Staggered swirler arrangement in two self-excited interacting swirl flames

Interference of acoustic and convective disturbances controls the development of self-excited combustion oscillations of a lean-premixed swirl-stabilized flame with a central bluffbody. How this interference mechanism influences the dynamics of multiple interacting flames in a multi-nozzle environment is currently unknown. Here we present observations of a multi-nozzle system's response to staggered swirler arrangements (ξsw, 1 ≠ ξsw, 2) as compared to non-staggered arrangements; the distance between the swirler and the flame is the dominant length scale of vortical disturbances. Our results demonstrate that a slight modification of the swirler arrangement in the streamwise direction – staggered or non-staggered – has a remarkable influence on the stability map of the whole combustion system. Phase-resolved flame imaging measurements indicate that under non-staggered conditions interacting swirl flames feature a coherent motion during a period of oscillation. By contrast, the staggered swirler combination creates significantly non-symmetric flame dynamics, disturbing the development of well-organized motion over the entire reaction zone. Flame surface modulations in the lateral direction are particularly pronounced due to the formation of non-symmetric convection delays of vortical disturbances between adjacent swirl nozzles. For a given swirler arrangement, the system's response to a wide range of combinations of mean nozzle velocities, including symmetric (u¯1=u¯2) and non-symmetric (u¯1≠u¯2) conditions, were explored to account for the simultaneous effects of the two convection parameters. Our data show that a major determinant of the onset of the instability is the combination of the Strouhal numbers, 〈St1, St2〉, which can be even or uneven depending on the manipulation of the convection time of each nozzle.