Comparison of flame response characteristics between Non-premixed and premixed flames under acoustic excitation

A comparative study was conducted on the response characteristics according to the increase in velocity perturbation intensity (u'/u) under acoustic excitation conditions of non-premixed and premixed flames. In addition, the flame dynamic characteristics and flame structure according to acoustic excitation were analyzed, and the response characteristics were compared according to the two flame types through the flame transfer function (FTF) and flame height perturbation analysis. In a non-premixed flame, the number of waveforms (undulations) on the flame surface increased according to the forcing frequency in the flame structure according to the acoustic excitation. On the other hand, as the velocity perturbation intensity increased, the flame height decreased. Thus, in the low forcing frequency range, the magnitude of the flame fluctuation increases, and in the high forcing frequency, the magnitude of the flame fluctuation tends to decrease. Therefore, the Strouhal number and FTF gain are nonlinear properties, and the nonlinear effect depends on the magnitude of the gain and the critical Strouhal number. In a premixed flame, various flame structures are shown according to acoustic excitation. The average flame height does not change much with increasing (u'/u¯). The correlation between the Strouhal number and FTF gain is linear. The flame height perturbation parameters were introduced to examine the flame dynamic characteristics according to the acoustic excitation, and non-premixed and premixed flames were analyzed. As a result, it shows the trend and similarity of the FTF gain, suggesting its potential as a parameter for simple analysis of the heat-release rate. In the correlation between the Strouhal number and FTF gain, the non-premixed flame has a nonlinear characteristic and agrees with the analytical study. On the other hand, the premixed flame is not consistent with analytic studies when the Strouhal number is greater than 1. The reason for various flame structures is to consider parameters such as the unusual flame structure and laminar flame speed by external perturbation.