ENGINEERING AND CFD

Research in combustion noise is increasingly gaining interest for mainly two reasons. First, combustion noise can lead to thermoacoustic instabilities. Especially the lean-premixed regime, which is favorable for a highly efficient combustion process with low emissions, is prone to an unstable feedback loop between the flame's unsteady heat release and its resulting acoustic emission that can lead to hazardous loads within the combustion chamber. Second, combustion noise is expected to become an important contributor to the overall sound emission of future jet aero engines, since the sound emission of the other components, such as jet noise, fan noise, etc., is being reduced through recent technological progress.

The goal of this project is to gain a deeper understanding of the acoustic source mechanisms, which is necessary for the prediction and the control of thermoacoustic instabilities in future combustor systems at lean premixed conditions. This is achieved by a hybrid simulation approach. First, the combustion process is predicted with a large-eddy simulation. The resulting flow field determines the noise sources, which are used in the second step to compute the generation and propagation of the acoustic waves. With this approach, the acoustic emission can be analyzed for various setups, i.e., depending on flow parameters, flame parameters and the burner geometry. The numerical data is used to further analyze the interaction between the turbulent flow field, the flame, and the acoustic field.

Figure 1 shows a juxtaposition of the flame shape of an experimental image (left) with LES results and a snapshot from the LES computation (right). The good agreement with the experiment is achieved with a highly resolved flow field and a level-set approach for the flame front which is especially suitable for the analysis of lean-premixed flames due to their thin reaction zones.