Direct numerical simulation of PRF70/air partially premixed combustion under IC engine conditions

A three-dimensional direct numerical simulation (DNS) is performed to study the ignition and the combustion process of a primary reference fuel (PRF) under conditions relevant to partially premixed combustion (PPC) engines. Detailed transport properties together with a skeletal PRF chemical kinetic mechanism are employed in the simulations. The initial partially premixed charge is prescribed according to a two-injection of fuel strategy with a 50% exhaust gas recirculation (EGR). The motion of the piston, hence, the effect of compression of the charge due to the piston motion is considered. The simulations are performed with a fine mesh with a spatial resolution of 1.2 micrometers in a cubic domain of 614 micrometers on each side. The results show that there are three distinctive combustion regions in a domain under PPC conditions, a lean homogeneous mixture region, a fuel-rich region, and in between a region with stoichiometric mixture. Auto-ignition occurs first in the lean and stoichiometric mixture regions, followed by a partially premixed flame in the fuel-rich region and at last a CO oxidation phase in the fuel-rich region. In the lean mixture region, the fuel and the combustion intermediates such as CO are consumed almost completely due to the relatively fast combustion and the abundant oxygen. The CO formed in the regions with the stoichiometric and the fuel-rich mixture is consumed in a mixing controlled mode, which requires a high-level turbulence mixing or a sufficiently long residence time. NO is formed mainly due to the premixed combustion in the region with the stoichiometric mixture.