On the role of cavitation in marine large diesel injector. numerical investigation of nozzle orifices eccentricity

Abstract

The injector geometry of large marine two-stroke diesel engines differs substantially from the configurations used in most other diesel engine applications, because the injector orifices are distributed in a highly non-symmetric manner. In order to investigate the impact of key features of such asymmetrical orifice arrangements on the liquid jet and mixture preparation, a simplified generic single-hole nozzle has been designed. In this work, the in-nozzle flow has been investigated numerically in the aim of identifying the role of cavitation in large marine Diesel injectors. Two configurations of such nozzles with different eccentricity of the hole have been computed. High Fidelity Simulations (HFS) of the entire process including cavitation and liquid jet atomization have been carried out using a newly developed two-fluid version of the software IFP-C3D [1, 2] which includes the advanced GERM (Gibbs Equilibrium Relaxation Model) cavitation model [3] and the Two-Surface Density (TwoSD) atomization model [4]. First, the results have shown that cavitation appears classically close to the orifice inlet edge, but it has a smaller size for the eccentric nozzle. For both configurations, the cavitation induces a flow deviation inside the holes which affects significantly the liquid jet atomization. The liquid flow has been deviated from the orifice axis by the cavitation pocket, leading to an off-axis spray in the combustion chamber. This asymmetry is at the origin of the spray deviation observed in the "Spray Combustion Chamber" (SCC) [5] experiments for an eccentric nozzle. Finally, the numerical results have shown a strong correlation between the in-nozzle cavitating flow and the spray direction and atomization that makes very difficult the simulations using a weak coupling methodology which consist of a preliminary in-nozzle flow simulation and then a second simulation for the spray injection, the mixture preparation and the combustion.