Flame Response of a Lean Premixed Swirl Flame to High Frequency Azimuthal Forcing

Abstract

This experimental study explores fundamental qualitative differences of the response of a lean-premixed turbulent swirl flame to azimuthal acoustic excitation in the low- (LF), medium- (MF), and high-frequency (HF) regime. An experimental test rig with a cylindrical combustion chamber is equipped with an array of azimuthally distributed acoustic driver outlets in the burner front plate, allowing for the excitation of the flame with a longitudinal wave, and different azimuthal wave types. Phase-averaged chemiluminescence images show that in the LF regime the flame responds to both longitudinal and azimuthal forcing schemes with hydrodynamically induced intensity fluctuation patterns at relatively small wavelength. Azimuthal forcing generates helical intensity patterns, at the same wavelengths but with lower intensities compared to longitudinal forcing. A fundamentally different flame response is observed in the HF regime: Longitudinal forcing generates only a very weak flame response, and the amplitude of the flame response to azimuthal forcing type depends on the forced azimuthal mode. No hydrodynamic structures are evident anymore, but the flame response now uniformly covers each of the flame sides, indicating a purely acoustic response.