Effect of split fuel injection on heat release and pollutant emissions in partially premixed combustion of PRF70/air/EGR mixtures. (1st July 2015) - Zhang, F.
Highlights: DNS study of PPC under IC engine conditions. Detailed reaction zones structures in PPC. Discovery of the two-stage combustion event in PPC under IC engine conditions. Mechanism for the trade-off between NO and CO emissions in PPC engines. Clarification of the effect of fuel mass split on PRR, heat release and emission. Abstract: Two- and three-dimensional direct numerical simulations are performed to study the combustion process of PRF70/air/exhaust gas mixtures under partially premixed combustion (PPC) conditions relevant to modern low temperature internal combustion engines. The aim is to gain improved understanding of the underlining physics in PPC engines. A skeletal PRF chemical kinetic mechanism is used together with full transport properties. The 2D and 3D numerical simulations are performed on respectively a 0.614 × 0.614 mm 2 domain and a 0.614 × 0.614 × 0.614 mm 3 domain with a 1.2 μm cell size. The results reveal the effects of the split of the fuel mass in different injections on the combustion and emission process in PPC engines. Increasing the amount of second fuel injection results in a retarded heat release and decreased NO emission, however, increased CO emission. While this CO/NOx tradeoff has been observed in previous PPC engine experiments the fundamental reason for this is clarified here. PPC is shown to consist of a two-stage combustion process: in the first stage the stratified fuel/air mixture auto-ignites, which results in partial oxidation of the fuel in the fuel-rich region and a mixture of radicals and hot products in the fuel-lean region. In the second stage the partially oxidized fuel/air mixture is oxidized in a thin diffusion flame where the diffusion and chemical reaction both play an important role. The split of fuel mass in different injections essentially affect the amount of fuel/air in the fuel-lean region and the fuel-rich region, thereby the relative importance of the diffusion flame. This in turn affects the emissions of NO and CO. The effects of turbulence on the heat release rate, pressure-rise-rate and emissions are a manifestation of the change of the reaction zone structures. At high turbulence intensities and large integral lengths a more homogeneous mixture is achieved, which promotes the volumetric ignition stage, speeds up the heat release rate, increases the pressure-rise-rate, and increases the NO formation and CO oxidation rates. The existence of both volumetric ignition and diffusion flame in PPC poses a great challenge for numerical simulations of PPC engines.