Flame structures and thermoacoustic instabilities of centrally-staged swirl flames operating in different partially-premixed modes

Flame structures and thermoacoustic instabilities of centrally-staged swirl flames fueled with methane at atmospheric pressure are experimentally investigated by varying the stratification ratio (SR) and covering four different partially-premixed modes, namely fully-premixed main and pilot stages, fully-premixed-pilot/partially-premixed-main stages, partially-premixed-pilot/fully-premixed-main stages, and partially-premixed main and pilot stages. The flame structures in the quiet cases at SR = 0–3 are studied, and three basic flame types can be defined with increasing SR: the lifted flame, the attached twin-flame, and the V-shaped flame. The above basic flame structures show deviations in different partially-premixed modes. The links between flame structures and thermoacoustic instabilities are investigated in the unstable cases by exciting thermoacoustic instabilities with higher thermal power at increased global equivalence ratio. The results show that the partial premixing in the main stage excites stronger thermoacoustic instabilities and thus leads to different flame structures. Simplified thermoacoustic network analysis is also conducted to provide an insight into the nature of thermoacoustic instabilities in the three basic flame structures summarized in the unstable cases. The present investigation reveals the effect of partially-premixed modes on flame structures and thermoacoustic instabilities, which can guide the early-stage design and the development of instability control strategies in practical combustion systems.