Acoustically Coupled Single and Coaxial Fuel Jet Combustion at a Pressure Antinode*

These experiments explore the combustion dynamics of single and coaxial methane laminar jet diffusion flames in the presence of a standing acoustic wave, conducted in the vicinity of its pressure antinode. As with earlier flame studies near a pressure node [Sim, et al., CST 2020; Vargas, et al., JFM 2023], flames are studied inside a closed cylindrical waveguide via high-speed visible imaging and analyzed using proper orthogonal decomposition (POD). A wide parameter space is explored here, including different burner geometries, annular-to-jet velocity ratios, and fuel jet Reynolds numbers (below 100), in addition to varying applied acoustic frequencies and pressure amplitudes. Flame-acoustic coupling processes differ significantly based on different operating conditions, producing sustained oscillatory combustion (SOC), multi-frequency periodic lift-off and reattachment (PLOR), permanent flame lift-off (PFLO) with low-level oscillations, and eventual flame blowoff (BO). Trends in instability transitions are identified, and spectra and phase portraits extracted from POD mode coefficients capture distinct signatures associated with these transitions. Comparable flame dynamics are observed in experiments at the Air Force Research Laboratory involving much higher pressures at the PAN and fuel jet Reynolds numbers over 5,000 [Plascencia, et al., AIAA Journal, July2024].