Measurements of the flow field around an axial fan with acoustic resonance in an upstream duct

Abstract

For a small axial-flow fan installed in a duct, the effects of the acoustic resonance occurring in the duct on the flow field and aerodynamic characteristic were investigated. The measurements of the sound pressure level around the fan installed in two ducts with different widths showed that more intense acoustic resonance occurs for the narrower duct at the blade passing frequency for a specific rotational speed. Moreover, the static pressure coefficient of the fan was found to drop at the specific rotational speed where the intense acoustic resonance occurs (resonance condition). To clarify the effects of the acoustic resonance on the immediately downstream wake flow of the fan, velocity measurements were conducted by a hot-wire anemometer. As a result, the time average velocity became lowest at the resonance condition for the narrower duct. This is possibly because the circumferential velocity became low due to a decrease in pressure load causing the curvature of the flow by the fan blade. Moreover, the power of velocity fluctuations around the blade tip in the frequency range lower than the blade passing frequency became largest at the resonance condition, which indicates the intensification of disturbances in the tip leakage vortices. The decreased pressure load and increased turbulence lead to the deterioration of the fan pressure coefficient.