During the last decades, significant efforts have been invested by the scientific community in minimizing exhaust emissions from aeronautical gas turbine engines. In this context, many advanced ultra-low NOx combustion concepts, such as the Lean Direct Injection concept treated in the present study, are being developed to abide by future regulations. Numerical simulations of these devices are usually computationally expensive since they imply a multi-scale problem. In this work, non-reactive Large Eddy Simulations of a gaseous-fueled radial-swirled Lean-Direct Injection (LDI) combustor have been carried out through the CONVERGE™ CFD code by solving the complete inlet flow path through the swirl vanes and the combustor. The geometry considered is the gaseous configuration of the CORIA LDI academic combustor, for which detailed experimental measurements are available. The macroscopic analysis of the main turbulent features related to the swirling flow and the Central Recirculation Zone (CRZ) generated within the combustor is well established in the literature. Nevertheless, neither a theoretical approach nor experimental data are yet able to predict which unstable mode dominates the flow. This work aims at using Large Eddy Simulation for a complete characterization of the unsteady flow structures generated within the strong non-reactive swirling flow field when fuelled with a gaseous methane injection. To this end, a spectral analysis of the flow field is performed in order to identify the frequency, intensity and instabilities associated to the phenomena occurring at the swirler outlet region. A coherent structure known as Precessing Vortex Core (PVC) is identified both at the inner and outer shear layers, resulting in a periodic perturbation of the pressure and velocity fields. The pressure and velocity fluctuations predicted by the CFD code are used to compute the spectral signatures at multiple locations through both the Fast Fourier Transform (FFT) and the Power Spectral Density (PSD), in order to investigate the complex behaviour of the PVC and the associated acoustic phenomena. The acoustic characteristics computed by the numerical model are first validated qualitatively by comparing the acoustic spectrum with available experimental data. In this way, the use of dimensionless numbers to characterize the most energetic structures is coherent with the experimental observations and the characteristics of the PVC. Then, the numerical identification of the main acoustic modes in the chamber allows a better understanding of the propagation of the hydrodynamic instability perturbations. This investigation on the main non-reacting swirling flow structures inside the combustor provides a suitable background for further studies on combustion instability mechanisms.


Spectral Analysis of an Aeronautical Lean Direct Injection Burner Through Large Eddy Simulation

Division / Sponsor:
International Gas Turbine Institute (IGTI)

Paper Proceedings: https://asmedigitalcollection.asme.org/GT/GT2020/volume/84058

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