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

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 hightemperature-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.