## 3 Experimental Results
For the remainder of the measurement, however, this effect played a subordinate role, since the deposits only formed locally and appeared to be dissolved as soon as a continuous film was present [41]. To better understand the influence between film formation, wall temperature and heat flux, the wall temperature in Figure 6b is plotted logarithmically. Two exponential fits were fitted to the curve in the time prior to the formation of a closed film and afterwards. It is evident that the exponent at which the wall temperature changes coincides with formation of a quasi-stationary film. This effect was expected, as the heat flux reaches its maximum, when the substrate is completely wetted by the liquid. Here, the distinct change in the exponent indicates the transition from spray cooling (separated wetted regions) to pool boiling (closed film). Spray cooling causes a stronger cooling of the wall than a closed liquid film. The timing of the regime change is reflected in both the wall temperature and the film measurements. However, there is an offset of about 13 s between the apparent stagnation of the film thickness (ca. 140 s) and the change in the wall cooling rate (ca. 153 s). This time offset can be explained by the measurement location 0.3 mm below the surface, which leads to a low-pass filtered measurement of the wall temperature. In order to investigate the reproducibility of these findings, three repeated independent measurements were conducted at this operational point (C1) Table 1. In Figure 7, the results of these measurements are compared. All three measurements show similar behavior in terms of film buildup, convergence towards a quasi-stationary thickness and decrease of film thickness. In the first phase of spray cooling, in which a closed film has not yet formed, deviations were observed between the three individual measurements. These differences are a result of the 0 D measurement, which can only provide information on the film thickness within an area of 1 mm\({}^{2}\) area, while the spray impacts on an area of 84 mm \(\times\) 100 mm. During the initial phase of AdBlue injection, single droplets are formed. Driven by the shear flow, they move in the flow direction. The thickness measured during this phase therefore depends on whether a droplet initially forms in the investigated area or not. The location at which droplets form and their movement paths is different during each individual measurement, which explains the differences. Nevertheless, the standard deviation after convergence (200-300 s) of the film thickness is 3.4% and thus below the maximum specified error (see Table 2). Despite the differences in the spray cooling phase, the cooling rate through spray  
Figure 7: Repeated measurements of the first operating point C1 (180 \({}^{\circ}\)C, 6.5 m 3 \(\frac{m}{s}\), \(\alpha=1\)). The dashed vertical lines marked the beginning and end of AdBlue injection. cooling show a good comparability with a standard deviation of 5.5% in relation to the gradient of the wall temperature. The remaining six operating points are shown in Figure 8. As a supplement, the deposition after the experiments at the corresponding operation conditions is shown in Figure 9.  
All seven cases show different film-formation behavior, which is also evident in the deposits. The formation of a quasi-stationary film as in case 1 (Figure 6a) can be also seen in the cases 2, 3, 6 and 7 (Figure 8a,b,e,f). For the two hyperstoichiometric cases (case 6 and 7, Figure 8e,f), the increased mass flux of impinging spray leads to an enhanced wall cooling. Together with the larger mass flow of AdBlue, a closed film forms almost initially at the start of the injection. In the second case (Figure 8a), the lower convection velocity results in lower heat transfer by convection, which reduces the initial wall temperature compared to case 3. In both cases, the gas temperature is 250 \({}^{\circ}\)C and the wall temperature is constant before the injection starts, but the velocity is 3 m/s, which is much lower than the 6.5 m/s used for case 3.