Abstract
This paper presented a numerical study of flow instability effects on the sound generation by jet diffusion flames. Numerical simulation coupled with the acoustic analogy based on Lilley’s equation was employed to predict the sound radiation. A parametric study by varying the Froude number was carried out, and simulations were performed with different levels of external forcing to shed light on the competing acoustic responses of the buoyant and jet preferred instabilities. The detailed sound source structure was provided by the flow details. It is shown that the increase of buoyancy is responsible for the development of the acoustic source in the downstream flame region. When the Froude number is larger than unity, only low-frequency noise will be affected, and when the Froude number is less than unity, combustion-induced buoyancy has a further positive impact on the noise level in the high-frequency region. The jet preferred instability caused by external disturbance can improve the intensity of the sound source and may further be amplified as the noise peak, depending on the buoyancy level. An acoustic transfer function was therefore analyzed to characterize the different acoustic responses. The results showed that there is a positive correlation between the buoyancy level and the acoustical response rate of external disturbance, and the high-frequency disturbance is much easier to be amplified by flame than that with low frequency, especially in the downstream flow field.
Highlights
Combustion sound is the natural characteristic of most energy producing systems, such as aircraft engines, industrial burners, and gas turbines.1,2 The consecutive rise and fall of pressure will have an ongoing impact on those exposed to noise over periods and can further reduce the energy productivity due to the unexpected thermo-acoustic instability.3–6 To reduce the noise level in such devices, the mechanisms of sound generation by reacting flows should be understood
This paper presented a numerical study to investigate the combustion-related instability effects on the sound generations by diffusion flames
Direct simulation coupled with an acoustic analogy based on Lilley’s equation was utilized to seek insights into the sound problems
Summary
Combustion sound is the natural characteristic of most energy producing systems, such as aircraft engines, industrial burners, and gas turbines. The consecutive rise and fall of pressure will have an ongoing impact on those exposed to noise over periods and can further reduce the energy productivity due to the unexpected thermo-acoustic instability. To reduce the noise level in such devices, the mechanisms of sound generation by reacting flows should be understood. In the case of combusting flows, Strahle speculated that the flow dilatation caused by a chemical reaction is the dominant cause of noise generation. In his subsequent work, he argued that the fluctuation of heat release rate is synonymous with flow dilatation and concluded that the combustion sound can be estimated by viewing the whole flame as a single sound radiator. He argued that the fluctuation of heat release rate is synonymous with flow dilatation and concluded that the combustion sound can be estimated by viewing the whole flame as a single sound radiator This theory simplifies the prediction of far-field sound because of the neglect of flame structure and fuel type. The simplified model continues to yield useful insights, it leaves considerable confusion regarding the near-field noise mechanisms
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