Accurate thermometry using NO and OH laser-induced fluorescence in an atmospheric pressure flame (Checked by narrow-band N2 coherent anti-stokes Raman scattering)

Abstract

Planar laser-induced fluorescence (PLIF) using OH and NO facilitates non-contact two-dimensional temperature measurements, and is therefore expected to be an effective technique for combustion thermometry. However, the accuracy of the measurement has not been clarified. The relaxation in the upper rotational levels of the tracer molecule is not easy to be predicted, and the rotational dependence of the molecular constants, such as absorption and emission coefficient, has not been fully clarified. Furthermore, it is not confirmed whether the rotational distribution of the tracer molecule maintains the quasi-equilibrium states or not. In this study, we investigated the accuracy of thermometry by means of laser-induced fluorescence (LIF) using NO: A - X (0, 0) and OH: A - X (3, 0) excitation for a premixed methane -air laminar flame in atmospheric pressure as a measuring target. These were checked by narrow-band N2 CARS with high spectral resolution, which is believed to be the most reliable temperature standard in flames. NO LIF temperature showed several % lower temperature than N2 CARS one, and OH LIF temperature 30% lower. Both methods were calibrated under exponential dependence assumption of molecular constants, and the calibrated temperature of NO and OH LIF agreed well with N2 CARS one. Furthermore, two-dimensional thermometry in a flame cross section using calibrated NO PLIF was demonstrated.