Large eddy simulation study on the effects of swirl number on dynamic combustion characteristics of stratified methane swirling flame

The effects of burner swirl number (SN) on the dynamic combustion characteristics of the Cambridge methane premixed stratified swirling flame has been numerically studied by large eddy simulations (LES), following a satisfactory validation of the models through comparing the temperature and velocity values of the experimental and LES results. Proper Orthogonal Decomposition (POD) and Phase Space Reconstruction (PSR) methods were employed to analyze the LES results. Key findings reveal that, as the SN increases, the internal recirculation zone (IRZ) expands into a dual-vortex structure while the outer recirculation zone (ORZ) gradually diminishes. Flow continuity issues, heat release rate (HRR) fluctuations, and non-periodic migration of high HRR regions are all alleviated. The energy proportion of the first-order flame mode decreases with weaker consecutive shedding effects. The limit cycle range of pressure oscillation signals narrows and shrinks, showing significantly reduced pressure amplitudes that effectively suppress combustion chamber oscillations, though oscillation frequency only slightly increases. Additionally, the high-temperature zone of the flame contracts as the SN increases, while the high-OH region and high HRR region exhibit the opposite trends. CO emission concentration first decreases then increases with higher SNs. Enhanced tangential velocity at high SNs hinders CO-oxygen reactions, causing local flame quenching and reduced CO2 emissions. This study demonstrates that increasing SN significantly improves combustion stability, effectively suppresses pressure oscillations, and moderately reduces pollutant emissions.