Laser-induced fluorescence measurements of NO and OH mole fraction in fuel-lean, high-pressure (1–10 atm) methane flames: Fluorescence modeling and experimental validation

A method for quantifying laser-induced fluorescence (LIF) signals and planar laser-induced fluorescence (PLIF) images of the OH and NO radicals in high pressure flames is presented. The fluorescence signal per unit radical mole fraction is modeled as a function of temperature, pressure, overall flame stoichiometry and laser spectral bandwidth. A recently developed model (JQSRT, 51, 511; Appl. Phys. B, 57, 249)1,2 for electronic quenching cross-sections of OH and NO is utilized to express the fluorescence yield as a function of these parameters. The models are confirmed using single-point measurements in the burnt gas region of a flat flame burner at up to 10 atm. The measurements are performed at points in the flame where the temperature, pressure, OH and NO mole fraction are all known. For fuel-lean flames at elevated pressure, interference from the O2 Schumann-Runge system was found with NO A ← X(0, 0) fluorescence measurements. This interference must be considered when selecting an appropriate NO transition in this type of environment.