Investigation On Jet-to-jet Interaction Of Multi-hole Spray In GDI Engine

Flash boiling is viewed as a promising way to promote atomization.However,its occurrence in Gasoline Direct Injection（GDI）engines,where multi-hole injectors are mainly used,could lead to the spray collapse,which significantly changes the fuel distribution and brings negative effects.Therefore,this dissertation studies the collapse process of multi-hole flash-boiling（FB）spray and the jet-to-jet interaction mechanism,to deepen the understanding of spray collapse.With regard to the collapse process,the spray behaviors have been observed in two view directions using a five-hole GDI injector with a larger envelop angle under FB conditions.From the front view,the spray collapse gradually occurs as superheat degree（Rp）increases.However,it is also found that the spray under higher ambient pressure（Pamb）shows stronger contraction for the cases with similar Rp,which can be attributed to the stronger diffusion resistance.From the bottom view,the adjacent jets with the closest hole-to-hole distance firstly merged with the increase in R_p,then more jets with relatively larger distance started to merge and finally the sprays collapsed as R_p further increased,i.e.,the collapse is orderly.Further analysis into the temporal spray development showed that the adjacent jets has merged at the beginning of injection,and the merged jets could attract other jets to some extent with the spray developing.This attraction occurs in the far field and influences the spray morphology,but has limited role in the spray collapse.The above results show that the nozzle-exit interaction between adjacent jets at the beginning is the main reason for spray collapse.Further,the orderly collapse process proves the feasibility of the nozzle arrangement to control the collapse.With regard to the interaction mechanism between adjacent jets,the spray characteristics under FB and non-flash-boiling（NFB）conditions have been observed using a two-hole GDI injector.The sprays collapse under both conditions,but their characteristics changing with P_amb and fuel temperature(T_fuel)show the opposite trends.Unlike the FB collapse enhances as T_fuel increases or P_amb decreases,the NFB collapse weakens as P_ambdecreases,and gradually disappears once T_fuel is beyond 70°C.Further,the NFB collapse gradually occurs with the time elapsed,which needs more time than the FB collapse completed at the beginning.The NFB collapse is attributed to the low-pressure zone caused by the jet entrainment.The gradual NFB collapse with spray evolving and the weakening NFB collapse with the increase in T_fuel process respectively demonstrates a certain time is needed to form the low-pressure zone caused by jet entrainment and the evaporation could counteract this low-pressure zone.These further indicate that this low-pressure zone caused by the jet entrainment cannot be used to explain the FB collapse because there is no time to form it and it can be counteracted by the stronger flashing evaporation.Additionally,the schlieren images show that there is an under-expansion shock wave at the nozzle exit under high-superheat FB spray.Combined with previous studies,it is believed that the occurrence of under-expansion and its shock wave interaction are the more essential reasons for the collapse of high-superheat FB spray.