Experimental And Numerical Study On Multi-Jet Spray Collapse Under Flashing Condition In GDI Engines

The increasingly stringent emission legislation casts the demand for higher fuel atomization quality in gasoline direct injection(GDI)engines.As a promising method to improve atomization quality,flash boiling injection has received interests from both academia and industry.However,for multi-hole injectors,the jets discharged from different holes would deflect towards each other and even merge into one solid jet under flashing conditions,which is named ‘spray collapse'.The occurrence of spray collapse could significantly change the fuel distribution and bring negative effects such as fuelwall impingement.This dissertation studies the spray collapse under flashing condition in three aspects,i.e.the influencing factors of spray collapse,the mechanism of spray collapse,and the mitigation of spray collapse.With respect to the influencing factors,the spray collapse under flashing and nonflashing conditions have been compared,and a comprehensive map has been proposed to describe the intensity of spray collapse in terms of thermodynamic conditions.The results show that under non-flashing condition,the intensity of spray collapse increases with ambient pressure,but is insensitive to fuel temperature.In contrast,under flashing conditions,three regions can be identified according to the ambient pressure.In the collapse region,the spray transits from non-collapse to fully-collapse with the rise of fuel temperature.In the non-collapse region,the spray transits from collapse to noncollapse as fuel temperature increases.In the transitional region,the spray transits from collapse to non-collapse as ambient pressure increases.With respect to the collapse mechanism,numerical simulations have been carried out to study the flashing spray behaviors of single-hole and two-hole GDI injectors using commercial CFD software CONVERGE v2.3.The results show that the flashing jets become under-expanded near the nozzle exit due to drastic vaporization,and then violent expansion occurs downstream.During the expansion,the velocity increases and the pressure reduces.Due to the inertia effect,a low-pressure region in contrast to the ambience is formed.For two interacting flashing jets,the center area is affected by the low-pressure regions inside the two jets,and the local pressure drops.Consequently,a pressure differential between the inner and outer sides of the spray is formed,which is responsible for the occurrence of spray collapse.With respect to the mitigation approach,two novel ideas have been proposed to mitigate spray collapse.Firstly,the results show that the width of internal flashing jets is determined by the chemical potential of phase change and the ambient pressure.Based on the prediction of jet width,the jet-to-jet interaction could be mitigated by changing the nozzle configuration.Secondly,it is proposed that spray collapse can be mitigated by generating external flashing jets,which is realized for a single-hole GDI injector.In the external flashing regime,the jet expansion does not occur at the nozzle exit.Hence,the low-pressure region caused by the expansion is not formed.Besides,the jet width reduces significantly near the nozzle exit,which leads to weaker jet-to-jet interaction.