This paper presents a phenomenological investigation of a new pattern of lean-premixed hydrogen-flame and flowfield dynamics under thermoacoustic instabilities (TIs) observed inside a low-swirl combustor (LSC) with two combustion chamber lengths of 300 and 500 mm. In this study, the initial bulk inlet velocity was set to V m = 15 m/s. The fuel flow rate was linearly increased while the air flow rate was kept constant, causing a transition from stable operation to TI. Simultaneous time-resolved electronically excited hydroxyl radical (OH *) chemiluminescence imaging and two-dimensional particle image velocimetry were performed along with pressure-fluctuation measurements to investigate spatiotemporal relationships between fluctuating quantities, i.e., OH * chemiluminescence, velocity fields, and pressure, before and after TI onset. A typical TI pattern characterized by a lifted, inverted conical flame caused by diverging, decelerating flow near the injector exit was observed in the long combustor. In contrast, the flame/flow dynamics pattern under TI in the short combustor had strong pressure oscillations and anomalous LSC flame structures which were characterized by a radially flat flame region and counter-rotating vortex pair (CVP) flame structures. In this case, the radial profile of the axial velocity assumed an atypical top-hat-like shape. This feature resulted in thermoacoustic coupling that satisfied the Rayleigh criterion over an almost entirely flat flame region and further strengthened the TI. Thermal expansions of CVP structures served as "converging nozzles" constricting the central flow region. Periodic appearances of this Venturi-like effect caused an alternating diverging (decelerating) and converging (accelerating) flowfield, with a significant peak-to-peak velocity-oscillation amplitude of 15 m/s, equivalent to V m . These pioneering observations regarding LSCs provide further insights into TI mechanisms in hydrogen-fueled gas turbine combustors.
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