A low-swirl burner operating in premixed mode with methane and air is studied using high-speed acetone and OH-PLIF and advanced post-processing methods. In a transient operating point where the equivalence ratio is slowly increased while keeping the bulk velocity constant, a thermo-acoustic instability is found to grow and generate large pressure fluctuations in the combustion chamber. The instability is maintained by bursts of flame in phase with the pressure fluctuation cycle. It is found that these bursts are amplified by the fact that the boundaries of the flame arms are fluctuating in phase. This phasing of the arms' boundaries is due to the change in flame speed coming from the increase of equivalence ratio.
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