In Situ Laser-Induced Fluorescence and Ex Situ Cavity Ring-Down Spectroscopy Applied to NO Measurement in Flames: Microprobe Perturbation and Absolute Quantification

Abstract

Flame-sampling experiments allow for describing the species profiles as a function of the height above the burner. Even with microprobes as those generally used for gas chromatography measurements, the perturbations induced by the microprobe can be non-negligible. Not only will the heat losses as a result of its presence affect the temperature and the spatial position of the species profiles, but the composition of gas sampling may also be altered. To attempt to clarify the induced perturbations by microprobes, NO species profiles are obtained in stoichiometric low-pressure flames using either in situ laser-induced fluorescence (LIF) or ex situ absorption measurements. Gas samplings are performed with two uncooled quartz microprobes characterized by their tip angle (6.6° and 16°) and analyzed in an external absorption cell by cavity ring-down spectroscopy (CRDS). The absolute quantification of the NO mole fractions is undertaken in the burned gas using either the standard addition method by LIF or after gas sampling from the integrated spectral absorptivity by CRDS. In the burned gas, it turns out that the gas sampling is altered by the microprobe having the smallest tip angle, where the absolute mole fraction of NO is found 40% lower than when the quantification is performed either after gas sampling with the largest tip angle microprobe or by in situ LIF. As expected, the comparison of the relative NO profiles along the flame indicates that the profiles measured after gas sampling are shifted downstream. This is all the more so that the tip angle is large and the flame is clearly attached. Kinetic simulations based on the probe geometry and the residence time do not allow us to discern possible homo- or heterogeneous reactions inside the probe. The most disturbing factor is attributed to a cooling effect as a result of the probe