The paper describes a numerical simulation methodology based on CFD for deriving and analyzing the spectral and spatial characteristics of combustion noise in industrial gas turbine combustors. ENEL is testing the combustion section of a medium size pure hydrogen-fed gas turbine, during which pressure fluctuation levels are also measured to asses the combustion stability. Pressure probes are located in colder zones only, but information on the fluctuations levels throughout the whole component are also desired. For this purpose, a simple empirical and, to the knowledge of the author, original method, based solely on CFD modelling, was developed for reproducing in a realistic way the spatial and transient characteristics of the acoustic flow field inside the combustor volumes. The method employs a sparse and persistent excitation on a wide frequency spectrum, by imposing a stochastic fluctuating component to the velocity computed on each grid node throughout the entire computational domain. The intensity of this additional component is proportional to the local level of the modelled turbulent velocity, so the method will be shortly designated as PRMT (Partial Refluctuation of Modelled Turbulence). This method requires that all the acoustically connected volumes be included in the computational domain and that transients be protracted for a time sufficient to provide meaningful spectral information. KIEN, an in-house low diffusive URANS code capable of simulating 3D reactive flows, was used. The adopted Very Rough Grid approach made it possible to protract the simulated transient for a long time, with an affordable computing time. Comparison with in-plant measured data shows that the refluctuation method provides a realistic qualitative description of the noise spectrum. The spatial distribution of computed acoustic field is also derived and analyzed.
Experimental investigation of entropy waves’ evolution for understanding of indirect combustion noise in gas turbine combustors
