Planar temperature imaging using thermally assisted laser induced fluorescence of OH in a methane–air flame

The present study seeks to demonstrate the use of vibrational thermally assisted laser induced fluorescence of the hydroxyl radical to obtain planar temperature measurements. This technique utilizes a simple two-level vibrational model to describe the relationship between the population ratio of excited states and temperature. The quenching to vibrational transfer ratio between the excited states was used as a calibration parameter to fit the thermally assisted fluorescence measurements to known temperature data. The measurements presented here are from a premixed methane–air flame. A rectangular shaped burner allowed for calibration and comparison of the thermally assisted temperature results with sodium line reversal data available in literature. Excellent agreement between the two approaches was achieved for three different equivalence ratios. A single calibration was sufficient for the range of conditions tested in the present work. Two detection schemes were also tested, the first using the (0–0) and (1–0) vibrational bands and the second substituting the (0–1) fluorescence in place of the (0–0) band. The weakness of the fluorescence signal from the (0–1) band was very restrictive to temperature imaging with the current setup. Overall, the applicability of the thermally assisted technique to temperature imaging was positively demonstrated from this work.