Spectral characteristics of the heat release rate in bluff body and swirl-stabilised flames*

Direct combustion noise results from pressure fluctuations produced by the unsteady heat release rate (HRR). Predicting this broadband noise is challenging and its mitigation is important, as jet noise levels have declined significantly. The power spectral density of the direct noise —a measurable quantity in experiments, is related to the product of local spectral densities of a Green's function and unsteady HRR (Ψ_q), intregrated over flame volume appropriately. The quantity Ψ_q is challenging to measure but easy to obtain from large eddy simulation (LES) results. The behaviour of Ψ_q for a wide range of thermochemical and turbulence conditions is investigated. Three combustors operating at atmospheric conditions with CH₄-air mixtures are studied: the dual-swirl burner developed by DLR, the PRECCINSTA single-swirl burner and a premixed bluff body burner. These cases are simulated using the LES-FlaRe (Flamelets revised for physical consistencies) model for sub-grid scale combustion. The dependence of Ψ_q on spatial location and flame configuration and its relation to volume-integrated HRR spectra are studied. It is observed that Ψ_q can be modelled using velocity and scalar spectra which will be discussed in the presentation.