## 3 Experimental Results  
In this section, the results of the film thickness measurements are presented. A special focus is on the first operating point C1 (180 \({}^{\circ}\)C, 6.5 m/s and \(\alpha=1\)). Figure 6a shows the results of a single measurement at this operating point; on the ordinate on the left, the film thickness (black) is plotted in \(\upmu\)m, on the ordinate on the right, the wall temperature (red) is given in \({}^{\circ}\)C. The wall temperature is measured with a thermocouple that is positioned 0.3 mm beneath the surface at the same location as the film thickness measurement. The elapsed time is plotted on the abscissa. The measurement results shown were taken at a frequency of 140 Hz. In postprocessing, the interference of the measurement signal by the spray is removed (Figure 5). The lasers are modulated with a top-heat profile at 140 Hz. Two phases of the modulated laser signal are thereby averaged and a moving average is then applied after calculating the film thickness to minimize noise. The effective measurement rate after averaging is 14 Hz. For all measurement points, the injection starts 30 s after the beginning of the measurements, as indicated in the plot by a dashed vertical line. The injection is stopped after 330 s and also marked by a dashed vertical line. Before starting the injection, the wall is not wetted and a constant signal of the clean stainless-steel surface is measured. This constant signal is used for the in situ determination of the offset-signal quantified by the calibration constant b (Equation (3)). After the beginning of the injection, the film thickness increases over time. Particularly noticeable is the decrease in the film thickness in the time range between 100 and 150 s and the subsequent increase, just before it reaches a quasi-steady state. This can be explained by the observation that droplets, driven by the flow, ran through the measuring area. At temperatures below the Leidenfrost point, separated spots of boiling liquid appear on the hot wall. Their relative area increases in time while the wetted substrate is cooled by the spray. The single drops flow together until a closed film is formed and the heat flux reaches its maximum [12]. Depending on the operating point, this effect varies in intensity. After the AdBlue injection is ended, the film thickness initially drops abruptly. This is to be expected because the film evaporates and is simultaneously drawn out of the measurement chamber due to the high temperature and shear flow. Particularly interesting is the time range between 380 and 400 s in which the film thickness increases again. This is a result of the gradual drying of the film in the direction of flow. The remaining film at the phase boundary between liquid urea-water solution and solid deposit is pushed forward. Subsequently, the film evaporates completely and only solid deposits remain. Immediately after the start of injection, an apparent \"negative\" film thickness can be seen. This can be explained with the observation that, at the beginning of the injection,  
Figure 6: (a) Film thickness (black) of AdBlue at 180 \({}^{\circ}\)C, 6.5 m 3 \(\frac{m}{s}\), \(\alpha=1\) and wall temperature (red) measured at the same location 0.3 mm below the surface. The dashed vertical lines marked the beginning and end of AdBlue injection, the uncertainty is indicated in gray. (b) Wall temperature compared to exponential fit. the wall temperature is very high. Since AdBlue is injected at 1 Hz, deposits can form and dissolve between individual spray impacts at the beginning of the measurement. Since the sensor was not designed to measure deposits, it provides the unphysical result of a negative film. In order to verify this hypothesis, another measurement was carried out in which the injection was stopped after 20 cycles. In a subsequent examination, it was found that deposits had already formed.