For an axial-flow fan installed in a duct, the effects of casing slit shapes on pressure rise, aerodynamic sound and flow field were investigated. By a tuft method, an inflow through the upstream part of slits and outflow through the downstream part of slits were found to occur for the baseline slit. The pressure rise by the fan was increased by closing the downstream part of slits, where the outflow of the fluid pressurized by the fan was suppressed. From the measurements of the noise and the wake velocity fluctuations, the turbulence in the rotor wake and related broadband noise were clarified to be intensified when the upstream or downstream part of slits of the casing is closed. The periodic interference between the rotor wake and struts was intensified due to velocity fluctuations at the blade passing frequency in the wake for the slits closed on the downstream part.
