Thierry S., Nicolas N., Daniel D., Sébastien C.

Experiments were carried out to investigate mechanisms leading to strong noise production in unconfined combusting systems. The experimental set-up comprises a premixed laminar burner equipped with an actuation system used to modulate the flow at the burner lips around the mean operating conditions. Four different flame geometries, stabilisation configurations and flow interactions were explored for premixed methane air flames in a mixture of constant equivalence ratio. Flame response to incoming flow modulations were analysed by simultaneous measurements of velocity perturbations at the burner outlet, variations of the flame light emission and noise radiation. These signals were correlated using classical results of combustion noise theory. Phase locked images of the flame patterns were also acquired to examine the history of noise emission and locate the flame patterns corresponding to the instant of maximum noise production. Events corresponding to large and fast rates of variation of the flame surface area were identified as the main contribution to the overall radiated noise in all cases explored. This was verified hereby for a set of laminar flame flow interactions, which constitute elementary mechanisms occurring in turbulent combustion~: a flame submitted to convective velocity waves, a flame interacting with vortices shed from the burner lips, mutual annihilations of neighboring flame elements and collective effects of an ensemble of small conical flames. These mechanisms investigated in unconfined configurations may also prevail in combustors operating at steady or slowly varying conditions, like those of gas turbines or jet engines, and may play an important role in acoustically coupled combustion instabilities.