Flow Dynamics in Single and Multi-Nozzle Swirl Flames

This paper describes an analysis of the unsteady flow structures in a single nozzle and triple nozzle swirl combustor (with nozzle spacing of s/D=2.9). It was motivated by a prior study by our group which compared the time averaged and unsteady features for a different swirling nozzle, and found that the single and triple nozzle flow dynamics were quite similar upstream of the jet merging region. This work is motivated by the fact that realistic hardware, whether in can or annular combustion systems, almost always contains several nozzles. However, it is common to use test facilities with a single nozzle to study flame dynamics, a key component of the combustion instability problem [1]. Simultaneous OH Planar Laser Induced Fluorescence (OH PLIF) and Stereoscopic Particle Image Velocimetry (sPIV) techniques were performed at 5 kHz on a swirl methane-air flame. Two transverse forcing configurations were applied, so that the flame nominally lies in a pressure node/transverse velocity antinode, and vice versa. The time averaged flow fields of the single and triple nozzle configurations are compared, and several key differences are identified. Most prominently, there are non-negligible differences in the recirculation zone reverse flow velocity and flame spreading angle. However, the spatial variation of the disturbance magnitudes along the shear layers exhibit quite similar growth and decay trends, and the convection speeds along the shear layer were nearly identical. These results corroborate the findings of Aguilar et al. [2] and Szedlmayer et al. [3], which show that despite differences in time average quantities, comparable flow dynamics occur in single and multi-nozzle flames. These results imply that useful insights into the dynamics of multi-nozzle systems can be gleaned from appropriately designed single-nozzle hardware, with appropriate accounting for the differences in time averaged flow/flame characteristics.