Effect of split injection on the macroscopic development and atomization characteristics of a diesel spray injected through a common-rail system

Abstract

This article describes the effects of split injection on the macroscopic behavior and atomization characteristics of a high-speed diesel spray injected through a common-rail system. In order to reveal the effects of the first injection on the second injection under various injection durations, experiments on the injection rate, spray behavior, and drop Sauter mean diameter (SMD) were conducted by using an injection rate meter, a spray visualization system, and a phase Doppler particle analyzer system. The test injector had a single bole with 0.3 mm bole diameter and 0.8 mm nozzle length. In split injections, the sum of the first and second energizing durations was set constant as 1.2 ms, and various split injections were studied. The effect of ambient pressure on the spay development was also investigated by using a high-pressure spray chamber. For the numerical investigations, the KIVA-3 code including a modified nozzle flow model, and a breakup model was applied to the simulation of spray atomization using the same calculation conditions as the experiments. The injection velocity was calculated based on the flow model that considers cavitation in the nozzle and the realistic nozzle geometry. The breakup model, which has a dominant effect in calculating the mean droplet size of the spray, was also mode zed to include the Kelvin-Helmholtz instability/droplet deformation and breakup (KH-DDB) model. In the KH-DDB model, both wave,instability and droplet deformation are considered as the breakup factors upstream of the jet breakup length. On the other hand, beyond the breakup length, only droplet deformation is regarded because the relative velocity between the droplet and the ambient gas is lower in comparison to the velocity above the breakup length. The results show that the effect of interference between injections on the injection pressure is little in comparison to the effect of gas flow. The entrainment of ambient gas by the first injection causes a longer spray tip Penetration and larger drop SMD for the second injection in comparison to the case of a single injection.