Lift-off and stabilization of n-heptane combustion in a diesel engine with a multiple-nozzle injection

This paper presents a joint numerical and experimental investigation of flame lift-off and stabilization mechanisms in heavy-duty diesel engines. The injection strategy, employing different nozzle configurations, allows for quantification of the impact of varying inter-jet angle spacing in the presence of swirl. For this purpose, three different inter-jet angles are chosen in this study; 45°, 90°and 135°. Large-eddy simulations are performed utilizing a detailed chemical kinetic mechanism for n-heptane to resolve the turbulent fuel and air mixing and to capture the important species surrounding the ignition and flame-fronts to describe the flame stabilization process. Measurements are carried out for OH chemiluminescence to identify the flame lift-off position in an optical accessible engine. In general, the swirl flow in the ambient air shows a great impact on the lift-off, with a 15% difference in the lift-off lengths on the upwind and downwind side of the jet. The LES results show that important ignition reactions undergo in a broad region in front of the lift-off position. With decreasing inter-jet angle, it is shown that the impact of transportation of hot products from adjacent jets becomes more prominent. Hot reservoirs surrounding the lift-off length increase the local ambient temperature and augment the auto-ignition process by mixing of the cold injected fuel and hot air.