It is important to be able to understand and control the transition to strong thermoacoustic oscillation associated with the swirl combustor of, e.g., gas turbines. We focus on the contribution of aerodynamic instability to the onset of thermoacoustic oscillation on a 33kW swirl stabilized combustor operating with lean premixed H2-enriched CH4 blend. Global CH* chemiluminescence and dynamic pressure shows that the combustion transits from a quiescent to a limit-cycle state as the H2 molar concentration increases from 20% to 40%. The global (CH4 and H2) equivalence ratio was kept at 0.55 and the thermal heat release was almost constant at 33kW. Linear stability analysis is applied to solve the linear perturbation mode of the non-reacting swirling jet, and shows the existence of a dominant, helical mode perturbation and a secondary, center mode perturbation. This is confirmed experimentally by using proper orthogonal decomposition (POD) and dynamic mode decomposition (DMD) to extract coherent flow structure from time-resolved PIV snapshots of both non-reactive and reactive flow. It is
found that the helical mode gives rise to the precessing vortex core (PVC) by inducing a sequence of downstream propagating vortical perturbations. The helical mode and its induced PVC motion is found to diminish as the system transits from quiescent state to a limit-cycle oscillation. In the transitioning case, a streak-structure perturbation, which resembles the center mode, becomes dominant. The center mode is distinguished from the helical mode in that it is localized near the centerline of the swirl and is marginally stable under non-reacting condition. It is inferred from the present research that the PVC motion is not the only coherent flow structure that the flame interacts with to trigger thermoacoustic oscillation; the center mode may play a role in the triggering event, which needs further investigation.