Experimental study of Mach-dependent combustion instabilities and associated performance variations in a combined-cycle scramjet

Stable combustion across wide flight envelopes is required for robust wide-range scramjet development. However, current research is often limited to narrow Mach ranges. The coupling between flight attitudes and unsteady combustion modes is not yet fully understood. This study addresses these issues through an experiment of a 20 kg class combined-cycle scramjet across flight Mach numbers 3.0–7.0 and varying angles of attack. It establishes a physics-based experimental benchmark for interrogating Mach-dependent combustion instabilities, spectral mode transitions, and flow–combustion coupling mechanisms in the flow of combined-cycle scramjet. The results show that the equivalence ratio of flameout decreases monotonically with the increase in Mach number. It indicates enhanced autoignition propensity at higher inflow total temperatures. At zero angle of attack, the flameout threshold decreases from 0.8 at Mach 3.0 to 0.3 at Mach 6.0–7.0. The positive angle of attack consistently extends the flame-stabilization threshold. Pressure oscillation characteristics exhibit clear Mach-dependent evolution. At Mach 3.0–4.0, the flow is dominated by a low-frequency mode near 130 Hz, with rocket jet forcing intensifying oscillation amplitudes and broadening the spectral bandwidth. At Mach 5.0, a distinct bimodal structure emerges with dominant peaks at ∼110 and 223 Hz. At Mach 6.0–7.0, pressure oscillations transition to pronounced broadband behavior, accompanied by substantial weakening of harmonic coupling between the isolator and combustor. Under high-Mach and large-angle-of-attack conditions, pressure dynamics are governed primarily by combustor-intrinsic sources. Additionally, steady-state propulsion performance peaks near Mach 5.0, where favorable aero-thermodynamic matching yields a combustion efficiency of 0.95, a net thrust of 8.54 kN, and a specific impulse of 1498 s. Performance deteriorates rapidly as Mach number approaches 7.0, despite the increase in inflow total temperature.