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High-Frequency Oscillatory Combustion in Tubular Flame Burners(Thermal Engineering)
High-frequency oscillatory combustion in tubular flame burners has been experimentally investigated using two prototype burners of 1MW and 2MW. Oscillatory combustion regions have been mapped in the plane of equivalence ratio and air flow rate, and pressures fluctuations also have been recorded. Results show that large-amplitude, high-frequency oscillatory combustion occurs for near stoichiometric mixtures and when the mean axial velocity exceeds about 5m/s. With increasing the air flow rate, the oscillatory combustion region gradually expands on both lean and rich sides. According to FFT analyses, the peak frequencies during the oscillatory combustion exceed 1000Hz. A simple analysis shows that these high frequencies cannot be explained on the basis of the Helmholtz resonator, or attributed to the axial mode of acoustic instability of the burner tube. These high frequencies can be explained on the basis of coupling of tangential and radial modes of acoustic instability of the burner tubes. Just before the onset of the intense oscillatory combustion, the fundamental tangential mode, which is of asymmetry, appears, and during the subsequent intense oscillatory combustion, higher radial modes appear coupled with the asymmetric and symmetric tangential modes. Since the first asymmetric tangential mode of acoustic instability seems to trigger the intense, high-frequency oscillatory combustion, it is suggested to design the burner as symmetric as possible, and in addition, to make the flame as round as possible.