Numerical Study of Flame Lift-off and Soot Formation in Diesel Fuel Jets

A detailed chemistry-based CFD model was developed to simulate the diesel spray combustion and emission process. A reaction mechanism of n-heptane is coupled with a reduced NOx mechanism to simulate diesel fuel oxidation and NOx formation. The soot emission process is simulated by a phenomenological soot model that uses a competing formation and oxidation rate formulation. The model is applied to predict the diesel spray lift-off length and its sooting tendency under high temperature and pressure conditions with good agreement with experiments of Sandia. Various nozzle diameters and chamber conditions were investigated. The model successfully predicts that the sooting tendency is reduced as the nozzle diameter is reduced and/or the initial chamber gas temperature is decreased, as observed by the experiments. The model is also applied to simulate diesel engine combustion under PCCI-like conditions. Trends of heat release rate, NOx and soot emissions with respect to EGR levels and start-of-injection timings are also well predicted. Both experiments and models reveal that soot emissions peak when the start of injection occurs close to TDC. The model indicates that low soot emission at early SOI is due to better oxidation while low soot emission at late SOI is due to less formation. Since NOx emissions decrease monotonically with injection retardation, a late injection scheme can be utilized for simultaneous soot and NOx reduction for the engine conditions investigated in this study. Introduction Diesel engine manufacturers are facing stringent emission regulations and a better understanding of the diesel spray combustion process is crucial to help design low emission diesel engines. Experimental data have been used to construct a conceptual diesel spray combustion image that depicts the flame structure and soot and NOx distributions [1]. It has been shown that the details of the flame structure are crucial to the soot formation process during the mixing-controlled combustion phase [2,3]. The lifted flame consists of a diffusion flame at the periphery of the fuel jet (where NOx is formed) and a rich reaction zone located downstream of the lift-off length in the central region of the fuel jet (where soot is formed). The lift-off length determines the time for fuel-air mixing prior to ignition and entering the reacting zone, and thus will affect the sooting tendency of diesel fuel jets. As a complement to optical soot and NO diagnostics, predictive numerical models can also help understand the diesel spray combustion process and provide insights to the details of flame structure. Development and applications of engine CFD models have become increasingly important and effective in analyzing the complex diesel combustion process [4—7]. The use of detailed chemistry is also essential to better predict fuel oxidation and emission formation, especially for the low-temperature HCCI combustion process which is of much interest [6,7]. This study develops a numerical model that uses detailed chemical kinetics to simulate the diesel lift-off flame, and its combustion and emission formation. The model is validated using experimental combustion and emission data from a combustion vessel and from a heavy-duty diesel engine under various operating conditions. Model Formulation The CFD code is a version of KIVA-3V [8] with improvements in various physical and chemistry models developed at the Engine Research Center, University of Wisconsin—Madison. The major model improvements include the spray atomization, drop-wall impingement, wall heat transfer, piston-ring crevice flow, and soot formation and oxidation models [9,10]. The RNG k-ε turbulence model was used for incylinder flow simulations. Since detailed reaction mechanisms for n-heptane were used to simulate diesel fuel chemistry, the CHEMKIN chemistry solver [11] was integrated into KIVA-3V for solving the chemistry during multidimensional engine simulations. The chemistry and flow solutions were then coupled. * Corresponding Author