Two-dimensional quenching lifetime measurement of OH:A ² Σ ⁺(υ′ = 1) and NO:A ² Σ(υ′ = 0) in atmospheric-pressure flames

Abstract

Planar laser-induced fluorescence is currently widely applied to research on combustion. However, conventional approaches for semi-quantitative measurement could not provide the satisfactory solution and the problem due to collisional quenching remains to be solved. This paper describes the two-dimensional (2D) quenching-time-constant measurement of OH:A ² Σ ⁺(υ′ = 1) and NO:A ² Σ(υ′ = 0) in flame cross sections at atmospheric pressure. These measurements involve 1–2 ns decay time of the excited state using a high-speed image intensifier and a tunable laser with a pulse duration of 3 ns at FWHM. The correlation factors of the exponential fits for the fluorescence decays after the laser pulse were larger than 0.999 in all experiments presented in this study. Furthermore, the measured Stern-Volmer plots of the quenching rate at 1.0, 1/2, 1/3, and 1/4 atm pressure using the same experimental apparatus was confirmed to have a linear relationship for both OH and NO, showing that the 2D decay-time measurements of 1–2 ns have been successful. For NO:A ² Σ(υ′ = 0), the obtained quenching rates inside the inner cone and in the outer flame in the NO-seeded methane-air Bunsen flame were 8.7 × 10⁸ and 7.8 × 10⁸ s⁻¹, respectively, and for OH:A ² Σ(υ′ = 1), the obtained quenching rate mapping in the outer flame was around 5.6 × 10⁸ s⁻¹ in the methane-air flame.