On the impact of H2-enrichment on flame structure and combustion dynamics of a lean partially-premixed turbulent swirling flame

Large Eddy Simulation (LES) with Conjugate Heat Transfer (CHT) is used to analyze the impact of H2-enrichment on the flame structure and combustion dynamics of a lean partially-premixed turbulent CH4/Air swirling flame. Experimentally, the combustor is operated at atmospheric pressure with H2 fuel fractions of up to 50%, by volume. LES-CHT results are compared and validated against time-resolved stereo PIV, OH* chemiluminescence, OH-PLIF imaging and acoustic pressure measurements. In terms of dynamics, for the pure CH4 and 20% of H2 enrichment cases, no thermoacoustic oscillation is observed in either the experimental or numerical data. As the fuel fraction of hydrogen is increased, the flame length reduces due to the increase in laminar flame speed and the heat release rate distribution becomes more compact. CHT simulations reveal that H2-enrichment leads to higher temperatures at the centerbody tip. At 50% H2, in agreement with experiments, LES predicts a bi-modal thermoacoustic oscillation, with two main frequencies corresponding to the quarter and chamber modes of the system. Dynamic Mode Decomposition is performed on the measured OH-PLIF images and LES 3D fields to extract each mode contribution to the overall flame dynamics. It is observed that both modes are characterized by local variations of equivalence ratio, while only the higher frequency (chamber) mode is characterized by vortices periodically detaching from the backplane and the centerbody walls causing a strong periodic wrinkling of the flame front during the thermoacoustic oscillation.