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 (R_p, 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 R_p 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 R_p. For individual flash-boiling jets, there were different behaviors in the near-nozzle region. In the case with R_p slightly larger than 1, no shock waves can be observed, but a set of compression-expansion chains. The further increase in R_p caused the generation of shock waves, and resultantly the primary cells were established. For the multi-jet sprays, the further increase in R_p enlarged the primary cells, leading to their interactions and the generation of secondary cells. When R_p 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 R_p, 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-R_p flash-boiling spray collapse in the far field; With the increase in R_p, the shock waves and shock-to-shock interaction become the main contributor to spray collapse, leading collapse appearing in the near-nozzle region.