Numerical Investigation of Combustion Noise and Sound Source Mechanisms in a Non-Premixed Flame Using LES and APE-RF

In this study, combustion noise and sound source mechanisms of an unconfined turbulent non-premixed flame, i.e., the DLR-A flame is investigated. A hybrid LES/CAA approach is thereby employed in which a low Mach number variable density large-eddy simulation (LES) is combined with the acoustic perturbation equations for reacting flows (APE-RF). In the first step of the hybrid analysis the flamelet/progress variable (FPV) model is employed as combustion model followed by the acoustic simulation in the second step using the acoustic perturbation equations for reacting flows (APE-RF). In the acoustic analysis, special emphasis is placed on the impact of the thermoacoustic source contributions within the pressure-density relation of the APE-RF system on the radiated acoustic field and the applicability of these source formulations in terms of a hybrid CFD/CAA approach. The flamelet/progress variable database has been extended in terms of acoustic source terms. The unsteady heat release rate, the source describing the effect of non-isomolar combustion, and the species diffusion term are described by two independent parameters, i.e., the mixture fraction and the progress variable. From the findings in the present study, the analysis of the acoustic field of low Mach number reacting flows induced by the thermoacoustic sources such as the unsteady heat release leads to a very stiff problem formulation, since the related sources require highly resolved regions in the source area, which restricts the possible time step during temporal integration of the equations. The numerical bottleneck is not so restrictive when a source term formulation based on the density distribution is used. Spectra obtained from the simulated acoustic field, using two different source term formulations involving derivatives of the density are in good agreement with the experimental data even in the higher frequency range.