Effect of Cavitation on the Nozzle Outlet Flow, Spray and Flame Formation in a Diesel Engine 2006-01-1391
Diesel injector nozzles with different geometries have been investigated, firstly to study the effect of cavitation on the flow at the exit of the nozzle, and secondly to see its effect on the spray and the consequent flame formation.
The flow at the exit of the nozzle was characterized by measuring both the injection rate and the momentum flux for a wide range of typical engine's rail and cylinder pressures. The injection rate is directly affected by cavitation and it will become choked when this phenomenon appears. The measurement of the momentum flux can be combined with the injection rate to derive the velocity at the nozzle exit. The momentum flux does not seem to be influenced by the appearance of cavitation. Apart from this hydraulic characterization, the different nozzles have also been mounted on an engine with optical access to the combustion chamber. During these tests, the spray, its vaporization and combustion were visualized and recorded with a high resolution camera. Also tests have been carried out in a single cylinder engine to study the differences in combustion law between the different nozzle geometries.
From the measurements it was possible to observe an important increase in the exit velocity, and a decrease of the flow density at the nozzle outlet, when cavitation occurred. The visualization on the optical engine allowed characterizing the effect of these exit velocity and density changes on the spray's liquid length and flame geometry. It was observed that cylindrical nozzles, as opposed to non-cavitating conical ones, show a significant decrease in liquid length when cavitation becomes stronger, due to the fact that the exit velocity increases and the aperture angle of the spray becomes larger. Studying the combustion law, showed that the cylindrical nozzle at non-cavitating conditions (low injection pressure) has a slower apparent combustion time, which is in agreement with the observations made from the visualization experiments.