Thermo-acoustic Mechanism Study On Vortex Shedding Induced By Flame Variation

Lean premixed combustion technology has been widely studied for its low emission, however, Thermo-acoustic oscillation has been the key problem which hindering its widely application. Although many mechanisms have been developed to interpret thermo-acoustic phenomenon, the mechanism which is responsible for thermo-acoustic has not been confirmed.In this paper, we developed a mechanism on the basis of former researchers, which integrates the flame variation mechanism and vortex shedding mechanism and try to confirm the mechanism by theoretical derivation, numerical simulation and experiment. In theoretical derivation process, we developed a heat release model derived from flame variation mechanism and study the contributing factors of the heat release model; in the numerical simulation process, we confirmed the modal analysis is powerful tool in thermo-acoustic study and solve the inherent frequencies, pressure shape and amplitude growth rate in frequency domain; stable and instable region of flame variation induced is simulated; in the experiment process, vortex shedding is confirmed to be the mechanism which is responsible for the thermo-acoustic oscillation, with pity that the vortex shedding is not induced by flame variation. We conclude three typical phenomenon named "sound not related to heat release", "sound related to low frequency heat" and "sound related to high frequency heat" and explain how the three phenomenon take place. Important conclusion is as below:In the process of theoretical derivation:1 Combustion expansion has important impact on amplitude-frequency-characteristic of heat release model related to flame variation, however it has no impact on phase-frequency-characteri stic.2 Height of combustor, height of central body and mean velocity have important impact on heat release model related to flame variation and the law is different at different combustion expansion condition.3 the turbulent flame speed is the most important parameter that influence the magnitude and phase of the heat release model, which can strongly change the time delay between heat release and sound.In the process of numerical simulation:1 Modal analysis is a powerful tool in thermo-acoustic study which can solve the inherent frequencies, pressure shape and amplitude growth rate in frequency domain2 Inherent frequencies of thermo-acoustic oscillation increase with equivalence ratio and we can confirm that thermo-acoustic oscillation has been taken place when the frequencies and pressure shape are right at the same time.3 The influence of inlet air temperature and equivalence ratio on the growth rate is strong. However, the influence of inlet air temperature in much complex, the growth rate of First-order and Third-order frequencies are much larger in low temperature and the growth rate of second-order frequency is much larger in high temperature.4 Stable and instable regions of flame variation induced oscillation are different for different air mass flow and equivalence ratio.In the process of experiment:1 vortex shedding is confirmed to be the mechanism which is responsible for the thermo-acoustic oscillation, with pity that the vortex shedding is not induced by flame variation.2 "sound not related to heat release", "sound related to low frequency heat", "sound related to high frequency heat" phenomenon are found and time delay between heat release and sound is the most out-standing different factor between them.3 Time delay between heat release and sound is the most important factor responsible for sound related to heat. When the time delay of high energy modal is in the instable region, the sound oscillation is strong; when the time delay of low energy modal is in the instable region, the sound oscillation is weak.4 Thermo-acoustic oscillation is an extremely condition of "sound related to high frequency heat" phenomenon. It can only take place when its frequencies are equal to inherent frequencies.In summary, vortex shedding is confirmed to be the mechanism which is responsible for the thermo-acoustic oscillation, with pity that the vortex shedding is not induced by flame variation and Thermo-acoustic oscillation is an extremely condition of "sound related to high frequency heat" phenomenon.