THE EFFECT OF HYDROGEN ENRICHMENT ON THE THERMOACOUSTIC MODES COUPLING FOR CONICAL METHANE/AIR PREMIXED FLAMES

Hydrogen enrichment flames exhibit faster burning speeds and greater sensitivity to high frequency acoustic perturbations, leading to that systems utilizing hydrogen as fuels more prone to exciting multiple unstable modes. This paper investigates the thermoacoustic instability and modes coupling in hydrogen enrichment premixed laminar methane/air conical flames within a Rijke tube. The flame dynamic responses under acoustic perturbations are expressed as the flame describing function (FDF), with the unstretched steady flame height maintained constant while varying the hydrogen enrichment level. The FDF is obtained using the G-equation model and solved through the level-set method. Results indicate that the increase in the hydrogen enrichment level augments the cut-off frequency of the FDF. Furthermore, within the Rijke tube, the increase in hydrogen enrichment level under the same external excitation conditions leads to more unstable modes in the system. These unstable modes further affect the flame dynamic response, with the hydrogen enrichment causing the flame to transition from responding to a single frequency to multiple frequencies, and even showing significant responses to coupled higher frequencies, which highlights the complexities of investigating thermoacoustic stability in hydrogen enrichment systems.