The effect of the induced outer flow on flame–acoustic interaction in channel-confined propagating flames

This study presents a quantitative analysis of the onset of self-induced thermoacoustic instabilities in a flame propagating through a channel open only at the ignition end. Direct numerical simulations are performed for a stoichiometric mixture to examine flame–acoustic interactions in two computational set-ups: (i) a conventional truncated domain that models only the channel, imposing a constant-pressure boundary condition at the open end; and (ii) an extended domain that more closely replicates experimental conditions by resolving both the internal flow and the surrounding atmosphere, where the jet emerging from the channel exit after ignition generates additional acoustic sources. In the extended domain, both pressure and velocity at the channel exit arise naturally from the calculation rather than being prescribed. The extended-domain simulations predict a markedly different flame response, with an earlier onset of thermoacoustic instability and an accelerated transition to a non-symmetric flame front. These features arise from vortices formed in the mixing layer between the pulsating jet and the surrounding atmosphere, which act as additional sources of acoustic perturbations and trigger the premature growth of flame instabilities. Although thermoacoustic generation has traditionally been regarded as the primary mechanism of acoustic energy production, the results demonstrate that aeroacoustic contributions play an equally significant role in amplifying acoustic perturbations. Our findings underscore the critical interplay between acoustics, combustion and fluid mechanics in determining the flame dynamics and emphasise the importance of accurately representing boundary conditions that reproduce the physical interaction with the surrounding environment to achieve reliable predictive simulations.