Analysis of eigenmodes in a swirling jet and flame: 3D PIV and PLIF study

The existence of spiral structures was directly confirmed experimentally on the basis of 3D Particle Image Velocimetry (PIV) and planar laser-induced fluorescence (PLIF) measurements both in a strongly swirling non-reacting jet and in a fuel-lean premixed methane-air flame. Flows were characterized by the presence of vortex breakdown and precession of the vortex core. The differences in magnitude and spatial distribution of flow eigenmodes with distance from the nozzle are analyzed by using Proper Orthogonal Decomposition (POD) based on Spatial Fourier Transform over azimuthal coordinate. The analysis of flow eigenmodes reveal that for a high-swirl isothermal jet, the vortex core co-existed with the pair of counter-rotating helical vortices, which were located in outer shear layer and inside the recirculation zone. This double-vortex helical structure was also detected in a swirling flame, but its magnitude was suppressed compared with isothermal flow. It was determined that the change in the shape of the chemical reaction area was associated with two types of large-scale coherent structures: nearly axisymmetric mode m = 0 of the flame front deformation, presumably due to the effect of buoyancy forces on the combustion products, and the quasi-solid rotation of the asymmetric global mode vertical bar m vertical bar = 1 in the form of double-vortex structure due to precession of the swirling flow. It was shown that the energy of the axisymmetric mode m = 0 in a reacting jet increased downstream by almost 10 %, unlike other modes, whose energy was only diminished.