A Comprehensive Study of Effects of Mixing and Chemical Kinetic Models on Predictions of n-heptane Jet Ignitions with the PDF Method

Abstract

The composition Probability Density Function (PDF) model is coupled with a Reynolds-averaged k − e turbulence model and three computationally efficient, yet widely used chemical mechanisms to simulate transient n-heptane spray injection and ignition in a high temperature and high density ambient fluid. Molecular diffusion is modelled by three mixing models, namely the interaction by exchange with the mean (IEM), modified Curl (MC) and Euclidean minimum spanning trees (EMST) models. The liquid phase is modelled by a discrete phase model (DPM). This represents among the first applications of the PDF method in practical diesel engine conditions. A non-reacting case is first considered, with the focus on the ability of the model to capture the spray structure, e.g., vapour penetration and liquid length, fuel mixture fraction and its variance. Reacting cases are then investigated to compare and evaluate the three different chemical mechanisms and the three mixing models. It is concluded that the EMST mixing model in conjunction with a reduced chemical kinetic model (Lu et al., Combust Flame 156(8):1542–1551, 2009) performs the best among the options considered. The sensitivity of the results to the choice of the mixing constant is also studied to understand its effect on the flame ignition and stabilisation. Finally, the PDF model is compared to a well-mixed model that assumes turbulent fluctuations are negligible, which has been widely used in the diesel spray combustion community. Significant structural differences in the modelled flame are revealed comparing the PDF method with the well-mixed model. Quantitatively, the PDF model exhibits excellent agreement with the measurements and shows much better results than the well-mixed model in all ambient O2 and temperature conditions.