# Effect of Fuel Mass Flow at the End of Injection on Cavitation and Gas Ingestion in the Nozzle
## 4 Results and Discussion  
### End of Injection (EOI)  
Figure 5 represents the diesel volume fraction (the volume of diesel as a percentage of the fluid volume in the nozzle) in the nozzle at different measurement times at the EOI, and the value is shown in the color bar below. From this figure, it can be clearly observed that the experimental images are very similar to the proposed simulation. Before the needle valve was seated (i.e., 0.2 ms), corresponding to the cavitation occurs only in the orifice. The cavitation occurred in the sac later than in the orifice, and the super-cavitation appeared in the orifice from 0.25 ms.  
At the beginning of the EOI, cavitation occurred at the orifice inlet, as shown by 0.02 ms. When the needle valve was seated, cavitation gradually developed along the orifice wall toward the outlet. This is because when the fuel entered the orifice from the sac, the flow direction and flow area of the fuel changed. As the cross-sectional area for the fuel flow was reduced, it caused the increase of flow velocity, which is called the throttling effect. When cavitation began to appear in the nozzle, it also meant that the pressure was already less than 2 kPa, which is the saturated vapor pressure of diesel. Analyzing the experiment and simulation results, from 0.15 ms to 0.25 ms, cavitation mainly extends along the orifice wall to the exit. This is because that the low-pressure zone was easily formed between the fluid boundary layer and wall boundary layer. However, the wall-attached flow was not obvious in the experimental nozzle until the middle of the needle valve was seated. Cavitation also appeared in the sac after 0.25 ms. This phenomenon meant that as the needle valve was seated, the cross-sectional area of the effective fuel flow near the sealing surface decreased, resulting in a decrease in pressure and an increase in velocity. Considering the viscous effect for the diesel, the speed of the real needle valve was different from that of the fuel. The result of this phenomenon was the same as the throttling effect,  
Figure 5: Volume fraction of diesel in the nozzle at the end of injection (EOI) (volume fraction of diesel means the volume of diesel as a percentage of the fluid volume in the nozzle).  
which led to cavitation. Assuming that the fluid in the nozzle was a continuous model, the central pressure of the vortex core gradually spread to the orifice, which led to a further decrease in orifice pressure and also meant an increase in cavitation.  
With the development of cavitation, cavitation bubbles eventually penetrate the entire orifice, as is shown at 0.3 ms. This phenomenon is called super-cavitation. However, the cavitation in the sac was still not at its maximum. The stretching velocity of fuel in the sac was smaller than that in the orifice, while the stretching degree was larger, which caused more cavitation in the sac, and this meant the total amount of cavitation in the nozzle was mainly up to the sac. In addition, due to the influence of the aerodynamic force and the pressure wave in the nozzle, the cavitation at the nozzle outlet was extremely unstable, which made the cavitation bubble burst and caused gas ingestion. This phenomenon has also been reported by [PERSON] [21].