Effects of flame structures on direct combustion noise produced by lean-premixed h₂ /air low-swirl jet flames

This experimental study explored the influence of flame structural characteristics on direct combustion noise produced by a lean-premixed gaseous H₂ /air low-swirl turbulent jet flame, with a focus on the mechanism resulting in characteristic peaks in combustion noise spectra. Ten-kHz OH* chemiluminescence and OH planar laser-induced fluorescence (OH-PLIF) imaging were used to study the spatiotemporal evolution of heat release and flame structure fluctuation, respectively, while two-dimensional particle image velocimetry (2D-PIV) measurements at 4 Hz were used to study the mean-based relation between the velocity/vorticity fields and flame structures. Pressure and global heat release fluctuation measurements carried out alongside these optical diagnostics revealed pronounced double peaks in both combustion noise and global heat release fluctuation spectra at global equivalence ratios of ϕ ≥ 0.45. Spectral orthogonal decomposition of the OH* chemiluminescence and OH-PLIF images revealed the first peak in the noise spectra to be caused by flame oscillations over nearly the entire flame region, while the secondary peak is attributed to periodically generated vortical flame structures near the flame boundary. The 2D PIV results suggest that vortical flame structures are probably generated by the interaction between the flame and the inner/outer shear layers.