Paper presented at the 7th International Conference on Heat Transfer, Fluid Mechanics and Thermodynamics, Turkey, 19-21 July, 2010.
Swirl stabilized burners are widely used in industrial combustion processes due to their benefits in terms of wider flame stability limits, improved mixing and high turbulence level . However, swirl flows are prone to hydrodynamic instabilities at critical operating conditions, which can lead to local velocity and pressure oscillations. These oscillations can modulate the heat release, giving rise to a space-temporal coupling between heat release and pressure, (Rayleigh criterion), leading therefore to thermo-acoustic oscillation. Industrial experience has shown that swirl stabilized wood powder burners can sometimes cause pressure fluctuations that result in high amplitude structural vibrations in the boiler and increased emissions. The qualitative experiments with a 150 kW wood powder burner that was perturbed with a loudspeaker in the secondary air register showed that a strong effect on the flame characteristic occurred when the perturbation frequency was about 17 Hz. Flame visualisations indicated that one or more processing vortices were present during stable combustion and that these vortices became unstable when the excitation frequency and amplitude had a critical value.