Large-eddy simulation analysis of turbulent flame periodic flashbacks in a backward-facing step (BFS) combustor

Highly turbulent premixed flame dynamics is studied in a backward-facing step combustor. Large-eddy simulations are performed for two operating conditions: (i) a stable case with the flame anchored in the vicinity of the sudden expansion and (ii) an unstable case with massive periodic flashbacks. This set of computational results is first assessed through comparisons with experimental data. Then, it is used to conduct a detailed analysis of the flashback process. Pressure and velocity fluctuations signals display (i) high frequency fluctuations associated to turbulence with similar amplitudes in both the stable and unstable cases, and (ii) a low frequency fluctuation, of acoustic origin, the amplitude of which is significantly higher in the unstable case. The occurrence of flame flashback is found to be closely related to the structure of the first longitudinal acoustic mode, which can be modified thanks to a throttling plug used to modulate the area of the combustor exit cross-section. In the unstable case, this corresponds to an axial-velocity anti-node at the backward-facing step location. Pressure variations are synchronized with the motions of the flame toward the combustor upper wall and the cross-correlation between pressure and heat-release rate oscillations is relevant to a thermoacoustic feedback.

Novelty and significance statement

The present computational study is devoted to the analysis of highly turbulent flame periodic flashbacks in a BFS combustor. It includes three principal contributions. First of all, it demonstrates the capability of the retained computational approach to satisfactorily retrieve flame dynamics and complex phenomena, as observed in experiments, for both conditions (i.e., stable and unstable). Then, it confirms that the presence of the needle at the outlet nozzle leads to the birth of a velocity anti-node in the BFS direct vicinity (i.e., at the flame location). Finally, it provides a detailed characterization of the thermoacoustic behavior of the combustor, putting into evidence the coupling of turbulence and acoustic-induced oscillations with the unsteady motion of the flame and the structure of the acoustic field.