[Objective] To reveal the influence of interaction between pilot and main flames on flow and thermoacoustic instability characteristics, the flow and thermoacoustic dynamic characteristics of the pilot stage, main stage, and centrally staged flames in the combustion chamber of the central staged model combustor are studied in detail through experiments. [Methods] High-speed measurement methods such as high-frequency particle image velocimetry, high-speed camera, pressure sensors, and photomultiplier tube are used to study the flow-flame-acoustic dynamic characteristics of the central staged burner. During the experiments, the unsteady flow field, flame CH∗ signal distribution, global heat-release rate, and pressure pulsation characteristics are measured under different testing conditions. [Results] The results showed that the pilot swirling jet had a significant influence on the flow-flame-acoustic dynamic characteristics of the stratified burner. Furthermore, when the pilot stage used a swirling flame, the main recirculation zone was formed downstream of the nozzle outlet and the high-temperature burned gas was rolled back to improve the combustion stability of the swirl flame. The peak value of pressure was 683 Pa, and the main peak values of dynamic pressure and heat release were both visible at a frequency of 120 Hz. However, the dominant frequency of the flow field proper orthogonal decomposition (POD) time coefficient spectrum showed that the dominant frequencies of large-scale shedding vortex in the flow field were 68 and 109 Hz, indicating that the interaction between pilot and main flames caused the thermoacoustic instability frequency to be inconsistent with large-scale vortex shedding frequency. Moreover, the dominant frequency of pressure and heat release were slightly shifted when the pilot stage was operated with a swirling air jet, and it was reduced by 5 Hz and got to 115 Hz. However, the main recirculation zone disappeared, resulting in a strengthening of the thermoacoustic instability and an increase in the peak pressure to 2 947 Pa. The dominant frequency of the flow field POD time coefficient spectrum showed that the frequency of the shedding vortex generated by the swirling flow shear layer of the main stage was 115 Hz, which was well locked with the frequency of the thermoacoustic instability mode under this condition. The results, in this case, showed that acoustic-velocity-flame instabilities were mutually coupled under the single-swirling flame self-excited oscillation condition. However, for the flame interaction case, the flow instability frequency induced by flame and pressure oscillations changed significantly, and the coupling between acoustic-velocity-flame instabilities was also destroyed. [Conclusions] When the main stage is operated with swirling air, whether the pilot stage is operated with swirling air or swirling flame, the stable main recirculation zone can always be observed at the burner exit plane. However, when the main stage is operated with a swirling flame, the swirling air from the pilot stage destroys the main recirculation zone, which is detrimental to the main-stage swirling flame's combustion stability. The main recirculation zone reappears when the pilot stage is converted into a swirling flame. The increased heat load also causes the accelerated expansion of the gas, which induces a stronger vortex in the swirling flow shear layer. However, the interaction between the pilot flame and the main flame accelerates the dissipation of the vortex. © 2024 Press of Tsinghua University. All rights reserved.
