Laser-induced breakdown; Scramjet; Cavity flameholder; Gated LIBS;REACTING FLOWS; INDUCED PLASMA; AIR; SPARK; CAVITY; MIXTURES; IGNITION; DENSITY; FLAMES; LIBS

Gas density and fuel mole fraction in a cavity flameholder of a supersonic combustor with ethylene (C2H4) fueling are simultaneously measured using nanosecond gated (10 ns) laser-induced breakdown spectroscopy (n-LIBS). Emission spectra from the laser-induced plasma over a range (550 nm-830 nm) containing multiple emission lines of O, H, N, and C are captured and used to quantify the gas conditions. A direct spectrum matching (DSM) process is employed for the gas property quantification in which an unknown spectrum is matched to a spectrum from a database, constructed through n-LIBS analysis of well-known gas conditions. A back-scattering collection method is implemented to allow for measurements using a single optical access point. Fuel mole fraction and gas density are mapped within a cavity flameholder on planes parallel to the Mach 2 freestream flow above the cavity. With C2H4 injection from the closeout ramp of the cavity toward the front step, a high fuel mole fraction region appears near the front step where density and flow velocity are low, and thus favorable for cavity flame ignition. The fuel mole fraction at a location in the cavity is found to be nearly proportional to fueling rate; the gas density, on the other hand, is not affected by the fueling rate, under the typical operating conditions. (C) 2016 by The Combustion Institute. Published by Elsevier Inc.