Investigation of flame and flow response in the swirler with different divergence cups and central body under external excitation

Abstract

Effects of swirl divergence cup and the central bluff body on premixed flame response with external excitation are experimentally investigated. Flame transfer functions (FTFs) associated with different swirlers are measured in 50–450 Hz. The corresponding flame and flow responses are examined with the help of chemiluminescence images and Particle Image Velocimetry (PIV) method. Results show that FTF gain curves of swirlers with different divergence cups are characterized by alternating regions with first a minimum and then a maximum value as the excitation frequency increases. Increasing the divergence cup may greatly reduce the corresponding FTF minimum gain. Dynamic mode decomposition and proper orthogonal decomposition analysis indicate that flames with large divergence cup angles are dominated by the flame angle oscillations at the minimum gain point, while the flame with zero cup presents both the flame angle oscillations and vortex shedding. PIV results indicate that vortical structures located at the outer shear layer (OSL) could induce high-flame response, while the impacts of vortical structures located at inner shear layer are much weaker. Increasing the divergence cup could largely weaken the strength of vortical structures at OSL. In addition, effects of the central bluff body on flame response are significant. The flame in the swirler without the central bluff body is mainly governed by flame angle oscillations, and the elongated flame induced by the swirler with a large body is almost not sensitive to acoustic excitations. These results are useful for the understanding of flame response mechanisms in premixed swirling combustion.