Numerical investigations on the beating behavior of self-excited combustion instability in a hydrogen-fueled Rijke type combustor

Abstract

Beating, a specific oscillation mode of combustion instability, occurred in a premixed hydrogen fueled Rijke tube was investigated. A two-dimensional transient numerical model for the self-excited combustion instability was developed and well validated. In a beating cycle, the amplitude of pressure oscillations increased to 1400 Pa and then decreased to 70 Pa, exhibiting an obvious feature of low frequency modulation. The joint time-frequency analysis showed that the dominant frequency shifted from 230 Hz to 330 Hz over time at each cycle. To shed lights on the inherent mechanisms of beating instability, the coupling processes between the combustion and flow dynamics were investigated. At each beating cycle, the shapes of flame were relatively stable in growing stage, while strongly fluctuated in transition stage and recovered to stable state in decaying stage. Notably, due to the large amplitude of pressure oscillations, an obvious recirculation zone was formed around the flame and sucked the high temperature exhausted gas to the flame root during the transition stage. Accordingly, the high temperature area was formed around the flame and affected the oscillation mode. By employing the proper orthogonal decomposition analysis, the apparent oscillations at 1 Hz were observed in the modes of heat release rate, velocity vectors and temperature. Later, it was found that with the increase of the amplitude of pressure oscillations, the mass flow rate of air supply at tube inlet dramatically decreased, which reduced the oxygen around the flame and changed the fluctuation mode of heat release rate. The coupling process was also identified by the phase difference and Rayleigh index.