**3. Results**

*3.1. Flow Test Results*

In Figure 2, the results obtained in terms of the mean injected quantity, parametrically varying both the injection pressure level from 5 to 15 bar,g and the water temperature at the injector inlet from 20 ◦C to 110 ◦C, are reported. On the left in the same figure, the results are reported for when Pinj 7 = bar,g was assumed as the reference injection pressure level.

**Figure 2.** Mean injected quantity for Tw range 20–110 ◦C and Pinj = 7 bar,g (**a**) and from 5 to 15 bar,g (**b**).

From the obtained results, the effect of the water temperature increasing on the injected mass is evident. For a given injection pressure level, the higher the fluid temperature, the lower the injected quantity. With the ET at 5 ms and the water temperature changing from 20 ◦C to 110 ◦C, the mass percentage difference ranged from 5.7 % at Pinj = 7 bar,g to 11.5% at Pinj = 15 bar,g. This is likely to be ascribed mainly to the reduced magnetic force exerted by the solenoid on the injector needle, caused by the increased coil resistance. As is reported in Figure 3, for the reference injection pressure level, the injector current was significantly reduced in high water temperature conditions. Furthermore, the current time history slope—and presumably the needle rise—was slowed at high water temperatures, as is suggested by the delayed occurrence of the needle's fully raised position, evidenced by the sudden slope change in the current profile around 1.5 ms after the ET started.

**Figure 3.** Injector current as a function of the water temperature. Pinj = 7 bar,g, and ET = 8.0 ms.

The effect of the fluid's temperature on the injector dynamics was particularly evident for short injection events, during which the injector operated in the so-called "ballistic regime". In ballistic operation, the needle did not attain the fully raised position, and consequently, the flow could not reach a static rate condition. As is reported in Figure 4a, where the flow results are plotted against the injection pressure level (with ET = 2 ms and Pinj = 15 bar,g), the injector's opening phase was drastically altered by high water temperature levels. In these conditions, the progressively reduced available magnetic force caused the mean injected mass to be lower at Pinj = 11 bar,g (and equal for Tw = 20 ◦C). Only for ET values longer than 3 ms was the expected rising dependence of the injected mass on Pinj obtained, given the presumable attained steady flow conditions and the reduced significance of the needle's opening phase with respect to the entire injection process.

**Figure 4.** Mean injected quantity vs. injection pressure with water temperatures from 20 ◦C to 110 ◦C. (**a**) ET = 2 ms and 3 ms. (**b**) ET = 5 ms and 8 ms.

The obtained flow results seem to sugges<sup>t</sup> that Pinj = 15 bar,g is not a feasible operating condition for this injector with a high water temperature and will no longer be investigated in terms of the spray evolution and drop size.

#### *3.2. 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 (Pinj = 7 bar,g, 20 ◦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 5.** Spray evolution sequence at Pinj = 7 bar,g and 20 ◦C (timing from ET start, scale tick = 10 mm).


**Figure 6.** Fully developed spray at 4 ms after the ET's 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 sugges<sup>t</sup> 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 Pinj = 5 bar,g to Pinj = 11 bar,g led the spray tip penetration to increase by 11% at Tw = 20 ◦C and by 12% at Tw = 110 ◦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 Pinj = 11 bar,g was smaller than for Pinj = 7 bar,g up to 1.5 ms after the ET's start, in which time the initial penetration gap was recovered. The effect of the injection pressure on the spray cone angle was even more evident; for all the examined water temperature conditions, the spray diffusion angle progressively increased with higher injection pressure levels, obtaining almost parallel trends for this quantity. It

is also interesting to observe how the monotonically decreasing trend for the spray cone angle changed its slope after the injector closure around 5.8 ms after the ET's start.

The effect of the water temperature at the injector inlet on the spray evolution is analyzed in Figure 8. As can be observed, only marginal effects were exerted by the water temperature on the spray's global structure for all the examined injection pressure levels. To be detailed, an unclear tendency was observed in terms of spray penetration for Pinj = 5 bar,g, while the effect was negligible for higher injection pressure levels. In terms of the spray cone angle, the increase in water temperature seemed to decrease the spray cone angle up to 90 ◦C, while a further increase to 110 ◦C seemed to attenuate or revert the trend. Globally, a moderate effect of the water temperature on the spray evolution was observed.

#### *3.3. Spray Drop Size and Velocity*

The PDA raw data for the measuring station corresponding to the injector axis projection on a plane at 50 mm from the nozzle (coordinates of X = Y = 0; Z = 50 mm) are reported in Figure 9a,b for the reference operating condition Pinj = 7 bar,g and Tw = 20 ◦C. In these plots, all the records relevant to 3000 consecutive injection events are reported (blue dots), along with the average values computed in 0.1 ms time bins (red dots). The ET's start was used as a time reference.

**Figure 7.** Effect of the injection pressure on the spray tip penetration and cone angle. (**a**) T w = 20 ◦C; (**b**) T w = 55 ◦C; (**c**) T w = 90 ◦C; and (**d**) T w = 20 ◦C.

