Experimental Investigation on Nonlinear Response of a Low-Swirl Flame to Acoustic Excitation With Large Amplitude

Abstract

Thermoacoustic instability is a major issue in developing high-efficiency low emission gas turbine combustors. In order to predict the amplitude of limit cycle oscillation, an understanding of the amplitude dependent response of the flame, i.e. the nonlinear response, to large acoustic excitation is needed. In the present study, the nonlinear response of a low-swirl CH4/air premixed flame to acoustic excitation is experimentally studied. Amplitude dependences of flame dynamic at 75 Hz and 195 Hz are discussed in detail over a wide range of excitation level. Experimental results show the gain of flame describing function of the low-swirl flame has a peak value at 65 Hz and a local minimum at 105 Hz which is caused by the destructive (out of phase) and constructive (in phase) of the axial and azimuthal velocity fluctuation. At low perturbation level, flame heat release fluctuation is in linear relationship with the normalized velocity driving level. Heat release fluctuation begins to saturate at a certain level which depends on the driving frequency. The low-swirl flame oscillates mainly in the axial direction at 75 Hz while it is in the radial direction at 195 Hz. The non-linear flame heat release response is a result of combination effect of flame rollup process and harmonic responses.

Experimental Study on Flame Response Characteristics of a Non-Premixed Swirl Model Combustor

“…Energies 2023, 16, 6834 5 of 19 15 mV/kPa, uncertainty: ≤0.3% F.S.). Heat release rate fluctuation, which can be used to determine the combustion state, was indirectly measured by capturing the fluorescence intensity of CH* chemical species of the flame [40].…”

“…They discovered that by arranging a series of small-diameter cylinders upstream of the bluff body, the gain and phase of the FTF at the target frequency can be adjusted, providing a method for suppressing thermoacoustic instability. In a study conducted by Liu et al [19], the nonlinear response characteristics of low-swirl CH4/air premixed flames toward acoustic excitation were investigated. The research findings indicate that the extreme values of the flame transfer function (FTF) gain for low-swirl flames are influenced by the coexistence or difference of axial and tangential velocities.…”