Annihilation events topology and their generated sound in turbulent premixed flames

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 (phi = 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. (C) 2019 The Combustion Institute. Published by Elsevier Inc. All rights reserved.