Leaner lifted-flame combustion with ducted fuel injection: The key role of forced two-stage mixing

•High-fidelity LES of the combustion process with ducted fuel injection (DFI) is performed.•The key role of forced two-stage mixing on the formation of leaner lifted flame is clarified.•The flame structure is analyzed by using combustion mode analysis.•A conceptual model for DFI combustion is presented. Ducted fuel injection (DFI) is a new approach to realize low-soot combustion for mixing-controlled ignition engines, while the underlying physics still remains inadequately understood. In this work, a high-fidelity LES of the combustion process with DFI was performed under engine-relevant conditions, with a particular interest in understanding the formation of the leaner lifted-flame combustion. Results show that, compared to free spray, both the ignition delay time and the quasi-steady flame lift-off length of DFI are prolonged, while the relative flame lift-off length from the duct outlet is shortened. The ignition process of DFI is typically two-staged in the mixture fraction space, which is fundamentally determined by the duct-forced, two-stage mixing process in the physical space. The mean soot volume fraction is synergistically controlled by the mixture fraction and the dissipation rate at the duct outlet and shows a nonmonotonic variation of “decrease-increase” with increasing duct length. To achieve leaner lifted-flame combustion, the duct outlet should pursue a balance between a globally rich (to avoid pre-ignition in the duct), spatially uniform (to avoid an excessively rich spray core) mixture and a certain large scalar dissipation rate (to extend the flame lift-off length). Compared to the conventional rich lifted flame of free spray, the leaner lifted flame structure of DFI exhibits a broader high-temperature-induced mode region, and the low-temperature flame is no longer circumferentially surrounded by the high-temperature flame. The decrease of the peak mixture fraction in the quasi-steady flame avoids soot formation especially under high-dissipation-rate conditions, and a conceptual model is presented to describe key features of DFI.