Large-eddy simulation of dual-fuel ignition: Diesel spray injection into a lean methane-air mixture

In the present study, large-eddy simulation (LES) together with a finite-rate chemistry model is utilized for the investigation of a dual-fuel (DF) ignition process where a diesel surrogate (n-dodecane) spray ignites a lean methane-air mixture in engine relevant conditions. The spray setup corresponds to the Engine Combustion Network (ECN) Spray A configuration enabling an extensive validation of the present numerical models in terms of liquid and vapor penetration, mixture distribution, ignition delay time (IDT) and spatial formaldehyde concentration. The suitability of two n-dodecane mechanisms (54 and 96 species) to cover dual-fuel chemical kinetics is investigated by comparing the predicted homogeneous IDTs and laminar flame speeds to reference values in single-fuel methane-air mixtures. LES of an n-dodecane spray in DF conditions is carried out and compared against the baseline ECN Spray A results. The main results of the study are: (1) ambient methane impacts the ignition chemistry throughout the oxidation process. In particular, the activation of the low-temperature chemistry is delayed by a factor of 2.6 with both mechanisms, whereas the high-temperature chemistry is delayed by a factor of 1.6–2.4, depending on the mechanism. (2) The ignition process starts from the spray tip. (3) There exists a characteristic induction time in the order of 0.1 ms between the start of the first high-temperature reactions and the time when maximum methane consumption rate is achieved. (4) The high-temperature ignition process begins near the most reactive mixture fraction conditions. (5) The role of low-temperature reactions is of particular importance for initiation of the production of intermediate species and heat, required in methane oxidation and (6) both applied mechanisms yield qualitatively the same features (1)–(5) in the DF configuration.