Tomographic reconstruction of an azimuthally forced flame in an annular chamber

Azimuthally forced spinning modes on a single ethylene-air swirl flame in an annular combustor were investigated using phase-averaged three-dimensional (3D) Computed Tomography of Chemiluminescence. Strongly spinning azimuthal modes in the clockwise (CW) and anticlockwise (ACW) directions were introduced by means of carefully controlled acoustic forcing. Tomographic reconstructions were calculated based on 25 independent views for an unforced, and CW and ACW forced cases. Using only a single flame in an annular enclosure resulted in a flame shape that differed from previous multi-flame investigations. The flame experiences significantly less flame-wall and no flame-flame interactions. Results at high and low equivalence ratio indicate that in the absence of significant enclosure effects, the global heat release rate (HRR) response is the same for both forcing directions. The 3D HRR oscillation distributions were investigated in detail, and the local flame response to modes with opposite spin direction was shown to feature 180∘ rotational symmetry. High magnitude HRR oscillations are formed close to the flame base, but due to the combined azimuthal and axial excitation the response on one side of the flame is significantly higher in magnitude, and concentrations of HRR oscillations are seen close to the inner or outer annular walls during CW or ACW excitation respectively. Swirl also causes these high magnitude structures to rotate slowly with downstream distance, although the spatial redistribution of HRR oscillations through this effect is subtle. This improves our physical understanding of previous observations of differing global flame responses to CW and ACW azimuthal excitation.