Design of Noise Reduced Large Fans for Wind Tunnel Application with CFD-based Optimization – a Case Study

Case Studies: Acoustics

Aeroacoustic wind tunnels require not only appropriate passive sound attenuation measures but also low noise fans. Due to innovative manufacturing methods large fans may now be equipped with highly skewed rotor blades – a proven method for noise reduction. In this case study we describe the development steps of a complete axial fan stage comprising rotor and guide vanes at model scale level.
The reliable prediction of the acoustic emission of a fan is still computationally extremely ex-pensive. Therefore, to achieve the low noise target a substantial skew of the rotor blades and a combination of the counts of blade and guide vanes for minimizing mode propagation in the duct system are chosen. Naive blade skew can introduce 3D-flow effects with a substantial deg-radation of efficiency. Therefore, the rotor blade shape and the guide vanes are optimized aero-dynamically via an automated CFD-RANS-based optimization scheme.
The model scale fan stage is manufactured and its aerodynamic and acoustic characteristics are measured. A comparison with experimental data from a model scale state-of-the-art benchmark wind tunnel fan with unskewed rotor blades reveals that the new optimized design has a consid-erably larger range of operation with good efficiency and without stall. The sound power emit-ted by the optimized fan is substantially lower as from the benchmark over the complete range of possible operation.