## 1 Introduction
The acquisition of raw images appropriate for post-processing can be still challenging under realistic operation conditions due to density and temperature gradients within the gaseous phase. The resulting refractive index fluctuations may lead to significantly degraded images [14; 17; 18]. Hence, the image quality may not be sufficient to extract droplet size distributions and velocity profiles. Nevertheless, macroscopic spray characteristics have been recorded at microscopic level, like, for instance, spray penetration rate and spreading angle in the work of [PERSON] et al. [19] or the impressive observation of single fuel droplets transitioning to a supercritical fluid by [PERSON] et al. [20]. The main goal of the work to be presented in this paper is the development of a methodology for analysing evaporation characteristics of urea-water sprays under realistic flow conditions. The evaporation behaviour of urea-water solution (UWS) is an important process of the selective catalytic reduction (SCR). SCR is an established technology for reducing nitrogen oxides (\(NO_{x}\)) emissions of diesel engines. It is capable of meeting recent and future emission regulations. In addition to a De\(NO_{x}\) efficiency of 90% and more, the technology offers the advantage of a very low fuel consumption [21]. Challenges involved in the design of an SCR system are the avoidance of deposits, especially melamine complexes, while ensuring optimal conversion rates of \(NO_{x}\) emissions in the catalyst [22]. Since both processes are highly dependent on spray and evaporation characteristics, experimental validation data are indispensable for the calibration of numerical simulations of urea-water sprays and finally the design process of the SCR system. Various experimental methodologies have been presented in the literature, which focus on urea-water spray characterization. Most of these studies investigate a specific UWS injection setup, which is closely linked to the actual SCR application. For instance, commercial 3-hole injectors are frequently used [11; 23; 24; 25; 26], the spray-wall interaction of UWS droplets is investigated [27] or the optimal mixer location downstream of the atomiser is evaluated [23]. The overall process of preparation of ammonia from UWS, including deposit formation, is reviewed in detail by [PERSON] [28]. In contrast to most studies, the experimental methodology to be presented is based on a generic test case, since the long-term objective is more fundamental: It is to provide experimental validation data, which can be used to study evaporating urea-water sprays under realistic thermodynamic conditions. A generic test case is advantageous for validating a numerical methodology, since other processes, like droplet-wall interaction and deposit formation, are excluded in order to solely focus on the modelling of spray evaporation. Furthermore, a generic test case provides well-defined boundary conditions, which enables a straightforward setup of numerical simulations. Obviously, the proper diagnostics of the droplet size and velocity distribution is a vital capability of the measurement technique to be presented. This enables an effective comparison to numerical predictions. A well proven shadowgraphy approach, which has been developed previously by [PERSON] et al. and [PERSON] et al. [8; 29], is extended to the microscopic level in order to enable the detection of droplet diameters as small as 4 \(\upmu\)m with high accuracy. The analysis of the evaporation characteristics requires high temperature conditions of the hot gas, which may be a major obstacle to obtain raw images with sufficient quality. Hence, a water-cooled window and an astigmatism correction device are adapted to the optical path in order to suppress image degradation due to refractive index fluctuations and astigmatism. Consequently, sensible quantitative results of the target spray parameters can be retrieved. The results to be presented will focus on the capabilities of the diagnostics for deriving vital validation data.