Experimental investigation of combustion instability in a centrally staged combustor under self-excited oscillation conditions

This paper presents an experimental investigation of combustion instability and flame dynamics in a laboratory-scale lean premixed prevaporized centrally staged combustor under self-excited oscillations. The macrostructure of the flame is captured by CH* chemiluminescence images using a high-speed camera, and the two-dimensional flow field of the combustor center plane is obtained by using particle image velocimetry. The effects of the pilot stage swirl number and main stage equivalent ratio on the thermoacoustic oscillations and flame dynamics are analyzed. The results indicate that there are intermittent oscillations, limit cycle oscillations, and mode switching in the combustor. As the pilot stage swirl number increases, the system transitions from intermittent oscillation to limit cycle oscillation. Additionally, the case with a pilot stage swirl number of 0.5 also occurs mode switching. Based on dynamic mode decomposition, the heat release fluctuation is primarily concentrated in the shear layers under different operating conditions. Furthermore, the intensity of thermoacoustic oscillations in a system is determined by the coupling strength between pressure and heat release fluctuations. From the results of the flow field, it is observed that as the pilot stage swirl number increases, the time-averaged axial strain rates and vorticities increase, but the time-averaged axial velocity is generally lower for the swirl number of 0.7 compared to the other two cases. On the other hand, the time-averaged axial strain rates and vorticities for the case with a swirl number of 0.5 decrease with the increase in the main stage equivalence ratio before and after the mode switching.