet data are captured, which may serve as initial conditions for numerical simulations.  
Evaluation of the data revealed that the mean gas velocity can be derived from the velocity of the smallest detectable droplets with reasonable accuracy. This gas velocity may be utilised to calculate the temporal mean slip velocity between individual droplets and the gas phase. Hence, the effect of this average slip velocity on secondary atomisation or evaporation can be taken into account. Additionally, the evaluation of the droplet velocity fluctuations in a radial direction reveals vital information about the turbulence of the gas phase, which may be used for validation of numerical predictions. Most importantly, the turbulent dispersion of droplets can be directly evaluated.  
Figure 17: Exemplary model results of UWS and water droplets with an initial diameter of 10 μm and 8 μm. The evaporation process of UWS is modelled based on the model of [PERSON] et al. [49]. Crust formation is considered using the reduction coefficient of [PERSON] et al. [50].  
In addition to UWS, distilled water was also used as a liquid under otherwise identical conditions. The direct comparison of the droplet diameter distribution between UWS and water revealed typical features of the evaporation characteristics of UWS for all positions downstream of the atomiser.  
In summary, the present methodology provides detailed spray data, which can be used to study the evaporation characteristics of UWS under challenging flow conditions. Future work will be focused on the variation of hot gas temperature, pressure and velocity, covering a range of technically relevant operating conditions.  
Conceptualization, C.L., R.K. and H.-J.B.; data curation, C.L.; formal analysis, C.L.; funding acquisition, R.K. and H.-J.B.; investigation, C.L. and R.K.; methodology, C.L. and R.K.; project administration, R.K.; resources, H.-J.B.; software, C.L.; supervision, R.K.; validation, C.L.; visualization, C.L.; writing--original draft preparation, C.L.; writing--review and editing, C.L. and R.K.  
This work was partially supported by MTU Friedrichshafen which is the core business of Rolls-Royce Power Systems, a division of Rolls-Royce plc.  
We acknowledge support by the KIT-Publication Fund of the Karlsruhe Institute of Technology. Additionally, the authors are grateful to [PERSON] and [PERSON], who assisted with the execution of the experiments as student assistants.  
The authors declare no conflict of interest.  
The following abbreviations are used in this manuscript:  
LDA Laser Doppler Anemometry  
LIF Laser Induced Fluorescence  
PDA Phase Doppler Anemometry  
PDF Probability Density Function  
PIV Particle Image Velocimetry  
PTV Particle Tracking Velocimetry  
SCR Selective Catalytic Reduction  
UWS Urea-Water Solution