Detection of trace NO concentrations using 1-D NO-LIF imaging

A measurement technique for trace nitric oxide (NO) concentrations in high pressure flames is investigated using 1-D spectrally-resolved imaging via laser-induced fluorescence (LIF). Experiments are performed in premixed methane and argon/oxygen flames with trace amounts of seeded NO (15 to 50ppm) and pressures between 10 to 60bar with an equivalence ratio of 0.9. LIF signals are dispersed with a spectrometer and recorded on a 2-D intensified CCD array yielding spectral resolution on the abscissa and 1-D spatial information on the ordinate. This enables isolation of NO LIF from the interference signals of other species (mainly hot O₂ and CO₂) while providing spatial resolution along a line. A fast algorithm is developed to evaluate pulse-by-pulse NO concentration from these images. To determine the detection limits for NO concentration and measurement precision, statistical analysis is used to characterize signal-to-noise ratio and shot-toshot fluctuations as a function of laser energy. Results show a single-shot detection limit of <10ppm for all pressure levels for pulse energies extrapolated to ∼16mJ/pulse. The measurement scheme developed here can provide a tool for high-pressure flame chemistry studies and for realtime monitoring of trace NO production in combustion systems.