Abstract
In a partially premixed methane/air flame, temperature and CO concentration measurements were conducted using coherent anti-Stokes Raman spectroscopy. Additionally, qualitative two-dimensional OH laser-induced fluorescence imaging was applied. Measurements were performed at different positions downstream of the burner's exit for equivalence ratios (φ) between 1.7 and 4.0. A distinct double-flame structure was observed. Accurate temperature and CO concentration results were achieved, even under sooting conditions. Temperature measurements close to the burner (5 mm downstream) indicated a temperature peak of 2000 K while further downstream at 10 and 20 mm a high-temperature plateau could be found. The CO concentration increases with decreasing φ starting below a particular threshold value. It was a maximum at the inner flame due to the formation of CO as a result of partial combustion. The maximum CO is found to be greater than that in a diffusion flame. The maximum CO concentration has been found for φ = 1.7, the upper ignition limit for CH4/air mixtures. OH measurements indicate that OH is mainly formed in the outer flame and is consumed there by reactions with CO and H2 diffusing from the inner flame.
The authors gratefully acknowledge financial support of parts of this work by Deutsche Forschungsgemeinschaft and by Volkswagen-Stiftung. Alexander von Humboldt Stiftung, Germany, supported A. Datta for his research stay at LTT-Erlangen where this work was conducted. The LIF measurements were performed by Volker Wagner.