Acoustic Optimization Approach for Annular Diffusers in Turbomachinery Applications Using Plane Wave Modelling

Abstract

The experimental investigation of unsteady aerodynamic effects in turbomachinery test rigs requires the establishment of anechoic boundary conditions in order to acquire data unaffected by acoustic reflections outside the test section. In this manner, controlled measurement environments can be achieved that allow for the acquisition of high-quality aeroacoustic and -elastic measurement data in turbomachinery test rigs. In the case of the initial design of the Aeroacoustic Wind Tunnel (AWT) at the Institute of Turbomachinery and Fluid Dynamics, acoustic reflections were observed in the diffuser section of the test rig, which interfered with the sound transmission measurements of low-pressure turbine airfoils. The present paper describes a one-dimensional semi-analytical modelling and optimization approach of sound propagation in the AWTs diffuser section for plane waves based on Webster’s horn equation. With this approach, a new hub diffuser was designed for defined geometric and aerodynamic boundary conditions, which compared to the original geometry, reduce acoustic reflections due to continuous impedance matching. The design of a new supporting strut configuration further reduces reflections due to scattering from installations in the flow path. Validation of the modelling and solution approaches are carried out based on a comparison with experimental data of the initial (reference) design, as well as numerical simulations of both designs. According to analytical and numerical models the optimized design reduces acoustic reflections by up to 21.2 dB compared to the initial design.