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Effect of Acoustic Excitation on Lean Blowoff in Turbulent Premixed Bluff Body Flames

机译:激振对湍流预混钝态火焰中稀薄喷吹的影响

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The lean blowoff characteristics of a premixed air-methane flame were investigated in a ducted combustor with a bluff body according to acoustic excitation. The blowoff equivalence ratio increases with the Reynolds number and changes depending on the extent of the recirculation zone. Using the relation between the Damköhler number and the Reynolds number, it was confirmed that the flow velocity at the downstream tip of the bluff body and the laminar flame speed are decisive blowoff factors. Although a periodic flame hole appeared far from the blowoff only with acoustic excitation, the blowoff observed by OH radical chemiluminescence occurred using a similar process regardless of the excitation. The recirculation zone collapses and the flame becomes small when it is close to the blowoff. Then, the flame is locally extinguished downstream from the bluff body and the recirculation zone completely collapses. Eventually, the unburned gas does not ignite and the flame is extinguished. The blowoff equivalence ratio rapidly increases at specific acoustic excitation frequencies. This was investigated using proper orthogonal decomposition analysis, the two-microphone method, and phase-lock particle imaging velocimetry measurement. Resonance occurs when the excitation frequency approaches the harmonic frequency of the combustor and it increases the velocity fluctuation in the combustor and the infiltration velocity of the unburned gas in the shear layer of the recirculation zone. Consequently, because the burning velocity must have a larger value corresponding to the enhanced mixture velocity for a sustained flame, the blowoff occurs at a higher equivalence ratio.
机译:根据声激发,在带有钝体的管道燃烧器中研究了预混气甲烷火焰的稀薄吹气特性。吹扫当量比随雷诺数增加而增加,并随再循环区的范围而变化。利用Damköhler数与雷诺数之间的关系,可以确定钝体下游端的流速和层流火焰速度是决定性的吹除因素。尽管仅通过声激发,周期性的火焰孔才出现远离吹扫的现象,但是无论激发如何,都通过类似的过程通过OH自由基化学发光观察到了吹扫现象。靠近吹气口时,再循环区破裂,火焰变小。然后,火焰在钝体的下游被局部熄灭,再循环区完全塌陷。最终,未燃烧的气体不会点燃,火焰熄灭。在特定的声激发频率下,吹扫当量比迅速增加。使用适当的正交分解分析,双麦克风方法和锁相颗粒成像测速仪进行了研究。当激发频率接近燃烧器的谐波频率时,会发生共振,这会增加燃烧器中的速度波动以及再循环区剪切层中未燃烧气体的渗透速度。因此,由于燃烧速度必须具有较大的值,该值对应于用于持续火焰的提高的混合速度,因此以较高的当量比发生吹散。

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