## 3 Results
### Spray Global Development  
The imaging set-up described in Section 2 was used to investigate the effect of the injection pressure and water temperature on the global spray's evolution. For the sake of brevity, only a short sequence of the spray evolution in the reference condition (P\({}_{\rm injj}\) = 7 bar,g, 20 \({}^{\circ}\)C) is reported in Figure 5 along with pictures of the fully developed spray under high injection pressure and water temperature conditions (Figure 6). Figure 4: Mean injected quantity vs. injection pressure with water temperatures from 20 \({}^{\circ}\)C to 110 \({}^{\circ}\)C. (**a**) ET = 2 ms and 3 ms. (**b**) ET = 5 ms and 8 ms.  
Figure 5: Spray evolution sequence at P\({}_{\rm injj}\) = 7 bar,g and 20 \({}^{\circ}\)C (timing from ET start, scale tick = 10 mm). As can be observed in Figure 5, four individual conical flow structures emerged from the nozzle at the beginning of the injection process, initially composed of relatively large ligaments. The four structures merged at a distance between 5 and 10 mm from the injector nozzle. After a short transition zone, at a 10-mm distance downstream, the primary break-up process seemed to be completed. The spray structure hereafter appears to be quite uniform and composed of relatively small drops; only in the most advanced and central part of the spray was the presence of significantly large ligaments clearly perceivable, probably originating from the initial injection transient when the flow velocity was restrained by the needle. When the operating conditions were changed by increasing the injection pressure from 7 to 11 bar,g (Figure 6a,b), the spray structure was evidently altered, with an increased penetration and cone angle. Furthermore, the spray structure appeared to be composed of smaller droplets, with a reduced presence of large ligaments in the spray's bulk. Only in the spray tip were some large blobs originating from the injector's opening transient still present. Conversely, the increase in water temperature (Figure 6c,d) did not seem to produce dramatic changes in the spray's overall structure with respect to the corresponding low-temperature operating condition. This evidence seems to suggest that in the tested operating conditions, flash boiling was not triggered despite the high temperature level at the injector inlet, possibly due to the progressive cooling of the water inside the injector body. The complete set of results in terms of the spray tip penetration and spray cone angle are reported in Figures 7 and 8 for the 12 examined operating conditions, evidencing the effect on the spray's global development of the injection pressure and water temperature, respectively. As reported in Figure 7, the injection pressure had a significant effect on both the spray tip penetration and global cone angle for all the examined water temperature levels. Globally, 5 ms after the ET's start, changing the injection pressure from P\({}_{\text{inj}}\) = 5 bar,g to P\({}_{\text{inj}}\) = 11 bar,g led the spray tip penetration to increase by 11% at T\({}_{\text{w}}\) = 20 \({}^{\circ}\)C and by 12% at T\({}_{\text{w}}\) = 110 \({}^{\circ}\)C. The spray tip penetration slightly increased from 5 bar,g to 7 bar,g, with this trend being more evident in the final part of the spray evolution and at a higher water temperature. When the injection pressure was raised to 11 bar,g, the final penetration increase was more evident, despite the relatively high injection pressure tending to slow the injector opening transient, consequently leading to the initial spray tip velocity being smaller. As a result, the spray penetration for P\({}_{\text{inj}}\) = 11 bar,g was smaller than for P\({}_{\text{inj}}\) = 7 bar,g up to 1.5 ms after the ET's start, in which time the initial penetration gap was recovered.