Influence of interaction with tangential lean blade and box type casing on blade passing frequency noise level in small axial-flow fans

Influence of interaction with a tangential lean blade and a box type casing on blade passing frequency (BPF) noise level in small axial-flow fans was investigated by measurement and computational fluid dynamics (CFD). Noise level of five fans having different tangential lean angles was measured. By increasing tangential lean angle, BPF noise level and broadband noise level reduced. An unsteady CFD simulation of tangential lean angle 31-degrees and 63-degrees was conducted to analyze the cause of BPF noise reduction. By increasing tangential lean angle, the inward blade force of the 63-degrees blade was increased compared to the 31-degrees blade. The increase of the inward blade force decreased centrifugal flow in radial direction at the exit of casing, because the inward blade force gave a momentum to the flow in radial inward direction. The decrease of the centrifugal flow suppressed interaction of the flow with the casing wall. The suppression of the interaction reduced static pressure fluctuation for BPF of the casing wall. The reduction of static pressure fluctuation for BPF was found to have a great influence on the BPF noise reduction. It is therefore clarified that increasing tangential lean angle was effective in reduction of the BPF noise.