An experimental study on the high frequency oscillatory combustion in tubular flame burners

In this study, high frequency oscillatory combustion in tubular flame burner was experimentally investigated using large scale 8- and 12-in. diameter tubular flame burners. The conditions for the high frequency oscillatory combustion were determined, and the pressure fluctuations were measured, on which spectral analyses were made. The results showed that a smooth laminar tubular flame could be established, however, high frequency combustion sound was emitted from the 8- and 12-in. burners when the air flow rates exceeded 650 and 1200 m³/h, respectively. Pressure fluctuation measurements and spectral analyses showed that high frequency pressure fluctuations occurred simultaneously during the high frequency oscillation. The flame images were also obtained with a high speed video camera. The flame surface was found to be notably corrugated and the symmetry was broken during the oscillation. By assuming that the tubular flame burner is a simple tube, the natural frequencies of the burners were determined using the fundamental theory of the acoustic resonance in a cylindrical cavity, and the experimental peak frequencies in the spectra were compared with the natural frequencies. As a result, it was found that the high frequency oscillations in both the burners were identified as the tangential/radial mode acoustic resonant oscillations. It was further found that the tangential first mode of oscillation, which had an asymmetric structure, preceded the higher modes of oscillation which indicated that the mode dominated the occurrence of the high frequency oscillation. To verify the occurrence of the tangential first mode oscillation, the pressure fluctuations were measured with two pressure sensors installed at opposed locations. The results showed that the phase of the oscillation was 180° difference, and the asymmetric structure confirmed the occurrence of the tangential first mode of oscillation.