Investigation on the Noise of an Axial Low Mach-Number Stage with a Heterogeneous Stator
2.1 Noise Generation Mechanisms
A low-Mach number ducted axial fan (LP3) from Safran Ventilation Systems is investigated in this study. This configuration has a short fan-OGV distance in the order of the blade chord. Moreover, three stator vanes of the stage configuration are thicker than the others. Those features will strongly modify the noise-generation mechanisms. First, rotor-stator interaction noise mechanisms will be stronger because of the shorter separation. For instance, the potential interaction noise generated by the interaction of rotor blades with the stator potential field is expected to be relevant. Secondly, the heterogeneity may cause the propagation of modes that would not be excited in a homogeneous configuration.
For this investigation, a numerical simulation of the LP3 fan-OGV will be performed using Powerflow, a Lattice-Boltzmann solver from Exa. The solver being naturally transient and compressible, the simulation will provide a direct insight into unsteady phenomena and thus acoustic propagation. The simulation will account for the full fan-OGV geometry and the fan configuration will be installed as in its experimental environment. For this reason, the simulation domain is chosen according to the real dimensions of the LP3 facility. The operating conditions are also chosen according to previous experiments performed at Ecole Centrale de Lyon.
A comprehensive modal analysis will be performed upstream and downstream of the fan in order to retrieve the modal content of this configuration. A detailed analysis of the aerodynamic excitations will also be provided by extensively studying the inter-stage. The authors want to assess the impact of an heterogeneous stator regarding the in-duct acoustic propagation. In fact, with the number of blades and vanes of this configuration, the first two blade passing frequencies (BPF) should be cut-off by the duct. Yet, both BPF have been observed experimentally. The presence of thickened stator vanes which are breaking the stator periodicity could explain this behavior. The final objective is to compare the simulation and experiments in terms of in-duct spectra and modal content. This comparison of the simulation with previous experiments should allow a better understanding of rotor-stator interaction noise in configurations with short fan-OGV distances and with a heterogeneous stator.