The Effect of the Exit Boundary Condition on the Acoustics of Premixed Flame Propagation in Channels

Abstract

Thermoacoustic instabilities play a fundamental role on the design and operation of the vast majority of combustion systems. The appearance of thermoacoustic instabilities is often unexpected, and their consequences can be critical. Its highly nonlinear nature, together with the wide range of time and length scales, makes numerical simulations challenging, as high-fidelity results are required for its accurate prediction. Fundamental studies of premixed flames propagating under confinement can be found in the literature, with the extensive use of the truncation of the computational domain with a boundary condition to close the problem. Although acoustic-friendly boundary conditions are available, these approaches overlook the complexity of downstream phenomena. This work questions the accuracy of the decoupling between the flow inside and outside the channel, which is implicitly assumed in these approaches, showing that the results can notably change when accurately solving these overlooked phenomena. For this task, the premixed flame propagation process in a semiopen slender channel is solved. The channel is initially filled with a stagnant fuel air mixture at standard conditions which is locally heated in the vicinity of the open end to mimic a spark-induced ignition. An auto-sustained combustion process starts allowing the flame to propagate from the open end to the closed end of the channel, consuming the reactants. The problem is numerically resolved using two different approaches: the first one truncates the computational domain at the channel exit whereas the second one extends the domain to include the atmosphere surrounding the channel. The latter case found violent oscillations in pressure and heat release rate (secondary thermoacoustic mode) right after ignition, as a consequence of the acoustic perturbations introduced by the sudden ejection of hot gas through the channel opening. In the truncated case, pressure remained constant at the channel entrance and flame oscillations were only observed in the second half of the channel and showed smaller amplitude than when the atmosphere was considered in the calculations. Our study suggests that different flame propagation regimes (symmetric and nonsymmetric flame) can be triggered depending on the boundary conditions used, anticipating the importance of using adequate boundary conditions to accurately predict the onset of thermoacoustic instabilities.