Combustors in gas turbines exhibit thermoacoustic instability marked by high-amplitude pressure oscillations, during which the characteristics of the flame undergo significant changes. In the present work, a laboratory-scale swirl combustor is characterized by its stable and unstable flame behavior. The chemiluminescence of the CH* radicals in the flame is recorded at high framing rates simultaneously with the measurement of the acoustic pressure excited in the combustor. The combustor is unstable at low fue-air equivalence ratio and high air flow rate (represented in terms of Reynolds number, Re). Under unstable conditions, the high-speed CH chemiluminescence images reveal the evolution of a mushroom-shaped flame structure at the pressure maximum in every acoustic cycle, which eventually fl4ares into an axisymmetric flame convecting downstream. The flame curling associated with the mushroom-shaped patt ern is investigated for the signature of vortex-driven combustion instability by means of rotational symmetry in the flame images. A scale-invariant feature tranform algorithm is employed to detect rotational symmetry for this purpose. The results reveal the appearance of rotational symmetry at the zero pressure crossings and pressure maxima. The number of frames bearing the rotational symmetry is correlated with the duration over which peak pressure prevails across several acoustic cycles, to reinforce the role of vortex combustion in exciting pressure oscillations in the combustor. © 2019 by Begell House, Inc.