A low swirl burner is used to examine thermo-acoustic instability under different perturbation level. For a constant forcing frequency of 125 Hz, a low swirl flame with the velocity of 5 m/s is used to examine the effect of the driving pressure amplitude (0.045–0.649 % of atmospheric pressure) on the global and local flame response. Rayleigh Index is used in current research to show the local and global coupling level between the heat release and the pressure oscillation. The toroidal structures found in the Rayleigh Index map indicate that the coupling strength increases with the perturbation level and then stays stable. At the same time, the coupling location doesn’t change much with the forcing amplitude. In addition, the root mean square of the Rayleigh index (RRMS) exhibits a linear region and a nonlinear region when perturbation magnitude rises. Besides the global response, the local RRMS and locally-weighted RRMS are studied. The contributions of the positive and negative toroidal structures on the global RRMS are similar in current case. Locally phase-averaged OH-PLIF data are used to investigate the local heat release variation trend. Phase analysis shows that the local phase is not sensitive to the perturbation level, which induce that the Rayleigh Index is more dominated by the oscillation magnitude of the heat release.

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