## 3 Results and Discussion
### Evaporation Characteristics of Urea-Water Solution  
The long-term objective of the present methodology is to provide experimental validation data, which can be used to study evaporating urea-water sprays under realistic thermodynamic conditions. In order to pin down the effect of urea on the evaporation process, distilled water is used as an alternative liquid in the measurement campaign. First of all, there is almost no difference in surface tension and viscosity between water and urea-water solution (UWS). Hence, it is presumed that the atomisation process is effectively not affected when using an air assisted nozzle. Therefore, it is expected that the same spray for UWS and water is generated. However, a recent publication by [PERSON] et al. [26] indicates that replacing UWS with water will influence the spray characteristics considerably. In this study, a pressure driven single fluid nozzle is used and the pressure in the liquid reservoir is preserved. In other words, the pressure difference over the whole injection system is kept constant. Therefore, the liquid volume flow through the atomiser may be affected by the fluid properties, most importantly by the viscosity. Hence, the velocity at the exit of the atomiser nozzle will also be affected, which is one major parameter affecting the primary breakup regime of a cylindrical jet. For the discussion of the effect of the Weber and Ohnesorge number, please refer to the review of [PERSON] [48]. In conclusion, the study of [PERSON] et al. [26] does not evaluate the influence of different fluid properties on the actual atomisation process but on the performance of a whole injection system. In contrast, an air assisted nozzle is used in the present study with an identical setting of the air mass flow and velocity, when replacing UWS with pure water. In addition, the liquid volume flow was kept identical. These provisions ensure that the relevant non-dimensional numbers are effectively identical for UWS and pure water. Therefore, the influence of slightly different liquid properties is considerably lower in comparison to simple pressure atomisers. It should be noted that the density difference between UWS and water of approximately 9% will affect the integral momentum of the spray jet. However, this effect is negligible using a twin-fluid atomiser. Hence, the overall effect of different fluid properties for UWS and water on the atomisation process can definitely be considered as marginal. In summary, the present experimental configuration provides identical droplet initial conditions, since the atomiser produces practically the same spray for UWS and water. By this means, experimental results for water can be used to calibrate numerical simulations in terms of starting conditions and the quality of submodels, since the complex evaporation behaviour of UWS can be excluded. Furthermore, the comparison of droplet diameter distributions of UWS and water further downstream of the atomiser may provide insights in the evaporation behaviour of UWS. Subsequently, it will be evaluated if evaporation characteristics can be retrieved using the present microscopic imaging technique. The measured droplet size distribution for UWS and water at the centre of the pipe and at a distance of 225 mm downstream of the atomiser are illustrated in Figure 16a. At first glance, the normalised number probability density function (PDF) is very similar for both fluids, which confirms the statement that the atomisation process is basically identical. Additionally, the average droplet velocity \(\bar{u}_{\text{d}}\) of the spray is basically identical. Nevertheless, a detailed comparison reveals a higher probability of small droplets for UWS, whereas most larger diameter classes are characterised by a higher probability of droplets for water. Furthermore, this characteristic effect becomes more prominent with increasing distance from the atomiser as shown in Figure 16b,c. The reason for this phenomenon can be explained by the effect of urea on the evaporation process.