Fuel stratification effects on flame dynamics and combustion instability for strongly coupled stratified flames

The strong coupling of the stratified flames, resulting from the Venturi design that prevents flame flashback, leads to complex flame dynamics and combustion instability. This study investigates thermoacoustic mode transitions and flame dynamics in a strongly coupled, centrally staged dual-swirl combustor, focusing on a range of global equivalence ratios and fuel distributions for both the main and secondary stages. Three distinct stability regimes are identified: a V-shaped flame, an M−shaped flame, and a double-branch stratified flame. The M−shaped and double-branch stratified flames are associated with thermoacoustic instability, while the V-shaped flame remains stable. Notably, the influence of fuel distribution in the main and secondary stages on flame structure and thermoacoustic properties varies, due to the distinct convective paths of the two flame types. An increase in secondary stage fuel does not significantly alter the flame shape, consistently maintaining a V-shaped structure and stability across a broad range of equivalence ratios from 0.584 to 0.935. However, excessive increases in secondary stage fuel result in interactions between the main and secondary flames, leading to combustion instability. Conversely, as main stage fuel increases, the flame structure transitions from a V-type to an M−type and eventually to a double-branch stratified flame. Main stage fuel increases are more likely to induce combustion instability compared to secondary stage fuel increases. This is primarily due to the interaction between the shear-layer flame and the wall, as well as the emergence of the flame in the external recirculation zone, which promotes interactions between the outer and inner shear-layer flames. The addition of secondary-stage fuel contributes to a sharp rise in NOx emissions. This study provides fundamental insights into thermoacoustic instability and flame dynamics in a strongly coupled, centrally staged swirl combustor, essential for guiding the design of practical low-emission combustion systems and developing effective instability prediction and control strategies.