Effects of EGR on the structure and emissions of diesel combustion

Effects of exhaust-gas recirculation (EGR) on the structure and emissions of n-heptane diesel combustion are numerically investigated. Two cases with different volumetric oxygen concentrations of 21% and 10% are considered. It is shown that the structure of reaction zone at the onset ignition is independent of the oxygen concentration in the ambient gases, namely and consistent with the experiments, the equivalence ratio in the premixed-burn region is around 2 for both cases presented here. A cool-flame is formed at almost the same distance from the nozzle in both cases. As the process is evolving in time, a premixed-burn flame is formed around the lift-off position. The distance between the location of premixed-burn flame and the cool-flame in the 10% oxygen case is much larger than that in the 21% oxygen case. In the mixing controlled burning stage, when the oxygen concentration in the ambient gas is at 21%, the equivalence ratio of the mixture at the premixed-burn region is significantly higher than that in the corresponding region in 10% oxygen case. This leads to higher soot yield in the 21% oxygen case. The mechanisms involved are discussed in details and the relation between the soot level in the reaction zone and the oxygen concentration in the ambient gas is explored