Lean blowout limits of centrally staged swirl flames

The lean blowout (LBO) characteristics of centrally staged swirl flames are systematically studied by using OH^* chemiluminescence and stereoscopic particle image velocimetry at a repetition of 5 kHz. First, the influence of the velocity ratio on the flow structures is evaluated based on the non-reacting flow fields. Three distinct types of flow fields are identified: main-stage swirl dominance, pilot-stage swirl dominance, and co-dominance of the main and pilot stages. This characterization establishes a basis for determining the parametric space in the subsequent flame LBO experiments and helps to interpret the LBO limits associated with the different flow features. Subsequently, a series of LBO experiments is conducted to explore the effects of critical parameters on LBO limits, including velocity ratio (R v), stratification ratio (SR), hydrogen fraction (X H 2), total equivalence ratio (ϕ), and the swirl number of the main stage (S). The experimental results demonstrate that as each of R v, ϕ, SR, and X H 2 increases, the flame stability limits are expanded. In particular, the influence of SR on LBO limits is found to be closely related to the characteristics of the flow field. Finally, using the LBO limit data and introducing the laminar flame extinction strain rate α e for all LBO conditions, the LBO prediction models are constructed based on linear regression and the Damköhler number within the multidimensional parametric space. These results emphasize the important role of swirl flow field control and air distribution design in ensuring the flame stability in centrally staged combustors.