Flame stabilization enhancement in a supersonic combustor by multi-scale cavities combination

This study investigates an enhanced passive control strategy for suppressing combustion oscillations in scramjet engines by introducing an auxiliary small cavity upstream of the primary flameholder. Experimental tests conducted on a direct-connect rig with Mach 3 inflow demonstrate that the staged-cavity design achieves remarkable stability improvements across equivalence ratios (Φ = 0.34–1.2). Key findings include: (1) 48.9% and 47.2% reduction in pressure oscillations at Φ = 1.2 and 0.6, respectively, with the mean normalized pressure increasing by approximately 8.57%; (2) Expansion of flammability limits (lean ignition at Φ = 0.34 vs baseline Φ = 0.48); (3) 81.2% suppression of flame front fluctuations through standing flame anchoring. Mechanistic analysis reveals five synergistic effects: (i) pre-combustion in the auxiliary cavity decentralizes heat release, (ii) the small cavity's shear layer sustains a persistent pilot flame without additional injectors, (iii) boundary layer stabilization prevents separation-induced oscillations, (iv) enhanced fuel–air mixing creates localized premixed zones within the diffusion flame, and (v) shock restructuring maintains flame anchoring despite pressure variations. High-speed imaging and schlieren visualization confirm the design's ability to maintain stable jet-like flames while eliminating chaotic combustion modes observed in conventional configurations. The auxiliary cavity approach demonstrates simultaneous improvements in combustion stability (oscillation suppression), performance (thrust increase), and operational envelope (extended ignition limits), providing a practical passive control solution for scramjet combustors. Residual low-frequency fluctuations (4%–5% amplitude) suggest future optimization opportunities in cavity-flowfield integration. These findings establish staged cavity combustion as an effective strategy for next-generation hypersonic propulsion systems.