Acoustically Coupled Fuel Jet Combustion Near Pressure Nodes and Antinodes

This experimental study explores the combustion dynamics of single and coaxial laminar jet diffusion flames in the presence of standing acoustic disturbances. The flames are studied inside a closed cylindrical waveguide under atmospheric conditions, observed through high-speed visible imaging and analyzed using proper orthogonal decomposition (POD). Different burner geometries with varying annular-to-jet area ratios, tube wall thicknesses, Reynolds numbers, and velocity ratios are explored, in addition to varying applied acoustic conditions, including flames in the vicinity of a pressure node (PN) and a pressure antinode (PAN). Differing flame-acoustic coupling processes resulted from the alternative conditions, including sustained oscillatory combustion (SOC), multi-frequency, periodic lift-off and reattachment (PLOR), permanent flame lift-off (PFLO), and eventual flame blowoff (BO). The phase portraits extracted from POD mode coefficients capture distinct signatures associated with these transitions, which can aid in developing topology-based reduced order models (ROMs) for flame-acoustic coupling. Machine learning tools are developed to improve denoising of flame images, in addition to nonlinear decomposition methods to further reduce dimensionality, with promising implications for the advancement of nonlinear ROMs.