On large eddy simulation of diesel spray for internal combustion engines

Large eddy simulation of diesel spray in a constant volume vessel as well as in an internal combustion engine has been performed. Spray is modeled using Eulerian-Lagrangian approach. The modeling involves primary and secondary break-up, the spray-induced turbulence (SIT) and the stochastic turbulence dispersion (STD) of parcels. Including SIT, based on Baharawaj et al. (2009), in coarse grids, e.g., Delta x = 0.5 mm, hardly affects the results. With finer grids, e.g., Delta x = 0.25 mm and Delta x = 0.125 mm, the local mixture fraction in the downstream flow field slightly decreases whereas the prediction of the spray tips and air entrainment dose not respond to the inclusion of SIT. In the constant volume vessel configuration, including STD in the model is shown to be crucial in obtaining predictive results. It is shown that a simulation excluding STD leads to an unrealistic prediction of the spray shape, over-prediction of the liquid and vapor lengths and erroneous distribution of the mixture fraction and air entrainment. Further investigation on the SIT and STD effects is carried out in practical internal combustion engine operating under a partially premixed charge (PPC) compression ignition condition where the PPC condition is obtained by employing a triple-injection strategy during the late engine compression stroke. It is found that when STD is not included, the fuel spray wall-wetting as well as the fuel spray parcels residing in the cylinder wall vicinity are increased. Furthermore, an unrealistic distribution of mixture fraction in the in-cylinder gases is observed. The results suggest that it is necessary to consider models for particle dispersion when LES of internal combustion engines is performed, in order to properly capture the combustion phasing. The significance of STD is found to increase when the ambient air density is low. (C) 2015 Elsevier Inc. All rights reserved.