Frequency domain predictions of the acoustic reflection coefficient of a combustor exit nozzle with Linearized Navier-Stokes equations

In this investigation the acoustic reflection properties of a combustor exit nozzle is characterized based on a coupled Computational Fluid Dynamics/Computational Aeroacoustic (CFD/CAA) methodology. This hybrid approach utilizes a stabilized Finite-Element method to solve the Linearized Navier-Stokes Equation (LNSE) in frequency space on the basis of a Reynolds Averaged Navier-Stokes (RANS) mean flow state. First numerical results are validated against measurements for different Mach numbers in the nozzle throat at ambient conditions. In this regard, the reflection coefficient of the exit nozzle is determined in an impedance test rig at ambient temperature. The resulting reflection coefficients are also verified against an analytical approach. Subsequently, the RANS/LNSE-approach is used to predict the acoustic properties at combustion conditions, considering exhaust gas temperatures of 1500K and various nozzle throat Mach numbers. The effect of the nozzle-throat Mach number on the predicted reflection coefficient amplitudes of the exit nozzle is demonstrated and acoustic loss mechanisms are discussed.