Hydrogen-assisted flame stabilization characteristics and oscillation mode transition mechanism in ramjet mode of RBCC engine

The combustion modes and oscillation characteristics in the ramjet mode of a rocket-based combined cycle (RBCC) engine are critical for reliable operation under flight Mach 3 inflow conditions. However, the coupling mechanisms between the equivalence ratio and flame stability remains unclear. A high-enthalpy inflow condition with a Mach number of 1.43 and a total temperature of 605 K was established on a direct-connected experimental platform. Comparative experiments and numerical simulations were conducted for four total equivalence ratios ( ϕ = 0.4, 0.5, 0.6, 0.7). Ethylene combustion was assisted by a hydrogen micro-jet injected into the cavity. Synchronized high-frequency pressure measurements, OH*/CH* chemiluminescence, and particle image velocimetry (PIV) techniques were employed. Results show the following key findings. First, as the total equivalence ratio increases from 0.4 to 0.7, the peak pressure in the combustor rises by 62.5%. This pressure rise drives the shock train upstream to expansion section. Consequently, the flame stabilization mode transitions from the supersonic cavity shear-layer mode to subsonic lifted shear-layer mode. Second, affected by multiple heat release factors including the main combustion zone, rocket nozzle exit recirculation zone, and throat thermal choking, combustion oscillations exhibit significant frequency domain transition. The dominant oscillation shifts from low-frequency small-scale pulsations (< 100 Hz) to high-frequency large-scale chaotic oscillations (> 200 Hz). Third, the high chemical activity of the hydrogen micro-jet ensures combustion stability even at high equivalence ratio condition. Fourth, the increased equivalence ratio enhances the fuel jet penetration depth, which shifts the premixed heat release core upstream by 46% compared to the ϕ = 0.4 condition. Furthermore, the thermal choking effect significantly improves the spatial concentration of energy release. These findings provide critical physical criteria and experimental support for wide-range combustion organization in hypersonic combined propulsion systems.