Experimental and Numerical Study of Thermoacoustic Instability Characteristics and Mechanisms of Swirling Flame With Outer Flame

This is the authors' abstract. We don't add key points for this paper.

Previous research has identified that a stable outer flame existing in swirling flame can enhance the flameout characteristics of swirl combustion. However, it may exhibit thermoacoustic instability under low equivalence ratio conditions. In this study, the combustion instability characteristics of swirling flame with outer flame under different operating conditions were investigated. The sound pressure was captured by microphone, and the OH* chemiluminescence was acquired at 10 kHz simultaneously. The results indicate that the amplitude of pressure pulsation induced by combustion instability undergoes a process of formation, development, weakening and disappearance starting from higher equivalence ratio until flameout. The instability frequency increases linearly from 560 Hz to 720 Hz as the airflow rate increases, and then gradually converges until the flow rate is sufficiently high. Subsequently, Reynolds stress model (RSM) and Flamelet Generated Manifold (FGM) combustion model were employed to conduct unsteady RANS simulation for an unstable condition. The flame pattern captured by simulation is generally consistent with OH-PLIF observations. By examining the flow field structure, it was observed that the air separated when it left the inner and outer swirl nozzles — a phenomenon not previously reported in other studies — suggesting its potential as a novel approach for studying swirling combustion dynamics. Furthermore, pressure spectrum characteristics obtained from simulation results exhibit similarities to experimental findings. The physical field of a single pressure pulsation period was investigated. It is founded that the pressure pulsation in the combustion chamber causes the fuel passage periodically releases fuel pellets into the combustion chamber, which resulting in variations in heat release. Dual spiral vortex and PVC in the inner swirl nozzle were captured. The fuel pellet reacts violently in the downstream of the flame under the enrolling action of large-scale coherent structure and the transport effect of jet air. This feature is highly consistent with the spectral proper orthogonal decomposition (SPOD) mode derived from the chemiluminescence data.