General understanding on spray collapse process of an asymmetrical multi-hole direct injection gasoline injector under wide flash-boiling conditions

How the under-expansion impacts the flash-boiling spray collapse process over wide superheat levels (Rp, defined as the ratio of saturation pressure to ambient pressure) is not well understood. In the present study, n-hexane flash-boiling sprays issued from a five-hole asymmetrical injector were experimentally and numerically studied to obtain a more general understanding of the spray collapse with wide Rp variation. The experimental results proved that the collapse in the transitional region occurs in the far field, unlike the fully collapse that occurred in the near-nozzle region. The numerical results demonstrated the complexity of individual jet evolutions and their interactions over wide Rp. For individual flash-boiling jets, there were different behaviors in the near-nozzle region. In the case with Rp slightly larger than 1, no shock waves can be observed, but a set of compression-expansion chains. The further increase in Rp caused the generation of shock waves, and resultantly the primary cells were established. For the multi-jet sprays, the further increase in Rp enlarged the primary cells, leading to their interactions and the generation of secondary cells. When Rp was sufficiently higher, the further interactions among primary and secondary cells could cause the generation of tertiary cell. Orderly interactions of shock cells were observed with increasing Rp, that is, the interactions initially occur between the adjacent jets with smaller distances, and then other jets were involved. Based on the results: It was found that the compression-expansion chains caused the low-Rp flash-boiling spray collapse in the far field; With the increase in Rp, the shock waves and shock-to-shock interaction become the main contributor to spray collapse, leading collapse appearing in the near-nozzle region.