# Modeling the Formation of Urea-Water Sprays from an Air-Assisted Nozzle
## 3 Results and Discussion
### Evaporation of the Urea-Water Droplets  
Prior to evaluating the predicted evaporation characteristics of the spray, the evaporation process of urea-water droplets will be discussed briefly. The evaporation can be subdivided into two main stages: The first stage is characterized by an almost exclusive evaporation of water. This means that there will be no evident difference between the evaporation of pure water and urea-water droplets. Once the water of the urea-water droplet is consumed by evaporation, the thermolysis and evaporation of urea sets in. This is the second stage, which can be described as an evaporation process with a considerably lower evaporation rate in comparison to water. At the same time, the droplet temperature increases drastically. A detailed investigation of the evaporation process of urea-water droplets is given by [PERSON] [33]. The important conclusion from the brief discussion of the evaporation process is that a designated droplet size exists during the lifetime of each urea-water droplet at which, the evaporation rate is drastically reduced. This distinct feature of the evaporation process was used to define the droplet size at the limit of water evaporation as a criterion for determining the progress of spray evaporation in a previous study [17]. Using this criterion, the comparison of experimental results to numerical predictions is straightforward, which will be directly demonstrated in the following paragraphs. The evaporation characteristics of the urea-water droplets are evaluated from the simulations by means of the droplet temperature and the droplet composition. Thereby, the droplet composition provides information about whether a droplet undergoes water evaporation or urea thermolysis. The composition and temperatures of the droplets as a function of the droplet size and the distance from the nozzle are depicted in Figure 16 for OP 2. At the first measurement position shown in Figure 16a, there is no significant evaporation recognizable. Only the smallest droplets start to evaporate. This behavior also proves the suitability to apply an averaged droplet spectrum from the first measurement position as the initial condition at the nozzle exit in the CFD model. After the droplets are heated up, the water evaporation is present for almost all droplet sizes shown in Figure 16b. The urea thermolysis indicated by a rapid rise of the droplet temperature and a significant reduction of the water content has already begun for the smallest droplets at this position. Further downstream of the nozzle, the droplet diameter from which the urea thermolysis is present increases continuously, as illustrated in Figure 16c,d. Thereby, the droplet temperature reaches a steady state for droplets smaller than 7 um at the last measurement position 495 mm downstream of the nozzle. To compare the findings from the simulations with the experimental observations, the characteristic diameter at which the urea thermolysis starts is evaluated. The evaluation is conducted by the increase of the droplet temperature as a function of the droplet diameter. In the case of a strong rise above 10 K from size class to size class, the corresponding droplet size is determined as the diameter at the limit of water evaporation. The results are compared with the experimental data in Figure 17 for the three investigated operating points as a function of the axial distance from the nozzle. It is obvious that the progress of the evaporation process of the present urea-water spray is underpredicted by the model. The diameter at the limit of water evaporation is approximately 2 \(\upmu\)m to 3 \(\upmu\)m below the experimental results. Nevertheless, the effects of the three hot gas operating conditions are predicted correctly. The evaporation rate is significantly decreased from OP 1 to OP 2 due to a decrease in the ambient density. For OP 3, the evaporation rate is further decreased due to a considerably lower ambient temperature.