Large Eddy Simulation of Pilot Fuel Soot Oxidation in Methane/n-Heptane Dual Fuel Combustion

In recent years, the University of New South Wales (UNSW) has produced high quality optical data of dual fuel spray combustion in a constant volume combustion chamber (CVCC) [1]. The present numerical work utilizes the diesel-methane dataset to develop and validate a computational fluid dynamics (CFD) model of the experimental setup. The CFD model incorporates large eddy simulation (LES) turbulence modelling with adaptive meshing, detailed chemistry and the addition of a phenomenological two equation soot model. The CFD model is benchmarked using the experimental data collected at a chamber temperature of 890 K and a chamber pressure of 52 bar. Quantitatively, the model replicates the fuel vapour penetration and apparent heat release rate of the experiments reasonably, although an overprediction of AHRR during pilot combustion is observed. Moreover, the spatial soot distribution matches the occurrence of observed luminescence within the CVCC qualitatively. The model is subsequently used to study the impact of dwell time between the pilot injection of diesel and the main injection of methane on the formation and oxidation of soot from the pilot fuel jet. Four dwell timings are investigated, representing a range of nominal electronic delays investigated experimentally. These are simultaneous injection (i.e. no delay), as well as 1, 2, and 3 ms relative to the pilot fuel injection. It is concluded that delaying the main fuel injection event by more than two milliseconds allows for complete oxidation of the soot produced from the pilot fuel combustion. This suggests that the consequent luminescence from the optical diagnostics originates from the main methane fuel combustion.