Role of Buoyancy Induced Vortices in a Coupled-Mode of Oscillation in Laminar and Turbulent Jet Diffusion Flames

The unsteady effects of buoyancy-induced instabilities on jet diffusion flames are investigated experimentally under normal gravity conditions. Methane and propane are used as test fuels that are lighter and heavier than ambient air, respectively. A similar Froude (Fr) and Reynolds (Re) number relationship is realized in both hydrocarbon fuels with different tube diameters ranging from 6 to 24.2 mm. The Schlieren visualization technique and highspeed imaging synchronized with chemiluminescence signal measurement are used to identify changes in global flame shape and dominant frequency. Buoyancy-induced instabilities generate two forms of diffusion flames with varying frequencies in space. Both laminar and turbulent jet flames exhibit natural and subharmonic frequencies, as well as a shift between them. The methane-propane Re-Fr relationship confirms the instability mode transition. In addition, Strouhal (St) and Froude number relations are obtained as St proportional to Fr-0.50, with a slope difference between natural and subharmonic modes in both fuels.

Automated summary

The experimental investigation on the unsteady effects of buoyancy-induced instabilities on jet diffusion flames reveals that the instability leads to frequency variations in diffusion flames, using methane and propane as test fuels ranging with different tube diameters. The results provide insights into the transition of instability modes in different fuels and the relationship between Strouhal and Froude numbers.