Tangential combustion instability and flame dynamics of a shear injector in a model rocket engine combustor

To investigate the tangential combustion instability of coaxial shear injectors, an experimental study on self-excited tangential combustion instability under high-speed and high-temperature inlet conditions was conducted in a model rocket engine combustor. The experiment captured time-series data of oscillating pressures and transient CH* chemiluminescence images of the flame, which were subsequently analyzed to determine the pressure and flame oscillation modes. The results indicate that self-excited tangential combustion instability can be achieved through an off-center injector configuration. The oscillation frequency within the combustion chamber was observed to switch from 2100 to 2700 Hz as the oxidizer velocity increased. Based on the analysis of pressure phase relationships and three-dimensional acoustic simulations, the oscillation modes were identified as 1T1L and 2T0L, respectively. In both cases, the flame exhibited a lifted state, where variations in lift height altered the phase relationship between heat release and acoustic oscillations, thereby influencing the instability conditions. Regardless of the specific tangential oscillation mode, the flame from a single injector consistently exhibits a symmetric longitudinal unstable mode, and the repeated formation and collapse of heat release clusters drive synchronized combustion oscillations.