Due to the lack of sufficient measurement methods, great difficulties are still present in clarifying the relationship between the nozzle internal flow and liquid-jet dynamics, especially when considering the needle motion effect. In this study, by utilizing the X-ray phase-contrast imaging (XPCI) technique, the in-nozzle needle motion characteristics and the liquid-jet dynamics in the near-nozzle field have been measured within a wide range of injection-pulse durations, based on which the needle-lift dependence of liquid-jet dynamics has been discussed in detail. The study is important for the nozzle related investigations, especially in view of the increasingly widely adopted multiple-injection strategies in the injection systems. The results indicate that, ruled by the mass flow equilibrium inside the nozzle sac, the liquid-jet flow dynamics in the near-nozzle field is closely related to the needle lift and injection pressure. Insufficient needle lift under short injection-pulse duration hurts the liquid-jet momentum that should be highly considered in the multiple-injection strategies. An analytical model has also been proposed to predict the jet axial velocity in the near-nozzle field. The model was found to have good agreement with the experimental results at different conditions.
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