Experimental study on thermoacoustic coupled oscillating combustion mechanism of swirl diffusion flame based on phase analysis

Since current research on the oscillation mechanisms in swirl combustors predominantly focuses on swirl-stabilized premixed flames, this study focuses on a single-stage swirling diffusion flame and conducts experimental research to explore the mechanisms of thermoacoustic coupled oscillatory combustion. The flame's dynamic response to velocity fluctuations is examined, and a novel phase analysis model is established. The closed-loop feedback relationship between velocity fluctuations, pressure fluctuations, and fluctuations in the heat release rate is verified by this model. Compared to existing studies, this research innovatively analyzes the impact of the swirl number on oscillatory combustion, while the influence patterns and mechanisms of the equivalence ratio, inlet Reynolds number, and combustion chamber length on oscillatory combustion are revealed systematically. The findings indicate that decreases in the equivalence ratio, along with the increases in the inlet Reynolds number and swirl number, reduce the phase delay in heat release rate fluctuations—key factors contributing to increases in both the dominant oscillation frequency and oscillation intensity. Furthermore, an increase in the combustion chamber length reduces the system's resonance frequency, enhancing the acoustic feedback strength associated with the oscillation main frequency and thus intensifying the oscillations. Overall, the phase delay in heat release rate fluctuations in response to incoming velocity disturbances and the acoustic characteristics of the combustion system are crucial in affecting thermoacoustic coupling. The study provides a comprehensive interpretative approach to the mechanisms behind swirl oscillatory combustion.