Tonal and broadband noise attenuation of an axial flow fan with forward and backward swept blades

A swept blade is one of the efficient ways to control unsteady flow and passively attenuate the noise radiated from an axial flow fan. Herein, an experimental study was conducted to investigate the effect of the blade sweep on the acoustic characteristics of an axial fan system operating under system resistances at an anechoic chamber facility. The noise level and fan performance of the axial fans with forward-swept, backward-swept, and straight blades were systematically measured using an acoustic fan tester, which can simultaneously measure them at various system resistance conditions. The variations in the overall A-weighted sound pressure levels (OASPLs) as a function of the static pressure and air flow rate, and acoustic spectra at the specific operating conditions were examined. The results showed that the forward-swept blade has superior aerodynamic and acoustic performance in a wide range of operating conditions to the other blades, except for the high system resistance corresponding to rotating stall conditions. In conclusion, the use of a forward-swept blade can achieve noise level reductions of 2–4 dBA under system resistance conditions since the amplitude of tonal noise associated with blade passing frequency is decreased.