Influence of the dump plane on vortex heat release modulation in a rearward-facing step combustor

The influence of the dump plane on vortex heat release modulations is studied experimentally in a two-dimensional, laboratory-scale, rearward-facing step combustor. A methane–air premixed-flame is acoustically forced using a compression driver mounted longitudinally to generate controlled vortical roll-ups of the flame-front to simulate conditions associated with self-sustained, longitudinal-mode, thermoacoustic oscillations in such combustors. Forcing at 200 Hz is seen to result in vortices that convect away from the influence of the dump plane for the inflow velocity, forcing amplitudes and equivalence ratios tested, providing a convenient baseline to compare with cases that show some near-field interaction with the dump plane. Forcing at 500 Hz, under the right amplitude of forcing, results in a counter-rotating, vortex–vortex interaction near the dump plane, leading to the sudden combustion of unburnt reactants entrained in the vortices. True flame images show the 500 Hz forced flame spreading out laterally near the dump-plane due to such interactions, resulting in a characteristic tri-lobbed ‘clover-flame’ appearance. High-speed schlieren images show flow accelerating from the recirculation region inwards toward the inlet tube, resulting in a mushroom vortex, the direction of rotation of which amplifies the counter-rotating vortices. Phase-resolved shadowgraph images with superimposed CH* chemiluminescence show the evolution of the vortex and heat release within it. Synchronized pressure and heat release data associated with the life-cycle of the 500 Hz “mushroom vortices” show positive feedback as evidenced through local Rayleigh-index calculations. While heat release modulations resulting from vortex impingement on the side-walls and the nozzle have been studied, the influence of the dump-plane in modulating vortex heat release has not been adequately addressed. Our work provides mechanistic details of the near-dump-plane, counter-rotating, vortex–vortex interactions and shows how they drive the observed oscillations.