Influence of needle-geometry design on in-nozzle flow characteristics for fuel flexibility on marine engines

The design and positioning of the needle are crucial factors that determine the morphology and dynamics of the flow within the nozzle flow of modern fuel injectors. Their optimization is crucial for enhancing the quality of the air-fuel mixture and ensuring compliance with stringent international emission regulations for marine powertrains. This study compares three needle-tip geometries with a reference design utilized in marine injectors. Numerical simulations were employed to analyze the flow characteristics induced by these needle geometries at two needle-lift positions. These correspond to two operational modes typically employed to ensure fuel flexibility: low-lift for dual-fuel mode and high-lift for Diesel-only mode. A multiphase model coupled with thermodynamic closure derived via the Perturbed Chain Statistical Associating Fluid Theory (PC-SAFT) Equation of State (EoS) was utilized. In order to minimize the computational cost associated with this EoS, the thermodynamic and transport properties of the fuel were computed beforehand and saved in a structured table. The results demonstrated strong predictive capability with computational costs typically accepted for industrial design time scales. In low-lift positions, the flow was attached to the inner surfaces, due to the high velocity imposed by the narrow flow area of the needle seat. The base needle design exacerbates cavitation, but two of the proposed designs significantly reduce the cavitation intensity. This led to a significant decrease in the amount of fuel vapor forming inside the injector compared to the base design. Furthermore, one design relocates most of the cavitation area beyond the spray hole. At higher-lift positions, the designs can be categorized into two groups regarding the overall flow structure. The first one exhibits flow patterns similar to the base design and the second one exhibits reduced nozzle flow coefficients.