Effects of secondary hydrogen injection on thermoacoustic instability of swirling premixed flames in a model combustor

Effects of pilot H₂ injection on the thermoacoustic instability of premixed CH₄/air flames were experimentally investigated with varied hydrogen injection rates and different injection strategies. The flames were stabilized on a swirl burner with a central pilot-fuel injection tube. The flame structure was captured using high-speed OH* chemiluminescence and OH-PLIF methods, while the CO and NO_x emissions were also measured. It is validated that a minimal flow rate of the pilot H₂ injection can significantly enhance flame stabilization. Comparatively, the other two strategies, including premixing H₂ with CH₄ and a pilot injection of the CH₄, all exhibit nearly no improvement in the flame stabilization. The underlying reason is that the pilot H₂ injection can modify the flame anchoring, reduce flame height, and even redistribute heat release in the flame. Moreover, the H₂ injection suppresses the flame front roll-up, mitigates the heat release fluctuations, and ultimately leads to decoupled acoustic pressure and unsteady heat release. Furthermore, it appears that the CO emissions decrease with the pilot H₂ injection while the NO_x emissions significantly increase. Thus, although the pilot H₂ injection can be effective to actively suppress the thermoacoustic instability, the elevated pollutant emissions should be a major drawback of this strategy. In the present work, the optimized trade-off between flame stabilization and NO_x emission is achieved using a pilot H₂ injection at a thermal power fraction of about 5%. These results are helpful to attain stable and clean combustion in the gas turbine based on hydrogen fuel.