Modeling of Combustion Noise in Turbulent, Premixed Flames

In regular operation all gas turbine combustors have a significant noise level induced by the turbulent high power flame. This noise is characteristic for the operation as it is the result of the interaction between turbulence and combustion. Pressure fluctuations may also be generated by thermoacoustic instabilities induced by amplification by the flame of the acoustic field in the combustor. This paper focuses on prediction of the former process of the noise generation in a premixed natural gas combustor. In order to predict noise generated by turbulent combustion, a model is proposed to calculate the power spectrum of combustion noise in a turbulent premixed natural gas flame on the basis of a steady state RaNS CFD analysis. The instantaneous propagation of acoustic pressure fluctuations is described by the Lighthill wave equation, with the combustion heat release acting as a monopole source term. For a semi infinite tube the solution can be written as a volume integral over the acoustic domain using a Green’s function. The source term is written as a function of a reaction progress variable for combustion. Finite chemical kinetics is taken into account by using the TFC model, and turbulence is described by the k-ε model. Subsequently the volume integral for the noise field is evaluated for the turbulent situation on basis of the calculated steady state combustion solution and presumed shape probability density function weighting. The k- ε model provides the parameters for the presumed spectrum shape. Experiments have been performed in a 100 kW preheated premixed natural gas combustor. Comparison of predicted sound spectra with experimental results shows that the model is capable of prediction of the Sound Pressure Level. The modeled spectrum agrees well with the trends observed in the measured spectra.