A Visual Investigation of CFD-Predicted In-Cylinder Mechanisms That Control First- and Second-Stage Ignition in Diesel Jets 2019-01-0543

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 1^st and 2^nd 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 1^st 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 1^st stage ignition transitions to 2^nd stage ignition. At this point, for the positive ignition dwell case, all of the fuel has already been injected and the 2^nd 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 2^nd 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.