Role of hydrodynamic instabilities in high-frequency transverse thermoacoustic instabilities in a dual-swirl H2 burner

High-frequency thermoacoustic instabilities pose a significant challenge to the development of new generations of combustion systems. This study investigates the interplay between helical hydrodynamic instabilities in a dual-swirl hydrogen-air burner, featuring a spinning thermoacoustic instability coupled to the first transverse acoustic mode of the combustion chamber in the absence of injector coupling. Particle image velocimetry coupled with OH planar laser-induced fluorescence, high-speed OH${}^{\ast }$ imaging, and pressure measurements are used to explore how varying the swirl level $S_i$ imparted to the hydrogen stream influences the flow and flame dynamics during self-sustained oscillations for a fixed swirl level $S_e=1.2$ of the air stream. A dramatic shift in flame response is revealed. At low swirl $S_i=0.2$, elongated flames with low-frequency self-sustained oscillations are observed, while compact flames dominated by high-frequency transverse instabilities are triggered at higher swirl levels $S_i=0.6$ and 1.0. In the latter case, the flow dynamics in the internal recirculation zone of the swirling flow is dominated by a transverse bulk oscillation due to acoustic displacement, while the shear layers are influenced by large-scale helical hydrodynamic structures. It is demonstrated that the amplitude of the high-frequency combustion instability depends on the synchronization between hydrodynamic $f_h$ and acoustic $f_a$ frequencies. When synchronization occurs ($f_a\simeq f_h$), large vortical structures synchronized with the transverse acoustic wave are formed. These structures strongly dominate flame deformation compared to the direct displacement caused by the transverse spinning acoustic wave, thereby substantially enhancing the amplitude of thermoacoustic instability. Conversely, when the frequencies are misaligned ($f_a\ne f_h$), transverse oscillations are weaker but persist, indicating that the helical hydrodynamic instability primarily acts as an amplifier rather than an initiator of the thermoacoustic coupling.