An insight on the spray-A combustion characteristics by means of RANS and LES simulations using flamelet-based combustion models

Advanced Computational Fluid Dynamics (CFD) modeling of reacting sprays provides access to information not available even applying the most advanced experimental techniques. This is particularly evident if the combustion model handles detailed chemical kinetic models efficiently to describe the fuel auto-ignition and oxidation processes. Complex chemistry also provides the temporal evolution of key species closely related to emissions formation, such as polycyclic aromatic hydrocarbons (PAHs) that are well-known as soot precursors. In this framework, present investigation focuses on the analysis of the so-called Spray-A combustion characteristics using two different flamelet-based combustion models. Both Reynolds-Averaged Navier-Stokes (RANS) and Large-Eddy Simulation (LES) predictions are combined to study not only the averaged spray characteristics, but also the relevance of different realizations in this particular problem. The discussion includes an evaluation of the performance of the models to predict the most relevant reacting spray macro-parameters: ignition delay and lift-off length (LOL). This is followed by the description of the temporal evolution and localization of key species during auto-ignition and flame stabilization in spatial coordinates and in the mixture fraction-progress variable space. The internal structure of the quasi-steady flame is also discussed so that the localization of key species, including PAHs, is investigated following a similar approach. Finally, the study is completed by analyzing the sensitivity of the results to the chemical mechanism and to the boundary conditions imposed for the applied manifold generation. Preliminary results show how the two models predict the auto-ignition and combustion onset in slightly rich conditions. Additionally, differences in LOL reflect on the structure of the establishing flame, so the proper prediction of the spray mixing field and the LOL is mandatory to model chemical species, especially PAHs. Dispersion effects observed analyzing different realizations are of second order for Spray-A reference conditions. Finally, it is found that the results are sensitive to chemical mechanism and evaporation must be taken into account in the simulation of the flamelets that determine the manifold.