In order to investigate the frequency shift,mode hopping characteristics and mechanisms of ther-moacoustic instability in a premixed weak swirling burner as the equivalence ratio changes,different combustion states,such as limit cycles,beating oscillations and intermittent oscillations,are identified and analyzed using Fourier transform,phase space reconstruction,OH* time series and average flame images.The"mode hopping"phenomena occurring in the process of thermoacoustic oscillations are simulated by combining the low-order ther-moacoustic network with the n-τ model,where the delay time τ is approximated by the convective delay time cal-culated based on the length of the unburned mixture.Experimental results show that the first-order frequency in-creases with increasing equivalence ratio(φ)in the range of 0.5～0.7.At φ=0.8,the dominant frequency switches from the first to the second-order mode.Furthermore,for φ within the range of 0.9～1.2,the first-order frequency decreases,resulting in a change of up to 41.48%.For a ratio of φ=1.1,the dominant frequency switches from the second to the first-order mode.The second-order frequency varies by less than 5%over the entire equivalence ra-tio interval.The simulation results are in agreement with the experimental tests,especially at φ=0.8,the growth rate of the second-order frequency exceeds that of the first-order,leading to mode hopping occurred in the exper-iments.Meanwhile,in some conditions,the predicted oscillation frequency also deviates from the experimental results a little bit,due to the increase in gain leading to an increase in deviation.The maximum error between the numerical and experimental results is about 26.4%for the first-order frequency and less than 12%for the sec-ond-order frequency.The current work shows that there is an inverse relationship between the convective delay time and the dominant frequency,which means reducing the convective delay time causes the first-order frequen-cy to shift to the higher frequency,and the instability modes switch from first-order to second-order modes in the process.
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