Examination of thermo-acoustic instability in a low swirl burner

•Increased LSB diameter led to higher natural mode frequencies and greater variation.•Three coupling modes are seen across the excitation spectrum.•FSD differs between base and shear layer generated coupling modes.•A critical driving pressure amplitude determines structural and heat release change.•Hydrogen addition to fuel causes less coherent thermo-acoustic coupling.

Low-swirl burners are of interest in industrial applications due to their low NOx emissions. In the present work, a 3.81 cm diameter low-swirl burner is acoustically forced with different fuel mixtures. The measured results are compared to 2.54 cm diameter low-swirl burner data to infer scaling properties. The experiments and analysis show that three coupling modes were present in the 3.81 cm burner: base mode coupling, shear layer generated coupling, and transitional coupling. The 3.81 cm burner was observed to have a critical acoustic driving pressure amplitude, similar to the 2.54 cm burner; however, the 3.81 cm burner had higher frequency natural acoustic modes. This counter-intuitive result is shown to arise because the modes are tied to shear layer behavior rather than burner size. It was also observed that adding hydrogen to the fuel stream resulted in less coherence. This is likely the result of increased flame speed and decreased ignition limit, allowing the flame to interact less with both large and small vortices. Small scale interaction leads to more small scale wrinkling while strong large scale vortices produce more flame role up and displacement of heat release zones. The ability of hydrogen addition to negate these effects suggests selective hydrogen addition as a possible method of inhibiting combustion instability.