Acoustic Dampers Effects on the Characteristics of Confined Swirling Partially Premixed Methane Flames

The effects of different baffled configurations of acoustic dampers placed downstream of the dump plane of a combustor on the mean flowfield, coherent structures, acoustics, stability and dynamics of partially premixed methane flames were experimentally investigated. Particle image velocimetry (PIV) was used to capture the instantaneous flowfield downstream of the dump plane or the baffles and proper orthogonal decomposition (POD) was used to analyze coherent structures. High-speed imaging was used to capture flame dynamics, and a Bruel and Kjaer microphone was used to perform acoustics measurements. Two interchangeable baffle configurations were tested; one consists of eight-blade radial baffles attached to a circumferential baffle and placed underneath of a circumferentially-slotted cup (AD#1), and another consists of eight-blade radial baffles attached to a circumferential baffle (AD#2). The results revealed significant improvement in flame stability when using baffles. This is attributed to several factors. First, the burner baffled configurations caused a significant reduction in the amplitude of coherent structures and acoustics. Second, the presence of baffles promoted either a very narrow CRZ (in case of AD#2) or no ORZ with a weak CRZ (in case of AD#1). This positive axial velocity convected vortical structures farther downstream of AD#1. Third, both flame-flame interaction in AD#1 configuration (resulting from the axial flow off the circumferential slots and that from the swirling flow in the core flow region) and flame-baffles interaction in AD#2 configuration (resulting from the interplay between baffles' tips and the swirling flame in the core region) helped reducing the azimuthal mean velocity and its rms component in the flow core region. This consequently resulted in a significant change in the azimuthal acoustic waves' behavior within the combustor/confinement. The results revealed that the baffled burner configurations enhanced mixing as witnessed by the more flame bluish color and consequently its stability.