Combustion instability effect on sound generation by jet diffusion flame

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.