Noise Source Identification for Ducted Fan Systems

Understanding combustion noise source mechanisms, designing efficient acoustic liners and optimising active control algorithms for noise reduction requires the identification of the frequency and modal content of the combustion noise contribution. Coherence-based noise source identification techniques have been developed which can be used to identify the contribution of combustion noise to near and far-field acoustic measurements of aeroengines. A number of existing identification techniques from the literature are implemented and evaluated under controlled experimental conditions. An experimental rig was designed and built to gain a fundamental physical understanding of the convection of noise through a rotor/stator set-up. The identification techniques were applied to this rig and the pressure field was separated into its constituent parts. The underlying assumption with these identification techniques is that the propagation/convection path, from combustion can to measurement point, is a linear one. It is shown with simulations that where the combustion noise propagates in a non-linear fashion the identified contribution will be inaccurate. The experimental rig, consisting of a vaneaxial fan mounted in a duct, allows potential non-linear interaction mechanisms between a convected sound source and the fan to be investigated. Tests carried out on the experimental rig allowed a non-linear interaction tone, between the rotor BPF and a convected tone, to be generated. An experimental technique was developed which enabled the non-linear interaction between the convected sound source with the vane-axial fan to be detected and identified when present. The technique was extended to allow the linear and non-linear acoustic contributions to be separated. The capacity to decompose the coherent output power into linear and non-linear components is a useful tool for the correct design of acoustic treatment for core noise and for an accurate identification of noise source generating mechanisms. The non-linear identification techniques, developed with the experimental rig, were applied to data from full scale turbo-fan engine tests. A Rolls-Royce engine was instrumented with pressure transducers at the combustor can and in the hot-jet pipe, and microphones were placed in the near-field. For a particular power setting, frequency scattering was seen to occur between the combustion noise and the high pressure turbine which was measurable in the hot jet pipe after convection through the many turbine stages. The techniques allowed the non-linear interaction to be successfully identified and linear and non-linear coherent output powers to be determined.