On the Generation of Direct Combustion Noise in Turbulent Non-Premixed Flames

Abstract

Generation of combustion noise in an unconfined turbulent non-premixed flame is investigated. For this, a model is developed, combining Lighthill's acoustic analogy with a flamelet-based combustion model to consistently express all thermochemical quantities by a set of reduced scalars. The model is applied in a large-eddy simulation, and the acoustic pressure in the far field is obtained from an integral solution. Three relevant acoustic source terms with different source characteristics and Mach number scaling are identified. The spatial distribution and spectral characteristics of the acoustic sources are analyzed, and it is shown that the acoustic source due to chemical reaction is the main noise contributor, and is located in the upper part of the flame. Contributions from the acoustic sources due to Reynolds stresses and fluctuating mass flux are found to be virtually insignificant at low frequencies. Discrepancies in the prediction of high-frequency sound pressure level in the jet forward direction were analyzed and are attributed to high-frequency acoustic refraction effects due to variations in sound speed. The directivity exhibits a weak directionality in the 30° forward direction, and some phase cancellation between individual acoustic sources is evident.