Investigation of Flame Response in a Swirling Micromix Hydrogen–Methane Combustor

Hydrogen-enriched natural gas combustion is a hot topic in industrial and academic communities due to the need for carbon emission reduction. However, thermoacoustic instability poses a major challenge for lean combustion development, particularly the higher frequency combustion oscillations. This study investigates the flame response in micromix hydrogen/methane flames within a low to medium frequency range of 50–1200 Hz, revealing a critical mixing ratio. Above this threshold, the flame response is dominated by hydrogen combustion, whereas below it, methane combustion takes precedence. Overall speaking, the hydrogen addition significantly enlarges the low-pass filter limit of the flame transfer function (FTF). Simultaneous OH* chemiluminescence and particle image velocimetry (PIV) experiments demonstrate that the gain is associated with the flame size and the number of acoustic-induced vortices on the flame surface. Higher frequency acoustic forcing leads to flame responses out of phase at different longitudinal positions, resulting in a low global flame response. Hydrogen addition reduces the flame length and enhances the FTF gain at specific frequencies.