Optical Spectroscopic Characterization of Plasma Enhanced Fueling and Supersonic Combustion

Effective mixing, ignition, and flameholding in a supersonic flow is an outstanding challenge critical for efficient operation of airbreathing supersonic combustion engines. Development and maintenance of these processes in high-speed flows is difficult due to compressibility effects, short residence times, and the complex reactive environment. In previous works, the Plasma Injection Module (PIM) has extended the operational envelope of a supersonic combustor to the unfavorable conditions of low stagnation temperature and pressure. However many details have remained unclear, including the mechanisms responsible for this effect and operational limitations in practical engine schemes.

This work applies a suite of optical diagnostic techniques to characterize the physical processes concomitant with plasma-enhanced mixing, ignition, and flameholding in supersonic flow. Fuel distributions, plasma parameters, and flameholding dynamics of the PIM system are studied using acetone planar laser-induced fluorescence (PLIF), optical emission spectroscopy (OES), Mie scattering, filtered chemiluminescent photo-sensing, and supporting instrumentation. This was performed in both a plane wall duct, and a cavity equipped geometry at the SBR-50 supersonic blowdown tunnel at the University of Notre Dame. Tests were performed at M = 2 and with typical conditions P0 = 1-3bar, T0 = 300-450K, and fuel jet momentum flux ratios J = 0.05-1.25.

Plasma impact on transverse jet injection is shown to significantly increase jet penetration depth and cross-sectional area. Data are presented for both non-reacting and reacting injected jets, to isolate the effect of plasma or interrogate the additional effect of plasma-stimulated chemical heat release. Extended downstream chemical reactions in fuel resulted in increased jet penetration and crossflow expansion over non-reacting cases.

Three PIMs were applied to study the effect of bulk combustion on plasma parameters and PIM ignition capabilities. Flow separation at strong combustion is shown to significantly modify PIM characteristics. Coupling of PIM characteristics to the flow pattern produces dynamic instabilities in the flowfield. Acetone PLIF characterized the unburned fuel pattern at these instabilities, revealing the coupling between chemical power, plasma power, and flow structures. A filtered chemiluminescent sensing technique is shown to detect modifications to the combustion mode using the ratio of radical species OH* and C2*.

This technique has significant potential to be employed in practical systems for combustion blow-off/flashback prediction and prevention through closed-loop control.