Impact of density stratification and azimuthal velocity on the growth of coherent structures in a convectively unstable swirl flame

Large-scale coherent structures resulting from hydrodynamic instabilities can interact with turbulent swirl flames and lead to combustion instabilities. The present work investigates the impact of density stratification and azimuthal velocity on the growth of coherent structures in a convectively unstable swirl flame. Flame structure and flow field are measured by simultaneous hydroxyl planar laser-induced fluorescence and stereoscopic particle image velocimetry (S-PIV) at a repetition rate of 10 kHz and are analyzed by using the spectral proper orthogonal decomposition (SPOD) and spatial linear stability analysis (LSA). The SPOD reveals that the dominant symmetric and anti-symmetric modes are within the frequency range from 156 to 585 Hz, accounting for more than 25% of the turbulent kinetic energy. The spatial growths of these coherent structures are quantified by the LSA that predicts large growth rates near the nozzle exit with the corresponding frequency band matching well with the SPOD analysis. The LSA results show that both density stratification and azimuthal velocity have little effect on the instability frequencies of the most spatially unstable modes. However, the flame-induced density stratification suppresses the growth of the coherent structures by altering the pressure gradient and viscous diffusion, whereas the azimuthal velocity promotes flow instabilities through the changes in convection and production of the coherent perturbations. The results also suggest that the conventional PIV technique with two-component velocity measurement is inadequate for linear modeling of coherent structures, and the density stratification should also be taken into account in convectively unstable swirl flames.