Suppression of combustion instability in a scramjet engine using unsteady fuel injection

Combustion instability remains a critical barrier to the reliable operation of scramjet engines, especially under high-speed, high-enthalpy conditions. This study investigates the use of unsteady fuel injection as a control strategy to suppress such instabilities in a kerosene-fueled scramjet combustor. Experiments were conducted in a direct-connect supersonic combustion facility at Mach 3.0, with fuel injection modulated at frequencies (171, 216, and 260 Hz) and equivalence ratios of 0.5 and 0.6. High-speed chemiluminescence imaging and pressure transducer measurements were employed to analyze flame dynamics and pressure oscillations. Results demonstrate that unsteady injection at 216 Hz, combined with an equivalence ratio of 0.6, induces a transition from an unstable mode to a stable ram mode, significantly suppressing pressure fluctuations and enhancing combustion intensity. In contrast, unsteady injection at lower equivalence ratios or non-optimal frequencies failed to mitigate instabilities and, in some cases, exacerbated them. To interpret these findings, a reduced-order model (ROM) was developed based on dynamic coupling and time-delay effects between the fuel supply and combustion zones. The ROM qualitatively predicts system stability by evaluating eigenvalues derived from experimental conditions and shows agreement with observed instability trends.