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The fuel flow in the diesel engine nozzle has a vital impact on the fuel atomization and spray, and the fuel mass flux affects the internal flow of the nozzle. The visual experimental platform for a transparent nozzle was built to obtain the image of fuel flow in a nozzle with a small sac combining the back-light imaging technology and a high-speed framing camera. A two-phase three-component numerical model, based on the OpenFOAM solver, was calculated to quantitatively analyze gas ingestion and cavitation in the nozzle. The results indicate that at the end of injection (EOI), fuel cavitation and external air backflow (gas ingestion) occur successively in the nozzle, and both phenomena first appear in the orifice and then transition to the sac. Cavitation collapse is the major factor of gas ingestion, and the total amount of gas ingestion and cavitation mainly depends on the sac. The outflow of fuel largely depends on the total amount of cavitation and the inertial outflow of fuel at the EOI. The type of cavitation in the nozzle mainly presents annular and bulk cavitation, the former primarily exists in the sac, while the latter is established within the orifice. Therefore, larger mass flows will contribute to stronger cavitation and gas ingestion.

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.

Lean premixed prevaporized combustors with a centrally-staged scheme are capable to reduce NOx emissions. Ignition is one of the key performances of the centrally-staged combustor. The present study proposes a novel method to improve ignition performance by using a flow deflector. The effects of various flow deflector lengths and pressure drops on ignition performance and flame kernel propagation are investigated in this work. It is found that ignition performance is significantly improved by the flow deflector. The ignition process is obtained using a high-speed camera under different operating conditions. The time scale of the successful ignition process is analyzed using a statistical method, revealing the effects of the flow deflector length and pressure drop on the time scale of each phase of ignition. The flame kernel propagation trajectory is extracted and analyzed by combining the flow and spray fields. The mechanism of the flow deflector is analyzed by numerical simulation. It is found that with the flow deflector, the local fuel/air ratio and droplet diameter are both improved, which benefits ignition performance. This work proves that the flow deflector is a potential method to improve ignition.