The effect of premixed modes of the pilot stage on combustion instability in a centrally-staged combustor

The present study investigates the effect of premixed modes of the pilot stage on combustion instabilities in a centrally-staged model combustor. Two categories of experiments, partially premixed mode (PPM) and fully premixed mode (FPM) of the pilot stage, are conducted with various global equivalence ratios (ϕ_glo) and stratification ratios (SRs). Three flame types, i.e., stratified flame (S-flame), V-shaped flame (V-flame), and lifted flame (L-flame), are observed in the FPM case at test conditions, while there are only S-flame and L-flame in the PPM case. The pilot flame of the PPM case has a broader flame front and shorter axial flame length than the FPM. Thermoacoustic instabilities are examined for FPM and PPM at test conditions. In the PPM case, a jagged distribution of dominant frequency is found at SR = 1.5 of ϕ_glo = 0.6. The PPM case has better thermoacoustic stability than FPM in certain conditions, especially at low SR, which may be caused by a more stable pilot flame. Flame dynamics are further analyzed using the Dynamic Mode Decomposition method. It is concluded that the PPM case has a more robust heat release rate fluctuation, even though there is a slight dynamic pressure fluctuation. A helical mode of precessing vortex core (PVC) with around 300 Hz is also observed in the PPM case, but not in the FPM case, which may suppress the PVC.These results have rarely been reported in previous work and may provide valuable data for designing the centrally-staged combustor pilot stage.