Numerical Study on Flash Boiling Spray of Multi-Hole Injector 2017-01-0841
Flash boiling spray is effective in improving the atomization and evaporation characteristics for gasoline direct injection engines. However, for a multi-hole injector the morphology structure of spray has an obvious change with the fuel temperature increasing or the ambient pressure decreasing, which influences the process of mixture formation and flame propagation. Specially, the spray collapses with both long penetration and a narrow spray angle above certain high superheat degree, which deteriorates air/fuel mixing and hence increases emissions. It is not desired for engine applications while the mechanism of spray structure transformation for multi-hole injector still remains unclear. In the present study, a systematic flash boiling spray model for multi-hole injector is built to investigate the flash boiling spray of multi-hole injector. Key physics involved in flash boiling, including bubble formation, bubble growth, as well as bubble breakup are added to the traditional spray model in KIVA-3V to describe the development process of flash boiling spray. The simulation results of spray structure and spray penetration for multi-hole injector agree well with experimental observation under various conditions. The numerical results of the pressure field, velocity field and vapor distribution are analyzed under different superheat degree. It is found that the low pressure region has a significant effect on the formation of eddies in velocity fields and the greater the pressure drop is, the more intense the eddy is. The interactions of low pressure regions and eddies change the trajectories of droplets and then change the spray structure. When flare flash boiling occurs, although the evaporation of fuel increases greatly, the fuel vapor is relatively concentrated in the spray center due to spray collapse, which deteriorates air/fuel mixing.