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

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 (P-amb) from 10 to 101 kPa, fuel temperatures (T-fuel) from 30 to 170 degrees C, and injection pressures (P-inj) 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 P-amb. With the increase in T-fuel, the shock length was relatively stable and the shock width overall increased, but a decreasing trend in shock width could be observed at 170 degrees C, possibly caused by the near-critical behaviors. Further analysis showed that the shock width shows a strong linear correlation with d mu.P-amb-0.5 (d mu denoting chemical potential of phase change) for a given P-inj, and the shock length shows a strong quadratic polynomial relationship with eta(iinj) (injection pressure ratio, defined as the ratio of P(inj )to P-amb). 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.