**Figure 8.** Effect of the water temperature on the spray tip penetration and cone angle. (**a**) Pinj = 5 bar,g; (**b**) Pinj = 7 bar,g; and (**c**) Pinj = 11 bar,g.

As can be seen, the drops' diameters and Z-velocity time histories evidence how the spray approached the considered measuring station about 4 ms after the ET's start. The main part of the spray structure flowed through the considered measuring station between 4 and 8 ms. During this time window, the drops' velocity ranged between approximately 5 and 22 m/s, while the observed drops' diameter range was predominantly between 15 and 150 μm. A significant number of drops with diameters up to 350 μm was also observed, possibly related to the defective primary break-up in the first part of the injection process, as was observed in the spray images. The presence of even such a restricted number of relatively large drops inherently had a non-marginal effect on the resulting Sauter mean diameter.

After 8 ms from the ET's start, the velocity data rapidly decreased to values around 2–3 m/s for the drops pertaining to the spray tail. This part of the spray's structure was composed of very small droplets (diameter values below 40 μm) featuring reduced momentum which continued flowing through the observed position for a long time.

When the injector's water temperature was raised to 110 ◦C, the drops' velocities and sizes time histories for the same measuring station, reported in Figure 9c,d, were obtained. Marginal effects due to the applied rise in temperature were observed in terms of both the drops' velocities and size ranges for the bulk spray evolution during the 4–8 ms time window, while a significant shift in the mean values was observed below 15 m/s and toward 50 μm.

**Figure 9.** Effect of the water temperature on the drops' sizes and velocities. Raw data in X = Y = 0, Z = 50 mm, Pinj = 7 bar,g. (**a**) Drops' velocity for Tw = 20 ◦C. (**b**) Drops' diameter for Tw = 20 ◦C. (**c**) Drops' velocity for Tw = 110 ◦C. (**d**) Drops' diameter for Tw = 110 ◦C.

In Figure 10, the results obtained with four different water temperature levels at the injector inlet are reported for the complete measuring travers at Z = 50 mm and crossing the entire spray structure. In these plots, both the mean and Sauter mean diameter values are included, along with the mean velocity and relative drop count. All these quantities were computed with data relevant to the entire 30-ms time-window to have a full portrait of the spray quality in the examined positions.

The effect of the water temperature on the drops' sizes, which was already commented on for the X = Y = 0 and Z = 50 mm position, was substantially confirmed for the entire measuring traverse; a significant but not dramatic effect from 20 ◦C to 110 ◦C was observed, with size reductions ranging between 8 and 15 μm in terms of the SMD and between 4 and 9 μm in terms of the MD. Minor consequences of raising the water temperature were observed in terms of the drop count and mean velocity, which were proven to change only for the central positions of the spray structure. As was observed when commenting on the spray images in the same operating conditions in Section 3.2, the water temperature's rise at the injector inlet from 20 ◦C to 110 ◦C, assumed to be compatible with standard PFI technology (e.g., injector body, rail and pressure sensor), was not adequate to trigger a net flash boiling mechanism for the injection process. Consequently, the injection process resulted in a spray with a basically unaffected shape but appreciably improved atomization quality. Nevertheless, the drops' size improvement was not as significant as was presumably attainable in the case of completely developed flash boiling conditions [25].

**Figure 10.** Effect of the water temperature on the drops' sizes and velocities over the measuring traverse at Z = 50 mm. Pinj = 7 bar,g. (**a**) Drops' mean diameter. (**b**) Drop count 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 Pinj = 9 bar,g drop size consistently reduced for the spray bulk (from 4 to 8 ms) for time bin mean values close to 50 μm 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 Pinj = 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 μm 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 Pinj = 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 11.** Effect of the injection pressure drops' sizes and velocities. Raw data in X = Y = 0, Z = 50 mm; Tw = 55 ◦C. (**a**) Drops' velocity for Pinj = 5 bar,g. (**b**) Drops' diameter for Pinj = 5 bar,g. (**c**) Drops' velocity for Pinj = 9 bar,g. (**d**) Drops' diameter for Pinj = 9 bar,g.

**Figure 12.** Effect of the injection pressure on drops' sizes and velocities over the measuring traverse at Z = 50 mm. Tw = 55 ◦C. (**a**) Drops' mean diameters. (**b**) Drops' counts and velocities.

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. 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 μm), while only marginal effects were obtained for the few very large drops produced during the initial part of the injection process. To be more detailed, it is interesting to point out how increasing the injection pressure (Figure 13b) from 5 to 11 bar,g also increased the PDF values in the range of 35–50 μm, confirming the modest atomization quality obtainable with low injection pressure values.

**Figure 13.** Effect of the injection pressure and water temperature on drops' diameter density probability functions. (**a**) Pinj = 7 bar,g. (**b**) Tw = 55 ◦C.
