Assessment of Scale Resolved CFD Methods for the Investigation of Lean Burn Spray Flames

Incoming standards on NOx emissions are motivating many aero-engines manufacturers to adopt the lean burn combustion concept. However, several technological issues have to be faced in this transition, among which limited availability of air for cooling purpose and thermoacoustics phenomena that should be managed to safely implement this burning mode. In this scenario, standard numerical design tools are not often capable of characterizing such devices. Thus, considering also the difficulties of experimental investigations in a highly pressurized and reactive environment, unsteady scale resolved CFD methods are required to correctly understand the combustor performances. In the last years Large Eddy (LES) and hybrid RANS-LES models such as Scale Adaptive Simulations (SAS) have undergone considerable developments. Such approaches have been already applied for gaseous flames, leading to a strong enhancement in phenomena prediction with respect to standard steady-state simulations. However, huge research efforts are still required when spray flames are considered, since all the numerical models chosen to describe spray dynamics and the related reactive processes can have a strong impact on the accuracy of the whole simulation. In this work a set of scale resolved simulations have been carried out on the DLR Generic Single Sector Combustor spray flame for which measurements both in non-reactive and reactive test conditions are available. Exploiting a two-phase Eulerian-Lagrangian approach combined with a Flamelet Generated Manifold (FGM) combustion model, LES simulations have been performed in order to assess the potential improvements with respect to steady state solutions. Additional comparisons have also been accomplished with SAS calculations based on Eddy Dissipation combustion model (EDM). The comparison with experimental results shows that the chosen unsteady strategies lead to a more physical description of reactive processes with respect to RANS simulations. FGM model showed some limitations in reproducing the partially-premixed nature of the flame, whereas SAS-EDM proved to be a robust modelling strategy within an industrial perspective. A new set of spray boundary conditions for liquid injection is also proposed whose realiability is proved through a detailed comparison against experimental data.