The Effect of Multiple Coexisting Convective Modes in Determining Thermoacoustic Behavior of a Partially Premixed Swirl Flame

Abstract

This study focuses on the effect of multiple heat release mode interactions on the amplitude of unsteady pressure oscillation for a partially premixed radial swirl burner, which uses methane as fuel. A range of operating conditions based on inlet airflow rate and global equivalence ratio is considered for this purpose. The pressure time series shows amplitude modulation at the dominant frequency for all the flow rates and equivalence ratios considered. Wavelet analysis based on continuous wavelet transform illustrates the presence of heat release rate fluctuations at multiple frequencies other than the dominant mode of pressure oscillation, with this more pronounced at low frequencies. Spectral proper orthogonal decomposition performed on time-resolved CH* chemiluminescence images reveal four dominant spatial modes of chemiluminescence, chosen based on the dominant wavelet coefficients for the same. The identified frequencies correspond to the duct-acoustic mode, helical mode (spectrally close to acoustic mode), low-frequency axisymmetric mode and low-frequency helical mode. The low-frequency helical mode (considered as the result of nonlinear interaction between acoustic and helical mode) and the low-frequency axisymmetric mode (considered to have independent existence) have similar spectral content. Amplitude modulation of unsteady pressure is found to be a result of the superposition of duct-acoustic mode and low-frequency axisymmetric mode, whereas reduction in overall pressure amplitude with the decrease in global equivalence ratio is seen to be a result of an increase in the dominance of low-frequency helical mode. The relative dominance of low-frequency helical mode over dominant pressure mode reduces the overall pressure amplitude.