An experimental study is conducted on the buoyant jet diffusion flames to explore the interaction between the fuel jet and the outer vortex and their impact on the flame structure. Dimethyl ether (DME) and liquefied petroleum gas (LPG) diffusion flames are investigated. A laminar diffusion burner setup is used to study the flame dynamics of three different tube diameters (6, 8, and 10 mm) over a wide range of Reynolds numbers (250–2000). Flame flicker frequencies are analyzed using chemiluminescence signals and image processing techniques, and high-speed Schlieren images visualized flame dynamics and structure. The spatiotemporal structure of jet diffusion flames is analyzed using the dynamic mode decomposition (DMD) tool. Analysis of the Schlieren images revealed that the outer vortex structure interacts with the fuel jet, which eventually leads to convective feeding to the outer vortical structure. The outcome is the radial expansion of the leading vortex, which allows the trailing vortex to merge and produce a subharmonic frequency. However, all test conditions, regardless of the presence of subharmonic frequency in the buoyant diffusion flame, follow the established relationship between Strouhal-Froude numbers. The dominant mode of DMD captured the spatial features of the outer vortex at associated temporal frequencies. The large-scale coherent structure is observed in the region of subharmonic oscillation due to the merging of successive vortices. This process occurs at the alternate flame bulge formed near the tube exit. © 2021 Elsevier Ltd