Self-induced instability of a premixed jet flame impinging on a plate

It is shown in this article that self-induced instabilities may arise when a premixed flame anchored on the rim of a burner impinges on a flat plate located in the vicinity of the burner and facing the outlet. Systematic experiments carried out on three burner sizes and three mean flow velocities provide the ranges of burner-to-plate distances which induce an unstable oscillation. The flame motion is imaged and the non-steady heat release, unsteady flow velocity at the burner outlet, and pressure fluctuations in the burner and in the far-field are characterized. The flow velocity is essentially sinusoidal while the heat release and far-field pressure feature a strong harmonic content. It is shown that the burner acts like a Helmholtz resonator but the fundamental oscillation frequency evolves around the resonator frequency as the burner-to-plate separation is changed. The sudden reduction of flame area produced by the perturbed flame impinging on the plate produces an intense source of sound which drives the system. A model describes the dynamics of the system and the predicted signal relationships and fundamental frequency of oscillation are in agreement with data in all experimental conditions. These experiments reveal that interactions between perturbed flames and solid boundaries could play an important role in the development of combustion instabilities.