PLIF study of arc discharge localization in mixing layer of high-speed flow

Demand for hypersonic airbreathing propulsion systems has led to intense study of the air-fuel mixing and ignition dynamics in high-speed combustors. In this work the plasma injection module (PIM), coupling an arc discharge (Quasi-DC) to a fuel jet, is used for ignition. Prior results demonstrated a significant extension of flameholding limits using PIMs, yet mechanisms yielding this effect need further study. This work focuses on one of the key aspects of PIM-related physics: longitudinal arc discharge localization in a fuel/air mixing layer. The PLIF technique along with high-speed visualization and optical emission spectroscopy were employed in this study. Typical test parameters were: flow M=2, P0=1.6-3.2bar, T0=300-750K, plasma filament length up to 100mm at current I=3-5A and gap voltage U=0.6-1.5kV with translational temperature Tg=3.5-6kK. Experimental data and analysis of the plasma composition/electrical conductivity indicate that, under favorable test geometry, the constricted plasma strives to locate within the mixing layer and not in the fuel jet or in the core airflow. The mechanism of arc plasma localization is considered as a trade-off between electrical and thermal conductivities in a carbon-containing plasma with a gradient concentration of the components.