Rotational symmetry-driven modal dynamics of high-frequency transverse instabilities in a lean-premixed multislit hydrogen combustor

Abstract

A cluster of small-scale multislit injectors, with aperture dimension of the same order of magnitude as the characteristic thickness of hydrogen flames, effectively mitigates flashback risk in extremely fast lean-premixed hydrogen flames. We recently reported that this approach is prone to triggering of high-frequency transverse combustion instabilities between 3.1 and 3.5 kHz even under typical laboratory-scale near-atmospheric pressure conditions. With the aim of testing the possibility that the high-frequency transverse mode can be controlled by manipulating multislit pattern-driven hydrogen flame interactions, here we investigate self-excited instabilities of four different multislit configurations over a broad range of operating conditions at 30–100 kW thermal power. Experimentally, we demonstrate that high-frequency spinning and standing modes are generated in connection with rotational symmetry, but the resulting modal dynamics and associated thermoacoustic properties vary considerably depending on the injector's spatial arrangement. While multislit injectors consisting of rotationally symmetric circumferential segments tend to promote the development of standing modes with noticeable amplitude modulations, the assembly consisting of rotationally symmetric radial segments prevents the nodal line from strongly locking at a single angular position, therefore undergoing intermittent transitions between standing and spinning modes. Using several probabilistic approaches in conjunction with wave decomposition and phase space analyses, we show that the preferred orientation angle of the (anti-)nodal line is determined by the existence of slight asymmetry and local deformation. When the breaking of rotational symmetry is large enough to produce conspicuously irregular patterns, the growth of symmetry-enforced multislit hydrogen flame dynamics can be completely suppressed. These results provide important insights into the nature of the modal dynamics of lean fully-premixed multislit hydrogen-air flames, and suggest that the observed correlations might be applied to the exploration of the high-frequency transverse instabilities frequently observed in high-pressure combustion systems.