Subfilter Scale Combustion Modelling for Large Eddy Simulation of Turbulent Premixed Flames

Large eddy simulation (LES) is a powerful computational tool for modelling turbulent combustion processes. However, for reactive flows, LES is still under significant development. In particular, for turbulent premixed flames, a considerable complication of LES is that the flame thickness is generally much smaller than the LES filter width such that the flame front and chemical reactions cannot be resolved on the grid. Accurate and robust subfilter-scale (SFS) models of the unresolved turbulence-chemistry interactions are therefore required and studies are needed to evaluate and improve them. In this thesis, a detailed comparison and evaluation of five different SFS models for turbulence-chemistry interactions in LES of premixed flames is presented. These approaches include both flamelet- and non-flamelet-based models, coupled with simple or tabulated chemistry. The modelling approaches considered herein are: algebraic- and transport-equation variants of the flame surface density (FSD) model, the presumed conditional moment (PCM) with flame prolongation of intrinsic low-dimensional manifold (FPI) tabulated chemistry, or PCM-FPI approach, evaluated with two different presumed probability density function (PDF) models; and conditional source-term estimation (CSE) approach. The predicted LES solutions are compared to the existing laboratory-scale experimental observation of Bunsen-type turbulent premixed methane-air flames, corresponding to lean and stoichiometric conditions lying from the upper limit of the flamelet regime to well within the thin reaction zones regime of the standard regimes diagram. Direct comparison of different SFS approaches allows investigation of stability and performance of the models, while the weaknesses and strengths of each approach are identified. Evaluation of algebraic and transported FSD models highlights the importance of non-equilibrium transport in turbulent premixed flames. The effect of the PDF type for the reaction progress variable on LES predictions of the PCM-FPI approach is also investigated. The improved prediction of the filtered laminar flame speed by the modified laminar flamelet PDF is shown for LES of turbulent premixed combustion in comparison with a more widely-used -PDF. Furthermore, the results of this study consist the first application of the CSE combustion model to LES of premixed flames. The feasibility of this approach in terms of stability and convergence is demonstrated and the possible improvements of this non-flamelet-based model over the more traditional flamelet-based approach, PCM-FPI, are assessed. The comparisons of the CSE and PCM-FPI models allow the relative importance of deviations from the standard flamelet assumption to be assessed for flames lying outside the flamelet premixed combustion regime.Ph.D