This paper studies the contribution of flame annihilation events to the sound radiated by turbulent, premixed flames. Previously published direct numerical simulation (DNS) datasets of stoichiometric and lean (φ = 0.7) flames (Haghiri et al. 2018) are first examined using an efficient formulation of the method of Griffiths et al. (2015) to identify the annihilation events. Four classes of annihilation event are observed. Three of these -pocket burn-out, tunnel closure and tunnel formation -were defined by Griffiths et al. A 'multi-feature' event is also defined in this paper as any combination of the other three annihilation events occurring close enough such that their radiated sound can be considered as originating from a single event. Further post-processing of these stoichiometric and lean datasets shows that the fluctuations in heat release rate associated with these 4 observed types of annihilation events are responsible for the broadband sound radiated by both flames. This, in turn, suggests that flame annihilation is the physical mechanism by which air-fuel ratio affects the radiated sound amplitude at high frequencies. This result is supported by previous works which have shown that the sound radiated from individual annihilation events scales with the laminar flame speed and the temperature ratio.

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Spectral proper orthogonal decomposition (SPOD) is applied to direct numerical simulation (DNS) datasets of a lean and a stoichiometric methane/air turbulent premixed jet flame. SPOD is used to extract the coherent structures that correlate with the radiated sound by using an inner product based on a linearized disturbance energy. Two types of structures are prominent in the data. The first type arises in the jet's shear layer and is linked to the Kelvin-Helmholtz (K-H) instability, which is an important mechanism of sound generation in non-reacting jets. These structures produce sound through deformation of the flame front in the shear layer. They contain most of the acoustic energy and are dominant at Strouhal numbers (defined based on the jet's diameter and the inlet mean velocity) less than unity. The second type of structures is found near the jet centreline, where large fluctuations of the flame surface are observed. The structures are linked to small non-linear flame dynamics and to the Orr mechanism. They travel at a speed close to the inlet mean velocity and are important at higher Strouhal numbers. Regardless of their energy content, both types of structures have important contributions to the broadband nature of combustion noise.

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