Influence of Temperature and Pressure on Droplet Sizes and Droplet Size Distribution

As described in literature, it was possible to show that the sudden phase transition from liquid to gaseous state takes place in relation to temperature and in a very small temperature window. It can also be proven that it is possible to inject fully vaporized fuel with a conventional injector — apparently without any major problems. However, the question arises as to what effect an increase in fuel temperature has on the injection cycle overall, particularly if no supercritical conditions are reached, i. e. only raising temperature at usual injection pressures. The assessment criteria identified as part of this study are droplet size as well as droplet size distribution. So, to evaluate the quantitative influence of raising temperature, droplet sizes and their distributions were determined by means of laser diffraction spectrometry (LDS) (also see Section 3.4). As droplet sizes were only ascertained for steady-state fuel spray, droplets produced during the opening and closing cycle are not included in analysis. The red circle in Figure 19 shows the part of the spray that was evaluated using the LDS system.

All of the measurements taken with LDS were conducted at different injection pressures, these being kept constant while varying fuel temperature from 25°C up to 400°C.

Influence of Temperature and Pressure on Droplet Sizes and Droplet Size Distribution

Figure 19: Limitation of measurement range; spray diameter 5 mm, exposure time 1.765 ms to 4.765 ms after SOI

Injection pressure was varied between 180 bar and 350 bar whereas ambient conditions were kept constant, TCH = 25°C, pCH = 1 bar.

As to be expected, a clear correlation is shown to exist between Sauter mean diameter and prevailing injection pressure. In the pressure range between 180 bar and 350 bar that was examined, SMD behavior is virtually linear, Figure 20.

□ 180bar

—— f 11,3-

подпись: □ 180bar
 f 11,3-

□ 220bar

подпись: □ 220barI260bar D350bar

10,0

~9,2~

8,3

0-I————————————— ^———————————- 1

SMD

Figure 20: Sauter mean diam. as fuel pressure rises; Tfuei = 25°C; RON 95

The higher injection pressure is known to alter spray breakup. As already described in Section 2.3, spray breakup is largely determined by the fluid discharge velocity which rises at high pressure, with chamber conditions remaining constant.

Evaluation of droplet size distribution corroborates the statement that a positive influence is brought about by increasing pressure, as can be seen from Figure 21.

Influence of Temperature and Pressure on Droplet Sizes and Droplet Size Distribution

Diameter [^m]

Figure 21: Droplet size distribution in relation to rising fuel pressure; Tfuel = 25°C; RON 95

It is also shown to have a clearly positive influence on droplet size distribution. Compared with 180 bar, for instance, a droplet size distribution is obtained at 350 bar that is far more homogeneous and contains few droplets larger than 15 |jm.

If fuel temperature is now varied at constant injection pressure, the Sauter mean diameter is shown to behave as follows for an injection pressure of 180 bar in relation to temperature, Figure 22.

B,4

C* —

Oi

E

Figure 22: Sauter mean diameter in relation to rising fuel temperature; Pfuei = 180 bar = const.; RON 95

Although the Sauter mean diameter shows a very similar overall tendency in relation to increasing fuel temperature at constant pressure as it does to increasing injection pressure, far smaller droplet diameters can be achieved. The same positive behavior is shown on evaluating droplet size distribution, Figure 23.

Influence of Temperature and Pressure on Droplet Sizes and Droplet Size Distribution

Diameter [^m]

Figure 23: Droplet size distribution in relation to rising fuel temperature; Pfuel = 180 bar = constant

Whereas the droplet distribution obtained at fuel temperatures of 25°C and 100°C is similar to that on increasing pressure, a pronounced concentration of very small droplets with an average size of approx. 6 ^m in the range from 3 to 9 ^m is revealed at a fuel temperature of 200°C. This highly astonishing result is corroborated by the studies carried out at fuel temperatures of 300°C and 400°C insofar as it was no longer possible to detect any droplets in the measurement range at these temperatures, Figure 24.

Influence of Temperature and Pressure on Droplet Sizes and Droplet Size Distribution

Figure 24: Measurement area on arrival of the fuel spray — at the fuel temperatures examined

As the LDS measurement area, as shown in Figure 19, is approx. 50 mm below the injector tip, it is not possible to say with absolute certainty that only vapor is injected at 300°C and 400°C because it could also be the case that very small droplets are injected, these being far smaller than 6 ^m and completely vaporizing on their way to the measurement plane. At 400°C, however, supercritical conditions definitely prevail that result in the injection of a supercritical fluid. This presumably prevents the occurrence of any liquid phase as transition goes directly into gaseous phase, as seen in the phase diagram, Figure 17.

To illustrate the huge potential of increasing temperature with regard to spray quality, studies were also performed with the schlieren method. The focus here was on observing the change in droplet size on injecting fuel at low pressure (pfuel = 8 bar) while increasing fuel temperature from 25°C to 300°C, Figure 25.

Influence of Temperature and Pressure on Droplet Sizes and Droplet Size Distribution

Figure 25: Spray patterns after the end of injection at different fuel temperatures and pfuel = 8 bar, Tch = 25°C, pch = 1 barA; RON 95

At an injection pressure of 8 bar, a standard GDI injector shows a sharp fall in visible droplet structures as temperature rises until, at approx. 300°C, droplets can no longer be visualized. These results open up many new avenues.

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