On electrode placement in plasma-assisted ignition of a scramjet flame-holder

A multi-scale model for plasma-assisted combustion is developed to investigate how the location of the electrodes in the cavity affects the ignition of supersonic flows in nanosecond repetitive pulse discharges. A new approach to plasma-fluid coupling is investigated that relies on solving the detailed plasma and photon transport equations on a near-electrode block partition of the overall mesh during the pulse and synchronizing the thermochemical balances with the reactive-fluid mesh by interpolation. The approach reproduces experimental observations of assisted ignition: the formation of trailing-edge flames in high-enthalpy conditions, the formation of localized ignition kernels near the cathode for medium-enthalpy conditions, and the presence of a distributed region of elevated OH mass fraction for conditions leading to no ignition. The approach matches the experimental measurement of plasma-energy coupling. The analysis emphasizes the significance of fluid strain rate in plasma-fluid coupling. The location of the electrode is found to affect ignition by supporting a larger radical turnover by plasma when the electrodes are placed in regions of lower strain, leading to a thicker reaction region. Novelty, Significance, and Contributions : A novel computational approach to plasma-gas coupling is developed and validated. This approach was applied to investigate the influence of strain rate on the focusing of pre-ionization electrons in the low-shear region of cavity stabilizers. The authors identify a correlation between strain rate and radical turnover number. This study led to the determination of the contribution of electrode placement to the efficacy of plasma actuation in supersonic flame-holders. The importance of the cathode location to supersonic ignition is investigated for the first time in detail. This research presents a significant advancement of previously published works: it models photoionization from first principles and includes the gas-plasma interactions in a three-dimensional turbulent flow.