A Visual Investigation of CFD-Predicted In-Cylinder Mechanisms
that Control1st and 2nd 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 investigation into mechanisms that are predicted to control 1st and 2nd stage ignition in diesel jets. Here, two n-dodecane axi-symmetric jets, injected into air, are simulated. One jet is injected into 900K air, which produces a classic-burning jet with a negative ignition dwell, -dwell. The other is injected into 760K air. It produces a more volumetric-appearing burn and a positive ignition dwell, +dwell.
The way fuel begins to burn for both cases is similar: very early reactions begin off-axis; reaction rates increase as reacting gases flow downstream; once beyond the point of complete fuel evaporation, 1st stage heat release (HR) transitions to 2nd stage as the HR zone starts passing through the premixed charge a second time and the rise in premixed burn spike forms. The chemical and thermodynamic environment surrounding the early-2nd stage reactants for each case are very distinct.
The +dwell initial 2nd stage burn-zone is surrounded by a relatively-large mass of premixed fuel/air mixture. Its 2nd stage burns rapidly, climbing the premixed-burn spike, consuming both fresh and partially burned premixed charge. Approximately when the partially-burned charge is consumed and only fresh-premixed charge remains, the heat release rate (HRR) slows forming the downward slope of the premixed-burn spike.
The –dwell 1st stage ignition consumes nearly all of its fresh-premixed charge and HRR temporarily slows forming a dip in the apparent heat release rate (AHRR) curve prior to 2nd stage HR. The head of the jet is now filled with a partially-burned mixture, the HR zone passes through this mixture a second time and, as in the +dwell case, forms the upward slope of the AHRR premixed–burn spike. Approximately when the downstream, partially-burned, premixed gases are consumed, the downward slope of the premixed burn spike takes shape. Now, no reactants remain downstream of the jet’s head and the burning transitions to quasi-steady, mixing-controlled HR.
For both cases, long after EOI, jet reactants continue to be consumed, but at a slower rate, forming the AHRR tail, with each case leaving 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.
Randy Hessel, Rolf Reitz, Mark Musculus
University of Wisconsin - Madison, Univ of Wisconsin, Sandia National Laboratories