Influence of sac structure on cavitation and gas ingestion at the end of injection

Abstract

A comparative study using different sac structures was conducted focusing on the cavitation morphology inside a diesel nozzle. Cavitation phenomena play important roles in the flow field during fuel injection at the end of injection, which causes high-temperature gas ingestion. This can lead to coke formation inside the orifice (a hole of the nozzle where cavitation mainly occurs), further influencing the injection atomization in the cylinder. Using visual experiments and a finite volume method two-phase, three-component multiphase flow solver, the flow process in the nozzle at the end of injection was tested and simulated, and the influence of sacs with three different wall curvatures on the cavitation and gas ingestion was analyzed. The results demonstrated that cavitation occurred successively in the orifice and sac at the end of injection. The cavitation amount in the sac was larger, and two cavitation forms were produced: vortex and bulk cavitation. For narrow and straight sacs, bulk cavitation played an important role, and the amount of cavitation increased. The prolongation of the lower pressure duration inside of the nozzle decreased the fuel outflow, eventually leading to a decrease in gas ingestion. When vortex cavitation occurred in the sac, the second matrix invariant Q of the velocity gradient tensor ∇V was positively correlated with the volume of the vortex cavitation. The smaller the vortex core zone was, the lower the friction dissipation in the flow field was; under these conditions, cavitation occurred more easily in the sac.