Study on the Combustion Roar and its Application to the Combustion Diagnostics

本文将燃烧系统中的声分为湍流火焰的非均匀结构所产生的“燃烧轰鸣声”和系统共振或反馈现象所产生的“共振声”，并结合离子流和OH自由基光发射的涨落对湍流本生火焰中的声进行了研究。人们认为，开放预混湍流火焰的燃烧噪声是由反应速率的波动，从而在火焰刷的热释放速率和波动特性的时间导数，如光发射强度或火焰中的离子电流，是很好地相关的热释放速率的波动。基于本研究的观察，提出了一个更详细的燃烧声的机制。结果发现，从开放的湍流预混火焰发出的声音不是由单个源产生的，而是由包括以下三个的多个源组成。(1)由于湍流涡流的燃烧和由此产生的热膨胀，在火焰元件中局部产生“局部燃烧轰鸣”。(2)火焰顶端的热释放速率波动引起的“火焰顶端燃烧轰鸣”，而上游火焰传播扰动增强了火焰顶端的热释放速率波动。(3)由于射流边界处的湍流而产生的“射流声”。上游火焰元件中的湍流燃烧以相对低的声级发出“局部燃烧轰鸣”。但是，不稳定性和燃烧产生的扰动向下游传播，并增强了火焰尖端附近的热释放率的波动，因此产生了“火焰尖端燃烧轰鸣”。结果表明，OH自由基的光发射和离子电流与局部热释放速率有很好的对应关系，声学诊断可以成为研究湍流火焰的有效策略。

The sound emitted from combustion systems is classified into the "combustion roar" generated by the inhomogeneous structure of the turbulent flame and the "resonant sound" produced by the resonance or feed-back phenomena of the system.The sound emitted from a turbulent Bunsen flame was studied in connection with the fluctuations of ion current and OH radical light emission. It has been held that the combustion noise of an open premixed turbulent flame is dominated by the fluctuation of the reaction rate, hence of the heat release rate in the flame brush and the time derivatives of fluctuating properties, such as optical emission intensity or ion current in the flame, are well correlated with the heat release rate. Based on the observations of the present investigation, a more detailed mechanism of combustion sound was proposed. It was found that the sound emitted from an open turbulent premixed flame is not generated by a single source but consisted of multiple sources comprising the following three.(1)"Local combustion roar" generated locally in the flame element due to the combustion of turbulent eddies and the resultant thermal expansion.(2)"Flame tip combustion roar" caused by the fluctuation of the heat release rate at the flame tip which was enhanced by the propagated flame disturbance originated in the upstream.(3)"Jet sound" due to the turbulence at the jet boundaries. The turbulent combustion in the upsteam flame element emits the "local combustion roar" at a relatively low sound level. However, the disturbance generated by the instability and combustion propagates downstream and enhances the fluctuation of the heat release rate in the vicinity of the flame tip, hence the "flame tip combustion roar". It was suggested that both the light emission of OH radicals and the ion current corresponded well to the local heat release rate, and the acoustic diagnostics can be effective tactics for the investigation of turbulent flames.