Thermoacoustic instability has been examined using the low swirl burner, which operates under lean, premixed combustion conditions. For a constant forcing frequency of 125 Hz, velocities between 5 m/s and 10 m/s were used to examine the effect of velocity and driving pressure amplitude (0.04-1.12% of atmospheric pressure) on the global and local flame response. As the velocity was increased, the toroidal structures within the flame increased in size. In addition, the root mean square of the Rayleigh index (RI_(RMS)) was analyzed as the amplitude of the driving pressure was increased. Weighted RI_(RMS) results exhibit a linear region and a nonlinear region. Although the slopes of the linear regions of different velocity were similar, an increase in bulk velocity resulted in an increase of the saturated point. For a constant velocity, when increasing the amplitude of the pressure perturbation, the local toroidal structures are found to move along the edge of the flame toward the flame base. Using a locally-weighted RI_(RMS), the contribution of the positive and negative toroidal structures on the global RI_(RMS) was examined. At low velocity, the positive and negative structures play similar roles in their contribution to the global flame response. However, as velocity is increased, the positive structures (with emphasis on the second positive structure) become more dominant than the negative toroidal structures in global flame response contribution.
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