Effects of Downstream Duct Length and Entropy Reflection on Self-excited Combustion Oscillations in a Twin-Stream Afterburner

Self-excited combustion oscillations, arising from the interaction between unsteady heat release and acoustic perturbations, can significantly undermine the stability of propulsion and power systems. Accurate prediction of their onset and nonlinear behavior is thus crucial. This study presents a theoretical investigation into the effects of downstream duct length and entropy reflection on combustion instability within a twin-stream afterburner featuring one open inlet, one choked inlet, and one choked outlet. Acoustic disturbances are modeled using a wave-based approach, while flame dynamics are represented via an empirically derived second-order lag law. The total heat release is assumed to be a linear combination of contributions from both supply streams. Unlike single-stream systems that typically exhibit periodic oscillations, twin-stream configurations display quasiperiodic behavior due to distinct acoustic modes in each stream. Reduced entropy attenuation raises the fundamental frequency but diminishes the effective oscillation amplitude. Extending the downstream duct further suppresses amplitude and lowers frequency. High entropy reflection can induce a transition from quasiperiodic to periodic oscillations through a saddle-node bifurcation. The influence of entropy reflection on amplitude is strongly dependent on duct length, highlighting the dual role of entropy waves in damping combustion instability and facilitating mode synchronization via coupled geometric and thermal mechanisms.