Numerical analysis of flow, mixture formation and combustion in a direct injection natural gas engine

摘要

Numerical simulation of flow, mixture formation and combustion in a direct injection natural gas engine was conducted using the large-eddy simulation method. The dynamic thickened flame model coupling with a skeletal methane reaction mechanism was implemented and applied for the combustion simulation. In Situ Adaptive Tabulation (ISAT) method was used for the efficient chemistry solving. It was found that in-cylinder large-scale vortices are principally formed as the fuel jet impinges on the cylinder wall, and they enter the combustion chamber due to the effect of the squish flow at the end of the compression stoke. Most of in-cylinder combustible mixtures begin to be produced after the jet-wall impingement. Under the stoichiometric condition, with the delay of the fuel injection timing, the distribution of in-cylinder equivalence ratio is wider at the ignition timing, and the inhomogeneity of mixtures increases. The fuel injection timing determines the in-cylinder dominant combustion mode and the flame displacement speed at the initial stage of flame propagation by affecting the distribution of mixture equivalence ratio at the ignition timing, thereby affecting the increase of the flame area. Mass fractions of CO, H-2, H-atom and N2O in the rich-burn region are higher than those in the lean-burn region. Mass fractions of O-atom, OH, NCN, HCN and NNH in the lean-burn region are higher than those in the rich-burn region. In the region with temperature below 1800 K, NO is principally produced by NO2 conversion, while in the region with temperature above 1800 K, NO is mainly produced by the thermal pathway.