Laser-Saturated Fluorescence Measurements of OH in Atmospheric Pressure CH₄/O₂/N₂ Flames Under Sooting and Non-Sooting Conditions

Abstract

OH concentration measurements have been performed in rich, atmospheric pressure CH₄/O₂/N₂ flames using laser-saturated fluorescence. Both sooting and non-sooting conditions have been investigated. OH is thought to be an important species in soot chemistry because of its role in the oxidation of soot and soot precursors. In the sooting flame, the centerline axial OH concentration drops sharply 3-4 mm above the burner surface. Visible soot emission begins only after the OH concentration decreases significantly from its peak value in the flame zone. Rotational Raman scattering was identified as a potentially significant interference in the fluorescence measurements at the low OH concentration levels (down to 5 ppm) typical of the rich flames.

Laser-saturated fluorescence and optical absorption measurements of OH concentration were also performed in a lean, atmospheric pressure CH₄/O₂/N₂ flame. A combination fluorescence-absorption calibration in the lean flame was used to determine absolute OH number densities from fluorescence measurements in the rich flame, where absorption techniques were inapplicable. Temperature measurements by two-line laser-saturated OH fluorescence in the lean flame showed excellent agreement with OH absorption and corrected thermocopule temperature measurements.