Experimental study on macro spray characteristics of ducted fuel injection under vaporizing conditions

This study investigates the impact of ducted fuel injection (DFI) on spray characteristics under elevated temperature and pressure conditions, using a constant-volume chamber (CVC) and a schlieren optical system. The optimized image processing method clearly captures the spray morphology, overcoming influences in high-temperature environments, such as background noise caused by thermal effects. The CVC and schlieren optical system provide enhanced image clarity, enabling precise analysis of spray characteristics. Spray images of DFI were captured and analyzed to assess key parameters, including spray morphology, spray cone angle (SCA), spray tip penetration (STP), and spray volume. The findings reveal notable variations in spray morphology depending on the duct type under high-temperature conditions. Specifically, the ST2 duct yields a “mushroom-like” spray morphology with a pronounced “large head and small neck” structure due to shear interactions with the duct walls. Conversely, the CD4.5 duct promotes enhanced radial diffusion and a thicker “neck” region, attributable to vortex motion and hot air entrainment within the divergent section of the CD duct. The study further examines the impact of DFI technology on the spray cone angle. The application of ST2 duct improves the SCA by 18.40 %–28.59 % relative to the FR spray, while the CD4.5 duct achieves even greater enhancements, with SCA increases ranging from 52.81 % to 79.59 %. Spray tip penetration is influenced significantly by both duct type and injection pressure. For same pressures, the ST2 duct increases STP by 2.80 %–4.73 %, whereas the CD4.5 duct reduces STP by 3.66 %–8.62 %. Improve injection pressures generally increase STP across all duct types, with the FR spray showing increases of 10.34 % at 120 MPa and 16.05 % at 160 MPa. The ST2 duct spray exhibits comparable increases, while the CD4.5 duct spray demonstrates more modest gains. In terms of spray volume, the ST2 duct spray consistently produces a larger volume compared to the FR spray, with increases ranging from 5.98 % to 14.48 %. The CD4.5 duct spray shows variable spray volume characteristics: compared with other spray, it generates the smallest volume at 80 MPa injection pressure, a moderate volume at 120 MPa injection pressure, and the largest volume at 160 MPa injection pressure, exceeding both the FR and ST2 duct sprays.