Progress in large-eddy simulation of premixed and partially-premixed turbulent combustion

In many practical devices such as gas turbines and internal combustion engines, liquid fuel is injected as a spray and mixed with oxidizer as it vaporizes, so that combustion takes place in a partially-premixed regime. While partially-premixed flame propagation has been the subject of extensive experimental investigations (Su 2000; Muniz & Mungal 1997), its numerical simulation remains a challenging task. The mechanisms by which turbulence, chemical reactions, and heat release interact are still under investigation. Results by Veynante (1994) suggest the importance of premixed flame propagation in the process of flame stabilization. While DNS of turbulent premixed flames using realistic chemistry is still restricted to very simple geometries, classical RANS modeling of reacting flows is often considered to lack precision, especially when highly-unsteady problems are considered. Different methods have been suggested to model turbulent premixed combustion in LES (Colin 2000; Kim & Menon 2000; Duchamp de Lageneste & Pitsch 2000). An approach based on a mixed level-set/diffusion-flamelet library applicable to premixed and partially-premixed combustion has been derived by Peters (1999) for RANS and validated by Herrmann (2000) showing good agreement with experimental data. Beside the fact that this method does not require solving any additional species transport equations or explicit modeling of chemical reaction rates, it also allows the use of arbitrarily complex chemistry without leading to prohibitive computational requirements. The work presented here focuses on the implementation and validation of a similar approach in the LES context. First, the governing equations to be solved are presented together with the sub-grid models used, including an improved model for the turbulent burning velocity. Then we discuss the application of this approach to two different test cases; a turbulent Bunsen flame (Aachen flame F3, (Chen 1996) and a lean, partially-premixed dump combustor, the so-called ORACLES geometry described in Besson (1999a) and Besson (1999b).

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