Macroscopic spray characteristics of methanol fuel enhanced by ducted fuel injection technology in single-hole injectors

Green methanol, a renewable and potentially carbon-neutral fuel, combines significant life-cycle carbon reduction with compatibility to existing fuel infrastructure. However, strategies to optimize its spray and combustion processes remain underexplored. Ducted fuel injection (DFI) is an emerging spray combustion strategy that can improve mixing of fuel with air and decrease exhaust emissions, yet its applicability to methanol has not been systematically investigated. This study presents the first experimental investigation of DFI for methanol, aiming to clarify its spray behavior and underlying flow mechanisms. This study systematically investigates the effects of injection pressure and two duct configurations—a straight duct with a 2 mm inner diameter (ST2) and a convergent–divergent duct with a 4.5 mm outlet diameter (CD4.5)—on methanol spray morphology, flow perturbations, and macroscopic spray characteristics, in comparison with conventional free spray (FR) conditions. Flow disturbance analysis revealed that the ST2 duct spray induced moderate unsteady perturbations along the spray boundary, leading to a slight increase in local turbulence intensity. In contrast, the CD4.5 duct spray significantly amplified unsteady flow structures, generating a broader region of elevated fluctuating values, which is favorable for enhancing fuel–air mixing. At 80 MPa injection pressure, the ST2 duct spray exhibited nearly a 50 % increase in spray cone angle and enhanced spray tip penetration by about 3.43 %–9.76 % relative to the FR spray. The CD4.5 duct spray, on the other hand, resulted in a 117.26 % increase in spray cone angle but reduced spray tip penetration by 12.91 %–13.56 %. This study provides the first validation of the applicability of DFI technology to methanol fuel, delivering quantitative evidence that can inform injector optimization and foster the development of combustion systems with enhanced efficiency and reduced emissions.