Experiments on Plasma-Assisted Combustion in a Supersonic Flow: Optimization of Plasma Position in Relation to the Fuel Injector

The results of an experimental study of plasma-induced ethylene ignition and flameholding in a supersonic model combustor are presented in the paper. The experimental combustor has a cross-section of 72 mm (width) x 60 mm (height) and length of 600 mm. The fuel is directly injected into the supersonic airflow through wall orifices. The flow parameters are: Mach number M=2, static pressure P_st=160-250 Torr, stagnation temperature T₀=300 K and total air flow rate G_air≤0.9 kg/s. The near-surface quasi-DC electrical discharge is generated by a series of flush-mounted electrodes, providing an electrical power deposition of W_pl=3-24 kW. The scope of the experiments includes the characterization of the discharge interacting with the main flow and fuel injection jet, a parametric study of ignition and flame front dynamics and a comparison of three plasma generation schemes: the first two examine upstream and downstream locations of plasma generators in relation to the fuel injectors. The third pattern follows a novel approach of a combined mixing/ignition technique, where the electrical discharge is distributed along the fuel jet, starting within the fuel injector. The last pattern demonstrates a significant advantage in terms of the flameholding limit. The experiments are supported by gas temperature and H₂O vapor concentration measurements by Tunable Diode-Laser Absorption Spectroscopy (TDLAS). The technique studied in this work has weighty potential for high-speed combustion applications, including cold start/restart of scramjet engines and support of the transition regime in a dual-mode scramjet and during off-design operation.