A comparative numerical study of plasma and spark assisted ignition in a cavity-based supersonic combustor

As the flight Mach and maneuverability demands increase, achieving effective combustion and reliable re-ignition becomes increasingly challenging due to inadequate mixing and the significant difference between the residence and ignition delay times. Rapid and efficient ignition in supersonic airflow is thus critical. Non-equilibrium plasma-assisted ignition technology offers substantial advantages, including reduced ignition delay time and enhanced flame propagation speed, improving the ignition reliability of scramjet engines. However, the plasma ignition mechanism in scramjet combustors, particularly the behavior of excited species with varying lifetimes in supersonic flows, remains poorly understood. This study develops a high-fidelity compressible supersonic plasma-assisted ignition solver that incorporates detailed electron collision, excitation reactions, and dynamics of excited species. Large eddy simulation (LES) is employed to model the ignition process of non-equilibrium plasma in a scramjet cavity combustor at a flight Mach of 6 with transverse C₂H₄ injection, compared with spark discharge ignition. The LES results reveal that plasma discharge generates a larger flame kernel with higher OH concentration and improved chemical activity, facilitating faster establishment of a stable cavity flame. In contrast, spark discharge generates high-temperature kernels with lower OH concentration, resulting in a prolonged ignition delay. The species spatial distribution in LES shows that the short-life, highly reactive N₂ electronic states are confined to the discharge zone, while long-life excited species, such as N₂(v1), are transported throughout the cavity. The zero-dimensional (0-D) simulation, using the mixture thermodynamic parameters within the discharge region obtained from LES as initial conditions, reveals that the short-life electronic states generated by plasma create new chemical reaction pathways, including the abstraction of H and O from C₂H₄ and O₂. The electron collision reaction, N₂ electronic states dissociation reaction and C₂H₃ oxidation reaction with O₂ promote O. The oxidation reaction of C₂H₄ with O and the chain branching reaction, H + O₂ = O + OH, are boosted to produce abundant OH, leading to the rapid flame ignition. Outside the discharge zone, the LES result suggests that the long-life N₂ vibrational states have a negligible effect on the development and propagation of the flame kernel. This work offers new insights into the ignition mechanisms of non-equilibrium plasma in supersonic flows and the roles of excited states with varying lifetimes under flow conditions. Novelty and Significance Statement The novelty of this work lies in developing a high-efficiency compressible supersonic plasma-assisted ignition solver, which incorporates detailed electron collision, excitation, and de-excitation reactions. The ignition process of non-equilibrium plasma in a flight Mach-6 scramjet cavity combustor is simulated using LES, and compared with the spark discharge ignition. The study analyzes the interactions between the flow field and temperature, and elucidates the role of excited species with varied lifetime in the propagation and development of the flame kernel in supersonic flows.