On the spatial characteristics of the flame transfer function for a GCH4-GO2 swirl coaxial flame

Abstract

In this study, the combustion instability characteristics of gaseous methane–gaseous oxygen atmospheric flame were investigated. A swirl coaxial injector was used under external modulation with a central methane flow and a loudspeaker. The flame transfer function with a photomultiplier tube was examined for possible errors, and an injector transfer function was defined to study the influence of the input perturbation on the cold gas flow without combustion. Both the results of flame transfer function and injector transfer function were affected by acoustic resonance. However, a higher mode of resonance did not appear in the flame transfer function. To analyze this difference, the flame movement was visualized using a high-speed camera by capturing the OH* chemiluminescence signal. Furthermore, a local flame transfer function was defined to examine the flame response zone-by-zone through a light signal. A higher-mode reaction appeared again in the local flame transfer function. It was found that flame separation due to modulation was the cause of the difference among the flame transfer function, injector transfer function, and local flame transfer function. By capturing the entire flame, its periodicity could be lost; hence, the local flame transfer function could prevent misinterpretation of the flame fluctuation.