Analysis Of Core-Noise Contributions In A Realistic Gas-Turbine Combustor Operated Near Lean Blow-Out

The relative importance of direct and indirect combustion noise in a realistic gas-turbine combustor is in-vestigated. While temperature fluctuations are commonly recognized as the primary source of indirect com-bustion noise, recent theoretical analysis has shown that mixture inhomogeneities and associated variations in the Gibbs free energy represent another indirect noise-source contribution that is further investigated in this study. To this end, a hybrid model is developed that combines large-eddy simulations for predicting the unsteady turbulent reacting flow field in the combustor with a linearized Euler solver to describe the trans-mission and generation of noise through the downstream nozzle. By considering an operating point near the lean blow-out limit at cruise conditions, it is shown that indirect noise has an appreciable contribution to the overall noise emission at low frequencies, and direct noise arising from a tonal instability in the combustor dominates at higher frequencies. At this operating point, indirect noise contributing from compositional in-homogeneities was found to be comparable in magnitude to entropy noise from temperature inhomogeneities. A modal analysis of the indirect noise sources showed that the entropy and compositional noise are shifted in phase, resulting in a cancellation of the indirect noise. Effects of Mach number and modal shape of the combustor-exit perturbations on the noise generation are investigated, demonstrating the importance of spa-tial inhomogeneities to the core-noise contribution. (C) 2020 The Combustion Institute. Published by Elsevier Inc. All rights reserved.