Abstract
A counter-rotating configuration can decrease the size and weight of fans by eliminating stators and increase the efficiency by recovery of the swirl velocity losses. However, that potential is not fully harnessed due to perhaps the issue of noise. This study explores one idea of passive noise reduction for a small axial-flow counter-rotating fan (120 mm in diameter) by the introduction of slitted trailing-edge for the forward rotor. The fan is designed with simple velocity triangle analyses which are checked by 3D steady-flow numerical simulation and experimental measurements of aerodynamic performance. The aerodynamic consequence and the acoustic benefit of such slit geometry are investigated experimentally when the separation distance between the forward and aft rotor is 4 mm. The results show that there is a reduction of total pressure compared with the baseline fan (without slit) at the same rotational speeds, but this is easily compensated for by slightly raising the rotational speeds. A reduction of about 5 dB in overall noise is achieved for the same aerodynamic output in all directions around the fan center. The spectral comparison at the fan inlet indicates that the most prominent interaction noise peaks are suppressed greatly by such trailing edge slits.
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