Flame regimes in DI diesel combustion: LES study for light- and heavy-duty injectors

Abstract

Experimental and numerical studies contributed significantly to the overall understanding of diesel spray combustion phenomena over recent decades. In particular, the conceptual model proposed by Dec (1997) [1] and its extensions, describing the mechanisms behind spray formation, vaporization, mixing, ignition, as well as pollutant formation and consumption, have provided new insights for DI diesel engine applications. However, the literature still lacks detailed studies on the local flame regimes of diesel spray combustion. This study presents a comprehensive flame regime analysis of two benchmark cases of the Engine Combustion Network, Spray A and Spray D, which represent light- and heavy-duty applications. To this end, the established gradient-free regime identification (GFRI) approach is extended to cover diesel spray flames and applied to numerical LES data, based on tabulated chemistry, using the unsteady flamelet progress variable (UFPV) approach. The applicability of the tabulated chemistry approach for flame regime identification is validated based on DNS data. For both light- and heavy-duty diesel injectors, a different ignition behavior and temporal evolution of the flame regime distribution is observed. In both configurations the main ignition is dominated by multi-regime combustion, which governs the flame stabilization, and non-premixed combustion, most prominent when approaching steady-state. However, the spatial separation of flame regimes, especially multi-regime and the low-temperature regime, significantly differs when comparing the two spray flames. Based on this qualitative analysis, the impact of the single flame regimes on the overall heat release is analyzed. This is followed by a conditional flame structure analysis, with respect to the mixture fraction and flame regime, for representative times, e.g., early and main ignition as well as quasi steady-state combustion. Finally, the outcome of the local flame regime analysis is consolidated, and an extended conceptual model is proposed, including a detailed understanding of the predominant flame regimes.