Use of laser-induced fluorescence of OH to study the perturbation of a flame by a probe

Abstract

The paper presents the first use of laser-induced fluorescence (LIF) of OH to overcome a problem irresolvable otherwise (owing to the spatial resolution needed). Previous work had shown a discrepancy between OH concentration profiles in a low pressure flame measured by mass spectroscopy and optical absorption, especially in the preheated zone. The first part of this work is devoted to a remake of the measurement by absorption of a pulsed dye laser beam tuned on a transition of OH, and to a LIF measurement of the ‖OH‖ profile. They both confirm the previous conventional absorption measurements. A detailed study by LIF of ‖OH‖ perturbation by the probe is then presented. It leads to the following conclusions: The quartz cone of a mass spectrometer sampling probe reduces the OH concentration in the preheating zone of the low pressure O 2 −C 3 H 8 , flame investigated. The extension of the perturbation is comparable to the probe penetration into the reactive zone of the flame. Therefore, the effect is attributed to a perturbation of the diffusion field by the probe. The OH concentration measured by mass spectrometry is the actual value in the sample. As predicted by the literature, the extension of the sampling zone is two diameters of the probe nozzle.