ダクト内に設置されたスリット付きケーシング内の軸流ファンの圧縮性解析

<p>Small axial fans are used for cooling electronic devices, and have been often installed in a casing with slits to improve aerodynamic performance in recent years. However, when the fan is actually installed in a product, the inflow region may be limited to narrow space due to other components of the product. In order to clarify the effects of the upstream duct width on the aerodynamic performance and aeroacoustic noise of the axial fan in a casing with slits, the flow and sound around the fan installed in a duct were directly predicted based on the compressible Navier-Stokes equations. The computational methods were validated by comparing the predicted pressure rise by the fan and the sound pressure spectrum with those measured. At the flow rate near the design condition, both the predicted and measured results showed that the pressure rise by the fan deteriorates for H/D = 2.0 and is higher for H/D = 1.0 compared with H/D = 4.0. Narrowing the duct width from H/D = 4.0 to H/D = 2.0 resulted in a 5% decrease in the efficiency defined by the ratio of the pneumatic power to shaft power estimated from the integration of the surface pressure on the fan blades. It is because the outflow of pressurized air through the downstream part of the slit increases for H/D = 2.0 comparing with H/D = 4.0. Both the computational and experimental results showed the sound pressure level in a wide frequency range became larger for the narrower duct width. The vortices caused by the blade tip vortex of a blade were found to be distributed in a larger region between the blades. The interference of the vortices with the pressure surface of the adjacent blade leads to the intensification of the power of pressure fluctuations on the pressure surface particularly in the low frequency range.</p>