Effect of porous casing on small axial-flow fan noise

Abstract A novel casing treatment is studied to suppress the tip leakage vortex and postpone stall inception, leading to significant noise reduction. The new casing is made by open-cell metal foam, and its effect on both aerodynamic and acoustic performance of an axial-flow fan is investigated experimentally. As much as 10 dBA noise reduction is achieved, in the otherwise stalled flow condition. The mechanism is qualitatively analyzed by computational fluid dynamic simulations for three-dimensional, transient flow. It is found that, with the porous casing, the strength of tip leakage vortex decreases mainly because of the suppression of the pressure difference between the pressure side and suction side of the blade and the flow momentum transport at fluid-porous interface. By experimenting with different casing surfaces, the roughness of the porous casing is found not to be responsible for the delay of stall. Moreover, direct sound absorption by the porous casing is found to be negligible due to the limited volume of metal foam. The working mechanism is thus identified as purely aerodynamic.