LAUSR

Abstract The mixing effect of the gas–air mixture and the concentration distribution of supersonic gas jets for natural gas engines are affected by the microscopic characteristics of natural gas jets. In this study, the schlieren method was used to record the injection process of natural gas from an outward-opening injector. Results showed that the average equivalent ratio φ decreased as the pressure ratio (PR) increased, and the effect of PR on φ was smaller when PR was large. Further, φ was large at the beginning of the injection, then decreased rapidly and finally stabilized in the range of 1.7–3. The effect of PR on gas concentration was insignificant. The concentration distribution of natural gas jet was uneven along the axial direction. The mole fraction decreased with an increase in the axial penetration Zt and entered a stable range of 0.1–0.2 when Zt larger than 20 mm. The steady penetration constant Γ for an outward-opening injector was 1 ± 0.06, which was smaller than that of a single-hole injector. Γ increased rapidly with time and stabilized after the injector valve was fully opened. The steady Γ was slightly affected by the PR, but the moment Γ reached stability advanced with the increase in PR. The PR imposed a weak effect on the self-similarity index S of the outward-opening injector. The steady S of the outward-opening injector was 0.7, which was larger than that of a single-hole injector (S = 0.25 ± 0.05).