Measurements of temperature and concentration of nitrogen monoxide in flames by LIF imaging spectroscopy with standard addition method

An NO-LIF imaging spectroscopy of A²∑⁺ → X²∏ (0, 0) excitation (225-226 nm) and A²∑ ⁺ → X²∏ (0,2) emission (246-248 nm) was applied to methane-air flames with equivalence ratio of 0.8, 1.0, 1.2, and a methane diffusion flame for measuring mole fractions of nitrogen monoxide (NO) and temperature. The mole fractions were determined comparing the NO-LIF intensities of non NO-seeded and NO-seeded cases. Temperatures in flames were also obtained using two-line method. Nitrogen monoxide was doped in both burner nozzle flow and surrounding air co-flow to enable two-dimensional measurements of mole fraction in the flames. The results of NO mole fraction images which should be closely correlated to the obtained temperature distributions showed good agreement with the well known NO formation mechanisms. Narrow-band filters for removing non-resonant lights were not enough to remove the strong Mie scattering of laser light by soot in the diffusion flame. Introduction of an imaging spectrometer provided high signal-to-noise ratio of NO-LIF imaging. The NO-LIF acquisition technique for the diffusion flame also revealed that NO A ²∑⁺ ← X² ∏ (0, 0) excitation was a good choice to capture strong NO-LIF while avoiding nonresonant lights such as LIF of soot precursors and Laser Induced Incandescence (LII) of soot.