An investigation of ignition behavior in diesel sprays

Ignition behavior in diesel sprays is explored by comparing computed and measured ignition characteristics in diesel sprays injected into a constant-volume chamber. The ignition parameters that are explored include ignition delay times and the location of ignition in the spray. The effect of changes in chamber density and temperature on these parameters is studied. The computations are carried out with a multidimensional model for diesel sprays. Heptane is used to represent the fuel employed in the measurements. The chemical kinetics of heptane is incorporated through an interactive flamelet model. The model is able to reproduce the trends in measured ignition delay times qualitatively. Improved quantitative agreement is achieved in cases where the ignition delay period is relatively longer. The quantitative disagreement is believed to be less important from the point of view of predicted overall heat release rates when the ignition delays are shorter. Ignition is predicted to occur closer to the orifice as chamber density is increased and also as chamber temperature is increased. Consistent with the measured results, ignition in the spray is shown to be a two-stage process. Ignition is predicted to occur in the rich mixture downstream of the maximum liquid penetration length where the equivalence ratio is about 3.0. As the chamber density and temperature increase, the location of ignition moves upstream in the spray and the corresponding equivalence ratio decreases.