In this paper, the application of the Equivalent Source Method (ESM) and the acoustical Boundary Element Method (BEM) for the prediction of the noise generated by an open diffusion flame is investigated. These acoustical methods have been coupled with a Large Eddy Simulation (LES) of the turbulent flow, which stimulates the flow and combustion processes in a source region in the vicinity of the flame. Among the hybrid methods which are being used to predict the sound produced by turbulent flow, the ESM and BEM have the advantage that only one acoustic variable must be known at a surface surrounding the source zone (fewer data has to be processed) and that the far field can be directly computed. The sound power generated from two open diffussion flames have been calculated with both the ESM and the BEM, using the velocity distribution over cylindrical control surfaces, which enclose the source region. The results of the calculations are presented and compared with the measured sound power of the same flames. For one configuration good agreement between measurement and simulation at low and middle frequencies is obtained. Possible reasons for the differences for the other configuration will be discussed.

Equivalent sources have been successfully used to calculate the sound radiation and the sound scattering from solid bodies lying in a homogeneous medium without flow. For the field determination an acoustic boundary condition at the body surface must be known. In an earlier work, the application of this method to compute the sound radiation of open turbulent flames was investigated in order to extend the range of use of this basic method to aero- thermoacoustic problems. It was assumed that outside a region surrounding the flame, the flow and temperature gradient had strongly decayed and approximate homogeneous conditions existed. The necessary data at a control surface (Kirchhoff surface) surrounding the combustion zone was delivered by an incompressible Large Eddy Simulation (LES). Measurements carried out of two simulated flame configurations showed that while the spectrum of one flame was well reproduced, the spectrum of the second flame was satisfactorily matched only in some positions. In the present work, additional calculations are made trying to explain and reduce these differences, for example, calculations using different control surfaces, using open surfaces and including a constant background flow. The results obtained are presented and discussed.

The Boundary Element Method (BEM) is a well-established and efficient tool for the calculation of sound radiation from vibrating structures. Up to now few attempts have been made to enhance this method for the use in aero- and thermoacoustical simulations. In this presentation, the BE method is applied to the calculation of the sound radiation of an open, turbulent flame. The calculation is realised by coupling of a Large-Eddy-Simulation (LES) with the BEM. The LES calculates the velocity field in the close vicinity of the flame. Based on these data BEM is used to calculate the radiated sound field. The results are compared with measurements. To determine the directivity of the sound field, the reflections from the ground or floor have to be taken into account. As long as the impedance of the reflecting plane is infinite or zero, this can be easily achieved by including image sources. A new approach for solving a finite impedance problem will be discussed. In this approach the image sources are equipped with complex source points. The results for selected test cases as well as the effect on the directivity of the sound field of the flame will be presented.

A hybrid approach based on large eddy simulation (LES) and the equivalent source method (ESM) as well as the boundary element method (BEM) is applied to evaluate the noise radiation of open turbulent non-premixed jet flames. Hybrid approaches are well known and often used for classical non-reacting turbulent flows. The extension to turbulent reacting flows is presented here. For an open flame there is no strong influence of the acoustic field onto the flame. This is in contrast to enclosed flames, where the acoustic waves can easily induce combustion instabilities. Therefore, the flow simulation can be decoupled from the acoustic simulation. Furthermore, the very low Mach number of turbulent jet flames allows for an efficient incompressible formulation of the LES. From the instationary flow field resulting from the LES, the required information on a control surface or interface to the

The equivalent sources method (ESM) was used to compute the sound radiation of an open flame from data of the velocity fluctuations obtained with a CFD code by means of a Large Eddy Simulation (LES). We thank Prof. Janicka and Mr. Flemming from the TU-Darmstadt for putting the data to our disposal. The results are compared with those obtained by a BEM program with the same input data. This work is integrated in the Research Project "Combustion Noise", supported by the German Research Foundation (DFG) (2).