Experimental Investigation of a Ramjet Combustor Using High-Speed OH* Chemiluminescence Imaging

Simultaneous wall-pressure and high-speed OH* chemiluminescence measurements were performed in an optically accessible, square-section, dual-side-inlet dump ramjet combustor burning compressed natural gas injected into the inlet arms, at four choked operating conditions. The pressure spectrum is dominated by a sustained, amplitude-limited oscillation at 2264–2417 Hz, which lies within 1–6% of the estimated first transverse eigenfrequency, scales with the equivalence ratio as an acoustic eigenfrequency does and is attributed to the first transverse mode of the chamber. Pressure-referenced coherence and phase maps and spectral proper orthogonal decomposition of the line-of-sight-integrated OH* signal show that the response at this frequency is a symmetric modulation of the brightness downstream of the jet impingement, in anti-phase with the wall pressure, accompanied by a weaker antisymmetric displacement of the flame 22–70 Hz below the peak; the mode is therefore inferred to be oriented along the line of sight, with a weaker companion mode along the inlet direction. Low-frequency activity at 350–410 Hz, close to the estimated first longitudinal mode, is a symmetric standing oscillation of the impingement region that leads the wall pressure. No spatially coherent jet-on-jet flapping is identified, whereas a symmetric OH* structure generated at the jet impingement and convected downstream at 581–636 Hz accompanies a weak pressure band in all tests; its origin is not established. Compared with the kerosene-fueled combustor of the same family studied by Boulal et al. (Combust. Flame 283, 114,535, 2026), the transverse resonance is common to both, whereas the jet-on-jet-mediated couplings reported there are not found.