Investigation of the Coupling Between the Dynamics of Vortical Structures and Flame Stability in Bluff-Body Premixed Combustion Using Extended Spectral Proper Orthogonal Decomposition

Large eddy simulation data of a bluff-body stabilized flame are analyzed using spectral proper orthogonal decomposition (SPOD) to investigate: (i) the role of flame-vortex interactions in the dominant flow dynamics and (ii) how the proper choice of the cross-spectral density (CSD) defining SPOD can assist in identifying the underlying dynamics. Bluff-body flame holders aim to achieve stable flames under lean premixed conditions to minimize pollutant emissions. The recirculation region induced by the body promotes the mixing of hot combustion products with unburnt gases, preventing the global blowoff. However, the coupling between the shear layers and flame-induced vorticity sources can result in large flow structures that either contribute to increased flame stability or exhibit features typical of the early stages of flame blowout. SPOD is a data-driven technique remarkably powerful in extracting low-dimensional models. For each frequency, it computes a basis of orthogonal modes that maximizes the content of a predefined CSD in the leading modes. By choosing physically relevant variables to construct the CSD, different physics can be explored, which is used here to investigate the coupled dynamics between the flame-induced baroclinic torque, vortical structures, and the temperature field. The results show that the vorticity and temperature fields exhibit low-dimensional dynamics characterized by a narrowband frequency and its harmonics; these dynamics are varicose oscillations of the flame region, governed by the baroclinic torque. Sinuous oscillations typical of wake instability for nonreactive flows are also present, suggesting a competition between them.