Transverse combustion instabilities observed in a multi-element oxidizer-rich staged combustion (ORSC) cycle based model rocket combustor operating at 1.15 MPa are presented. An oxygen-hydrogen preburner feeds warm gaseous oxygen to the oxygen manifold where it is injected through a linear array of nine oxidizer-centered shear coaxial injectors into a rectangular combustion chamber. Optical accessibility is provided for two injector elements at an end wall and three in the center of the chamber. Simultaneous 100 kHz CH*-chemiluminescence and schlieren measurements along-with co-located high-frequency pressure measurements are used to analyze the thermoacoustic behavior of the behavior. A representative test case where a transition from low amplitude (≈9.4%p'/pc) instabilities to a high-amplitude limit cycle (≈ 100%p'/pc) is presented. Analysis of the chemiluminescence and schlieren measurements highlight presence of an axial oscillation near the center injector of the chamber at 1300 Hz resulting in lifted flames at low amplitudes. Flame oscillation during this period is characterized by asymmetric varicose vortex shedding at the fundamental transverse (1T) frequency of the chamber. As the chamber pressure transitions to a high amplitude, the steep fronted pressure wave distorts the flame in a near momentary jump and spends a majority of period before the arrival of the next wave in slow relaxation. Due to an increase in the speed of sound during the compression phase, the compression side catches up with the expansion side of the pressure wave. This wave steepening also results in an abrupt increase in the amplitude of the pressure oscillation, alongwith an approximately 15% shift in mean chamber pressure and 1T frequency.
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