Source Term Evaluation of the APE-RF System using DNS Data

Acoustic perturbation equations for reacting flows (APE-RF) in conjunction with direct numerical simulations (DNS) are used to investigate in detail the thermo-acoustic effects resulting from turbulent premixed flames. The basic procedure is a two-step DNS/APE-RF method, where the flow is simulated by direct numerical simulations and the acoustic analysis is performed using the APE-RF system. Based on the DNS data, the source terms of the APE-RF system can be thoroughly evaluated, since the full chemical reaction is taken into account in the DNS. The acoustic impact of several source mechanisms are investigated, such as the effect of unsteady heat release, that of heat flux, viscous effects, the effect of non-isomolar combustion, and that of species diffusion. The study shows the unsteady heat release to be the dominant source. All source terms but the heat diffusion term possess a monopole-like structure in the low frequency range. At high frequencies a multipole-like pattern is also determined for the sources due to species diffusion and viscous effects. This deviation from the monopole structure is caused by the chemical reaction time scales. It is shown in this study that the radiated acoustic energy is in good agreement comparing the impact of the total time derivative of the density as major source term with the unsteady heat release rate