Flame visualization and spectral analysis of combustion instability in a premixed methane/air-fueled micro-combustor

Abstract

The present study experimentally explores the combustion instability characteristics of premixed methane/air flames in a planar micro-combustor by utilizing the high-speed camera and the noise data acquisition system. It aims to identify the sound pressure fluctuation of unstable flames and shed light on the relationship between flame structure and thermoacoustic instability under micro-scale conditions. Four types of flame structures, i.e., flame with repetitive extinction and ignition (FREI), weak flame, steady flame, and oscillating flame, are experimentally observed by varying operating parameters. Among them, the weak flame at low flow velocities and the oscillating flame at high flow velocities form thermoacoustic oscillation. For weak flames, the collision with the wall and the local deformation of the flame result in high-frequency harmonics. Varying the inlet velocity from 0.14 m/s to 0.15 m/s leads to the pressure fluctuation of the weak flame being reduced by 10 Pa, and the dominant frequency being changed from 125 Hz to 167 Hz. As for oscillating flame, the variation of the inlet velocity plays a decisive role in the sound pressure fluctuation. At the equivalence ratio of 0.7, an increase of 0.01 m/s in the inlet velocity reduces the pressure fluctuation by 4 Pa.