Large Eddy simulation of a swirling flame response to swirl modulation with impact on combustion stability

A swirl stabilized flame has been studied numerically using Large Eddy Simulation. The response of the flame to a large swirl number modulation has as been determined. Decreasing the swirl number moderately does not destabilize the flame. Decreasing further the swirl number induces a transition from stabilized to a jet flame with increase in the flame fluctuations. Increasing back the swirl restores the original configuration. However, the re-stabilization has been found to be considerably slower as compared to the burner aerodynamic time scale. Further studies, including identification of some characteristic turbulent structures, have been carried out. Both a helical and an axial mode have been identified. The helical mode arises in the central recirculation zone, whereas the axial mode arises in the outer shear-layer. Both modes are possible sources of thermo-acoustic instabilities.