Numerical Study On Flash Boiling Spray Characteristics Of A Direct Injection Six-hole Injector

Owing to the advantages of good power performance, fast transient response and low hydrocarbon emissions during cold start, gasoline direct injection engine has become a hot spot of the internal combustion engine research and application around the world. Flash boiling spray can greatly improve spray performance, such as spray cone angle, penetration and droplet Sauter mean diameter(SMD), and consequently improve the quality of spray atomization, therefore flash boiling spray attracts great attentions of researchers of internal combustion engine.A complete flash boiling spray model is first built and realized in the computational fluid dynamics program KIVA-3V. Then the bubble dynamics behaviors in superheated liquid is studied. O n the basis, the flash boiling spray of gasoline and single component alkane from a direct injection six- hole injector is investigated by numerical simulation. The main achievements and conclusions are as follows:Bubble nucleation modes are studied and the results show that the homogeneous nucleation requires high limit of superheat degree, and is not applicable in the ambient pressure range(0~1MPa) tested in this study. The superheat degree limit of heterogeneous nucleation is low in a wide pressure range(0~2MPa), so heterogeneous nucleation mode is only considered in the nucleation model.By studying the bubble growth model, it found that the numerical model can accurately predict the bubble growth process in a wide superheat degree range, and the prediction is more accurate than the analytical solution. The superheated liquid bubble growth properties is studied numerically. Computational results show that the different bubble growth characteristics are caused by the competition of surface tension, viscous force, fluid dynamic resistance, which inhibit the growth of bubble and pressure difference, as well as thermal feedback effects, which p romote the growth of bubble. Jacob number(Ja) has great influence on bubble growth characteristics in different stages. In the transition stage, the bubble growth gradually transforms from the heat transfer dominated regime to the inertial force dominated regime with increased Jacob number. At constant pressure, the bubble growth delay decreases and the maximum acceleration increases with increase of Jacob number in surface tension dominated regime, and bubble growth velocity increases with increased Jacob number in heat transfer dominated regime.The linear stability analysis(LSA) breakup model is studied, and results show that, under relatively low growth speed,bubble breakage mainly depends on the void fraction. Whereas, when the growth speed exceeds a certain level, the void fraction at breakup decreases with the increase of bubble growth speed and initial radius ratio. The broken time decreases with increasing bubble growth speed and decreasing initial radius ratio.Finally, the flash boiling spray of gasoline and alkane in a direct injection six-hole injector is investigated numerically. The results show that the fuel vapour concentration increases with higher intensity of flash boiling spray, and results in the drop of gas phase temperature in the same area. However, the inverse trend of temperature distribution characteristics of gas phase is observed under cold state. The formation of low-pressure area during flash boiling spray process has direct influence on the spray shape. The overlap between oil spray plumes under transition flash boiling condition results from the airflow movement formed by the low pressure area between the adjacent spray plumes. It is also found that the spray "collapse" phenomenon under flare flash boiling condition primarily results from the airflow movement formed by the low pressure area inside the spray. The flash boiling spray of n-pentane, n- hexane and n-heptane shows very similar macroscopic and microscopic characteristics under the same ambient pressure and superheat degree, which indicates that flash boiling spray is controlled by superheat degree at a constant ambient pressure, and is not significantly affected by fuel species and temperature.