Abstract
Tip-clearance noise induced by tip-leakage flow in axial flow fans, with the blades skewed circumferentially at angles of ±8.3°, has been investigated under off-design conditions by numerical simulation and experiments. Aerodynamic and aeroacoustic performances have been measured. Tip-leakage flow has been investigated by Navier-Stokes flow simulations at off-design conditions. The characteristics of noise sources in the tip-clearance region, obtained numerically, have been analyzed using vortex sound theory. Synergy between the tip-leakage vortex and the noise source in the tip region is discussed. Moreover, the noise source in the tip region has been measured by dynamic pressure sensors mounted in the casing wall near the blade tip and the acoustic far field has been obtained simultaneously as a function of flow rate. The frequency characteristic of the tip noise component is analyzed by measured results. The characteristics of noise source reveal that tip-clearance noise is associated with skewed blades, which are found to be effective in controlling tip-leakage flow and in eliminating the noise source in the tip-clearance region at off-design conditions. Forward-skewed blade are more effective than backward-skewed blades in reducing noise
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