## 3 Results
### Spray Drop Size and Velocity
A similar analysis is presented in order to separately analyze the potential effects exerted by an increase of the injection pressure in terms of the spray evolution and atomization quality. In Figures 11 and 12, the results obtained in terms of raw data for the measuring position X = Y = 0 and Z = 50 mm and in terms of the mean for the entire examined traverse are reported. The injection pressure's effect on the drops' sizes is evident from the comparison of Figure 11b,d, with the P\({}_{\text{inj}}\) = 9 bar,g drop size consistently reduced for the spray bulk (from 4 to 8 ms) for time bin mean values close to 50 \(\upmu\)m for a large part of the considered time window. Higher values were observed only for the very initial part of the injection process and around 7 ms when, presumably, the drops produced during the injector's closing phase approached the measuring station. Both the initial and final transients were characterized by a severe channel flow restriction, with consistent flow velocity reduction and drops breaking up the process penalization. For P\({}_{\text{inj}}\) = 5 bar,g, relatively large drops were attained at the Z = 50 mm measuring station with an almost constant velocity during the bulk spray time window, suggesting a moderate drag effect by the surrounding air. Conversely, with the higher injection pressure, the mean drops' velocities showed initially lower values, possibly due to a consistent drag exerted on the finely atomized drops, with the mean velocity rising only later when, presumably, the core of the completely developed spray attained the measuring station. The analysis in Figure 12 confirmed the significant effect exerted by the injection pressure on drops' characteristics. The pressure increase from 5 to 11 bar,g caused a reduction of the SMD in excess of 20 \(\upmu\)m in several stations, with minor effects in the spray's periphery. Smaller and more uniform effects were observed in terms of the mean diameter. The drops' mean velocities consistently increased for all the measuring stations as a direct consequence of the injection pressure's increase. As was observed in Figure 11, the velocity increase was observed mainly in the second part of the 4-8 ms time windows, when the fully developed spray crossed the measuring station. It is also interesting observing how, with P\({}_{\text{inj}}\) = 5 bar,g, the drop count profile was clearly asymmetrical, suggesting incipient off-design operation for the used injector. Correspondingly, the drops' atomization quality in the low-count region was particularly poor. Figure 10: Effect of the water temperature on the drops’ sizes and velocities over the measuring traverse at Z = 50 mm. \(\text{P}_{\text{inj}}\) = 7 bar.g. (**a**) Drops’ mean diameter. (**b**) Drop count and velocity. Finally, in Figure 13, the diameter probability density function (PDF) for the droplet population is reported for the examined ranges of the injection pressure and water temperature. Figure 11: Effect of the injection pressure drops’ sizes and velocities. Raw data in X = Y = 0, \(Z\) = 50 mm; T\({}_{w}\) = 55 \({}^{\circ}\)C. (a) Drops’ velocity for P\({}_{\rm inj}\) = 5 bar,g. (b) Drops’ diameter for P\({}_{\rm inj}\) = 5 bar,g. (c) Drops’ velocity for P\({}_{\rm inj}\) = 9 bar,g. (d) Drops’ diameter for P\({}_{\rm inj}\) = 9 bar,g. Figure 12: Effect of the injection pressure on drops’ sizes and velocities over the measuring traverse at Z = 50 mm. T\({}_{w}\) = 55 \({}^{\circ}\)C. (a) Drops’ mean diameters. (b) Drops’ counts and velocities. ture. As can be observed, both the temperature and pressure variations caused a significant improvement of the atomization quality, with a consistent reduction in the PDF values for the intermediate diameter values (50-150 \(\upmu\)m), while only marginal effects were obtained for the few very large drops produced during the initial part of the injection process.