High-frequency self-sustained oscillations of a hydrogen jet flame

Abstract

This work investigates a high-frequency thermoacoustic instability in an atmospheric laboratory-scale combustion test rig. The flame is stabilized by a single-jet burner operated with pure hydrogen at thermal powers between 60 and 120 kW. A parametric study is performed by varying the equivalence ratio and bulk flow velocity. The measurements show a clear correlation between the equivalence ratio and the occurrence of high-frequency thermoacoustic instabilities, and with increasing thermal power the amplitude of pressure fluctuations rises. A stable and an unstable operating condition with identical bulk velocity but different equivalence ratios are examined in detail using optical and acoustic methods. The power spectra reveal two closely spaced peaks in the 4 kHz range, indicating that multiple modes in close proximity may be simultaneously unstable. This is supported by a beating pattern in the response of azimuthally distributed microphones, which also suggests the presence of transverse acoustic modes. High-speed OH* chemiluminescence imaging from lateral and aft-end views is used to describe the mode shapes. The phase-averaged images show an axisymmetric OH* distribution propagating at approximately the bulk jet velocity, indicating a longitudinal or radial mode. A spectral proper orthogonal decomposition of the chemiluminescence signals confirms that the leading mode is rotationally symmetric, while a secondary mode of transverse shape carries less energy.