Structure and thermoacoustic instability of turbulent swirling lean premixed methane/hydrogen/air flames in a model combustor

The structure and thermoacoustic instability (TI) of premixed CH4/H-2/air swirling flames were experimentally investigated for a range of hydrogen fraction (eta(H2)) up to 80% under different equivalence ratio (Phi) and swirl number (S) conditions. It is shown that the onset of TI is enhanced when increasing either eta(H2), S, or Phi. The dominant frequency of TI increases dramatically with eta(H2). The higher dominant frequency in the hydrogen-enriched flames can be attributed to a shorter flame length which results in a reduced flame convection time. It is observed that the unstable flames are always accompanied by the appearance of outer recirculation zone (ORZ) flame. Therefore, the flame kernel residing in the ORZ can be an indicator of the occurrence of TI. The flame front of thermoacoustic unstable flames was observed to be more wrinkled, e.g., with larger mean absolute curvature (kappa (abs)) and local flame surface area ratio (delta Sigma(max)). Importantly, the phase-locked analysis shows that kappa (abs) and delta Sigma(max) can be modified at different oscillation phases, and their maximum and minimum values are simultaneously achieved at phase angles theta of about 0 degrees and 180 degrees, respectively. Variations of kappa (abs) and delta Sigma(max) are in phase with the heat release rate, indicating a strong correlation between the TI and flame structure modification; however, they show a phase lag of about 72 degrees behind the pressure in this work. These results are vital when understanding and predicting the TI based on the flame structure, especially when adopting a visual detection method of the instability.