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 NOx 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 NOx 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 NOx 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.