Investigation of the acoustic behaviour of modern combustion chambers using a hybrid LES/CAA approach
Due to the reduction of fuel consumption and new global emission limits especially for the pollutant emissions of NOx a change to lean premixed combustion technologies in aeroengines combustors are unavoidable.
Near to the lean limits combustion tends to be unstable and a coupling between unsteady heat release and acoustic perturbations lead to the phenomenon of thermoacoustic instabilities. These thermoacoustic instabilities occurs if the unsteady heat release fluctuations are in phase with the acoustic pressure fluctuations.
The coupled mechanism described by the rayleigh criterion leads to excessive vibrations, high levels of acoustic noise and high temperature caused by a change in reaction stoichiometry. The combustion chamber could be damaged or destroyed if the frequency of pressure fluctuations match the characteristic frequency of the combustor. To guarantee a reliable and efficient operation of the combustor in all situations the challenges of lean premixed combustion have to be investigate.
An incompressible unstructured turbulent reacting Large Eddy Simulation (LES) is used to simulate combustion and study the impact of unsteady heat release as a major source of acoustic noise. For a simultaneous simulation of the reacting flow with its acoustic properties a numerical code from the field of Computational Aero Acoustics (CAA) is coupled with the LES to a hybrid LES/CAA approach.
The advantage of a incompresssible hybrid LES/CAA approach compared to a compressible LES approach is accounted for by the possibility of using larger timesteps in the flow simulation which are not related to the timestep limitation depending of the speed of sound.
Extend the hybrid LES/CAA approach to investigate
- enclosed premixed or partially premixed swirl burner configurations
- accelerated entropy waves (indirect acoustic waves) and its contribution to the combustion
- two way coupling processes between acoustic waves and combustion