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Five different low-Mach large eddy simulations are compared to the turbulent stratified flame experiments conducted at the Technical University of Darmstadt (TUD). The simulations were contributed by TUD,

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Large eddy simulations (LES) show a good prediction accuracy at a decent computational cost for the simulation of combustion processes in complex geometries. However, the large grids required make the direct solution of detailed reaction kinetics impracticable. Therefore, the chemical reactions can be tabulated in a pre-processing step using detailed chemistry with one-dimensional laminar steady flamelets. These flamelets can be either non-premixed or premixed and are stored based on controlling variables like mixture fraction and reaction progress parameter, for example. In this work, a progress variable approach (PVA) using premixed flamelets was adopted to generate a manifold defined by mixture fraction and reaction progress variable. Since the computation of the flamelets is only feasible between flammability limits, the data outside these limits has to be extrapolated to obtain the complete manifold for all chemical states. The extrapolation influences the stability of the LES and its prediction quality and so four different extrapolation schemes were studied. A probability density function (PDF) model was applied to account for subgrid scale variances. Two methods of modeling the joint PDF of mixture fraction and progress variable in terms of their statistical dependence were investigated. Some results of a bluff body configuration comparing the PDF modeling approaches are shown. The results demonstrated that a diffusion flame can be simulated with both the progress variable approach based on premixed flamelets and classic non-premixed flamelets without progress variable.

Large Eddy Simulation (LES) and flamelet-based combustion models were applied to four bluff-body stabilized nonpremixed and partially premixed flames selected from the Sydney flame series, based on Masri's bluff-body test rig (University of Sydney). Three related non-reacting flow cases were also investigated to assess the performance of the LES solver. Both un-swirled and swirled cases were studied exhibiting different flow features, such as recirculation, jet precessing and vortex breakdown. Due to various fuel compositions, flow rates and swirl numbers, the combustion characteristics of the flames varied greatly. On six meshes with different blocking structure and mesh sizes, good prediction of flow and scalar fields using LES/flamelet approaches and known fuel and oxidizer mass fluxes was achieved. The accuracy of predictions was strongly influenced by the combustion model used. All flames were calculated using at least two modeling strategies. Starting with calculations of isothermal flow cases, simple single flamelet based calculations were carried out for the corresponding reacting cases. The combustion models were then adjusted to fit the requirements of each flame. For all flame calculations good agreement of the main flow features with the measured data was achieved. For purely nonpremixed flames burning attached to the bluff-body's outer edge, flamelet modeling including strain rate effects provided good results for the flow field and for most scalars. The prediction of a partially premixed swirl flame could only be achieved by applying a flamelet-based progress variable approach.