Visualization and simulation study on impacts of wall roughness on spray characteristics of ducted fuel injection

The creative proposal of ducted fuel injection (DFI) in the application of heavy-duty diesel engines is to improve the quality of gas-fuel mixture in the combustion zone. The present research employed experimental and simulation methods to investigate the impact of the roughness of the inner wall of the duct on DFI spray characteristics. The study was carried out under injection pressure of 120 MPa and high ambient density of 54.2 kg/m³. From the perspective of spray tip penetration (STP), it was found that DFI spray configured with the smooth-walled and rough-walled duct both accelerate the axial diffusion with respect to free spray, and the smooth-walled duct performs best. Under the duct confinement, the gas entrainment does not occur inside the duct, slowing down the spray momentum decay, resulting in stronger axial diffusion after the spray penetrates through the duct. However, from the perspectives of spray cone angle (SCA) and spray area (SA), for the two duct configurations, the smooth-walled duct shows better overall diffusion compared to that of free spray, but the rough-walled duct shows weaker overall diffusion while increasing axial diffusion with respect to free spray, highlighting the strong spray-film interaction of the smooth wall promotes the overall diffusion, while the weak interaction of the rough wall accelerates the attenuation of the overall diffusion momentum with respect to free spray. Regardless of the smooth-walled or rough-walled duct, the pumping effect caused by the huge pressure difference between the inside and outside of the duct formed by the duct confinement causes strong gas entrainment at the duct inlet, promoting the strong gas-fuel mixing inside the duct, which is beneficial to the formation of leaner combustion.