||本研究以聲導納(Acoustic admittance)量測法探討長方形管道燃燒室中, 預混火焰自發性之震盪。燃燒爐冷流及熱流聲導納之數據, 是以量測壓力振盪振幅, 速度振盪之振幅, 以及壓力與速度振盪之相位關係而直接獲得。實驗結果顯示, 在某此情況下, 火焰產生自發性之振盪, 而冷流聲導納扮演非常重要之角色; 熱流聲導納主要由冷流聲導納來主導, 火焰之存在僅改變壓力與速度振盪振幅比值及相位關係之值域, 但未改變其分佈; 冷流聲導納之虛部為正, 火焰則有可能產生自發性振盪, 且火焰提供火焰振盪能量之多寡, 可大致由冷流聲導納虛部來預測。此研究結果同時顯示, 火焰自發性振盪之頻率是並未非產生在火焰提供能量最大之頻率下; 火焰自發性振盪之存在必須同時滿足冷流聲導納虛部為正、火焰提供足夠能量、以及火焰與管道產生共振。
Self-excited acoustic oscillations in a combustor with flames stabilized on a burner surface and burning in a rectangular duct were investigated by using acoustic admittance measurements. The admittance data were obtained by directly measuring the amplitudes of the velocity and pressure oscillations, and the phase relations between the velocity and pressure oscillations. Results of this study show that the investigated premixed flames exhibit multi- frequency, self-excited flame and pressure oscillations at certain flow velocities and equivalence ratios. The nonreactive burner admittance plays an important role in these oscillations. Self-excited flame oscillations are favored for positive imaginary parts of the nonreactive burner admittance. The characteristic of the reactive burner admittance is mainly dominated by the acoustic property of the nonreactive burner. The presence of flames only modifies the values, but not the distributions of velocity and pressure oscillations, where the modification depends on the equivalence ratios. Only those flames possessing enough energy are able to cause the right modification and to sustain the self-excited acoustic oscillations in combustors. This research concludes that the existence of self-excited acoustic oscillations in combustors must simultaneously meet the requirements, including the positive imaginary parts of the nonreactive burner admittance, the flames possessing enough energy, and the flame oscillations in resonance with the combustor system.