Gas property measurements in a supersonic combustor using nanosecond gated laser-induced breakdown spectroscopy with direct spectrum matching

Gas density and fuel mole fraction in a cavity flameholder of a supersonic combustor with ethylene fueling were simultaneously assessed using nanosecond gated laser-induced breakdown spectroscopy. Emission spectra from the laser-induced plasma over at 550-830 nm containing multiple emission lines of O H N and C were captured and used to quantify the gas conditions. Fuel mole fraction and gas density were mapped within a cavity flameholder on planes parallel to the Mach 2 freestream flow above the cavity. A high fuel mole fraction region appeared 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 was almost proportional to fueling rate while the gas density was unaffected by the fueling rate under the typical operating conditions.