Characterizing under-expansion behaviors induced by rapid phase change of flash-boiling jets

Abstract

Flash-boiling sprays issued from multi-hole gasoline direct injection (GDI) injectors could collapse, which is recently proved to be caused by under-expansion, but very few experimental data are available on this sort of under-expansion. In this study, high-speed microscopic Schlieren photography was used to capture the near-field shock structures in flash-boiling n-hexane jets. The tests were carried out in a constant volume vessel with ambient pressures (Pamb) from 10 to 101 kPa, fuel temperatures (Tfuel) from 30 to 170 ℃, and injection pressures (Pinj) from 2 to 20 MPa. The shock structure was carefully analyzed in terms of shock width and length. Generally, both the shock width and length decreased with the increase in Pamb. With the increase in Tfuel, the shock length was relatively stable and the shock width overall increased, but a decreasing trend in shock width could be observed at 170 ℃, possibly caused by the near-critical behaviors. Further analysis showed that the shock width shows a strong linear correlation with Δμ·Pamb−0.5 (Δμ denoting chemical potential of phase change) for a given Pinj, and the shock length shows a strong quadratic polynomial relationship with ηinj (injection pressure ratio, defined as the ratio of Pinj to Pamb). Additionally, some new shock structures without Mach disk were observed, indicating that the phase change induced under expansion is different from the typical gaseous under-expansion.