Impact of H2-enrichment on the response of a partially premixed CH4–air flame to velocity and equivalence ratio fluctuations

The dynamics of hydrogen-enriched partially premixed methane–air flames are investigated, focusing on the frequency response to velocity and equivalence ratio fluctuations. Large-eddy simulations (LES) with conjugate heat transfer are performed on the partially premixed PRECCINSTA combustor for various methane–hydrogen mixtures, maintaining a constant thermal power and global equivalence ratio of 10 kW and 0.8, respectively. To reduce the computational cost of the LES, a three-step global mechanism is calibrated for each hydrogen-enriched partially premixed flame. The accuracy of the LES is assessed against experimental data, showing a good agreement with the mean velocity field and shape of the swirl-stabilized flame. The multiple-input–single-output structure for the flame transfer function is applied to separate the individual contributions of velocity and equivalence ratio fluctuations to the overall heat release rate oscillation in the partially premixed flames. System identification with broadband forcing is used to compute the various flame transfer functions. Hydrogen enrichment significantly influences the flame response to equivalence ratio fluctuations, explained by the respective impact on the response of the heat of reaction, the flame speed, and the flame surface area to equivalence ratio fluctuations. The interaction between velocity and equivalence ratio fluctuations is further characterized by the mixing transfer function, which describes the equivalence ratio fluctuations reaching the flame. The higher mass diffusivity of hydrogen results in lower equivalence ratio fluctuations reaching the flame as the frequency increases, attenuating the pronounced effect of hydrogen on the flame sensitivity to equivalence ratio fluctuations. This study contributes to the understanding of the impact of hydrogen enrichment on the thermoacoustic response of partially premixed flames.