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 investigated. While temperature fluctuations are commonly recognized as the primary source of indirect combustion 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 transmission 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 inhomogeneities 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 spatial inhomogeneities to the core-noise contribution.