Investigation into shock-to-shock interactions induced by flash boiling and the impact on spray behaviors

Abstract

Through recent numerical studies, the shock-to-shock interactions between flash-boiling jets are believed to be the reason for multi-jet spray collapse in gasoline direct injection (GDI) engines. But this sort of interaction was not well understood through the experimental investigation. In the present study, a two-hole GDI injector was used to examine this process via optical diagnostics and CFD simulations. With the increase in superheat level (Rp), the shock waves (primary cells) in individual jets became observable. Further increase in Rp could cause shock-to-shock interactions, and a secondary cell could be formulated between two primary cells. The different intensity of shock-to-shock interactions would result in different jet-to-jet interaction behaviors. In the moderate interaction region, the secondary jet appeared inside and downstream of the secondary cell, but the sprays did not collapse. In the highly interaction region, both the secondary jet and spray collapse could be observed, where the secondary jet would laterally develop perpendicular with the plane of the two jets. The detailed numerical results showed that the secondary cell was small in the moderate interaction region, but became much larger in the highly interaction region. The occurrence of secondary cell directed the fuel from primary cells into the secondary cell, contributing to the generation of secondary jet. After flowing out of the secondary cell, the incipient secondary jet could subsequently entrain the nearby low-velocity fuel. Further, the strong shock-to-shock interactions would alter the shape of primary cells, reducing the transverse velocity component of jets and promoting collapse. Based on the experimental and numerical results, the existence of shock-to-shock interactions and its important role in jet-to-jet interactions and spray collapse were fully illustrated, and this understanding would be of help in nozzle optimization.