Passive Noise Reduction for a Contrarotating Fan

There has been renewed interest in the contrarotating (CR) fan configuration in aviation and other applications where size and weight are important design factors. Contra-rotation recovers swirl energy compared with the single-rotor design, but this advantage is not fully harnessed due to, perhaps, the issue of noise. This study explores passive noise reduction for a small, axial-flow, CR fan with perforated trailing-edge for the upstream rotor and perforated leading-edge for the downstream rotor. The fan is designed with simple velocity triangle analyses, which are checked by 3D flow computations. The aerodynamic consequence and the acoustic benefit of such perforated blading are investigated experimentally. The results show that there is a reduction of total pressure compared with the baseline CR fan at the same rotating speeds, but this is easily compensated for by slightly raising the rotating speeds. A reduction of 6–7 dB in overall noise is achieved for the same aerodynamic output, although there is a moderate noise increase in the high frequency range of 12.5–15.0 kHz due to blade perforations. The effect of inter-rotor separation distance is also investigated for the baseline design. A clear critical distance exists below which the increased spacing shows clear acoustic benefits.