The long-term goal of this work is to develop a conceptual model for multiple injections of diesel jets. The current work contributes to that effort by performing a detailed modeling investigation into mechanisms that are predicted to control 1st and 2nd stage ignition in single-pulse diesel (n-dodecane) jets under different conditions. One condition produces a jet with negative ignition dwell that is dominated by mixing-controlled heat release, and the other, a jet with positive ignition dwell and dominated by premixed heat release.
During 1st stage ignition, fuel is predicted to burn similarly under both conditions; far upstream, gases at the radial-edge of the jet, where gas temperatures are hotter, partially react and reactions continue as gases flow downstream. Once beyond the point of complete fuel evaporation, near-axis gases are no longer cooled by the evaporation process and 1st stage ignition transitions to 2nd stage ignition. At this point, for the positive ignition dwell case, all of the fuel has already been injected and the 2nd stage ignition zone is surrounded by a relatively large mass of premixed gas, which results in the premixed-dominated heat release mentioned above. Conversely, relatively little premixed gas surrounds the 2nd stage ignition zone of the negative ignition dwell case, its small premix charge burns rapidly and the remaining charge is supplied via injection during the heat release process yielding a mixing-controlled dominated heat release.
After end-of-injection, both cases leave a distinct residual jet. Gaining a deep understanding of the aforementioned processes is the purpose of this paper. Understanding how a second pulse of fuel burns when injected into residual jets of different character is the subject of future work.