Numerical analysis of swirl variation effect on cyclone burner flame transfer function

The effect of swirl variations on the combustion dynamics of a lab-scale cyclone burner, fuelled by a 40-60 vol% methane-hydrogen blend, is investigated. This burner was designed to simulate the thermoacoustics of a large industrial furnace. Since a large fraction of industrial combustion systems are swirl-stabilized, it is of interest to know if a change in swirl of the incoming fuel-air mixture has an effect on the thermoacoustic stability of the system. In this study, forced vibration simulations are performed at five different swirl numbers and the Flame Transfer Function (FTF) is analysed. In a parallel study, the effect of the change in FTF on the system stability is studied. SAS-SST URANS simulations are performed in Fluent in combination with an FGM combustion model. The flow is forced at the air inlet by pulsating the mass flow rate at given frequencies. To reduce the computational cost, each simulation is forced with a superposition of sine waves at distinct frequencies. Results show a reduction of the combustion time delay constant with increasing swirl number, as the flame becomes more compact. The gain is largest for smaller swirl numbers with a maximum around 400-600 Hz, after which it starts to roll off. For larger swirl numbers, the gain shows behaviour more similar to a low-pass filter as the compact flames are less responsive to low-frequent forcing. An increase in the gain was found for these cases when the forcing frequency is close to the vortex shedding frequency.