Entropy Transport and Noise Generation Mechanism in Modern Aero-Engine Combustors

A large-eddy simulation (LES) of a combustor rig is utilized to estimate the direct and indirect noise generation of a modern N+1 lean-staged aero-engine combustor. The combustor flow physics is first studied by using a Spectral Proper-Orthogonal Decomposition (SPOD) of the LES flow-field to identify the primary hydrodynamic and acoustic modes. The SPOD shows that the high frequency tone around 1900 Hz is due to the longitudinal acoustic mode within the combustor. The acoustic tone at 350 Hz and the lower frequencies are shown to be associated with convection velocities of the entropy perturbations and hence due to indirect noise. The total noise of combustor is a result of both the unsteady heat release (direct noise) as well due to accelerating entropy perturbations within the combustor. These are computed using the Helmholtz equation and the Linearized-Navier Stokes equations and the LES mean flow. The difference between the total and the direct noise is considered the indirect noise and shown to be dominant primarily at the lower frequencies below ~400 Hz for this combustor.