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

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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.