Experimental Study Of Flash Boiling Spray Characteristics Of A GDI Based On IPI

In flash boiling spray, the superheat degree of fuel is increased, and the phase of fuel is changed during the injection process. Under a certain fuel superheat degree, the tip-penetration of flash boiling spray becomes shorter with increased spray cone angle and smaller droplet diameter. Consequently, the droplet breakup is enhanced and the mixture preparation is optimized. In recent years, flash boiling spray has attracts increasing interests around the world, but the spray characteristics especially the microscopic behaviors of flash boiling spray are still not entirely understood. In order to study the microscopic characteristics of flash boiling spray, the Laser Interferometer Particle Imaging(IPI) was used to measure the droplet size distribution and droplet velocity of gasoline and n-hexane using a six-hole injector in a constant volume chamber. The droplet size distribution of n-pentane, n-hexane and n-heptane were also compared in a single hole injector.First, the droplet size distribution under different fuel temperatures and ambient pressures was studied in a six-hole injector. The results show that the droplet size of flash boiling spray is more uniform than the cold spray, and the droplet size distributes within a smaller range. The Sauter Mean Diameter(SMD) of the droplets are reduced when the fuel temperature is increased and the ambient pressure is decreased. By comparing the droplet size distribution of gasoline and n-hexane, the results show that both the range of droplet size distribution and SMD of n-hexane are smaller than those of gasoline under each experimental condition.Then, through the study of the droplet velocity in the six-hole injector, it was found that superheat degree of flash boiling spray is an important factor that affects droplet velocity distribution. The velocity distribution is essentially the same at the same superheat degree. For cold spray, the droplet speed at spray tip ranges from 30 m/s to 60 m/s at the injection pressure of 10 MPa. When the fuel is heated to the transition flash boiling stage, the droplet speed at spray tip ranges from 20 m/s to 30 m/s due to the spray entrainment. Further heating the spray to the flare flash boiling stage, the spray collapses, and the number of droplet with the velocity over 90 m/s at spray tip increases significantly.Moreover, the droplet size distribution of n-hexane under different injection pressure was studied in a single hole injector. The results show that increasing injection pressure can reduce the droplet SMD at low fuel temperatures. At the fuel temperature of 20?,when the injection pressure is increased from 4 MPa to 10 MPa, the SMD is decreased from 21?m to 17?m. However, at the fuel temperature of 80?, the SMD is as small as 13?m at the injection pressure of 4 MPa, and further increasing of injection pressure cannot reduce the SMD anymore. Increasing fuel temperature can reduce the dependence of the engine on high-pressure fuel injection systems.Finally, the droplet size distribution of n-pentane, n-hexane and n-heptane under the same superheat degree was studied in a single hole injector. The result shows that superheat degree is an important factor affecting droplet size distribution. In the flash boiling spray, the droplet size distribution and SMD of n-pentane, n-hexane and n-heptane are nearly identical under the same superheat degree.