Effects of Tip Clearance on the Noise Generated from Low Pressure Axial Flow fans : Difference Caused by the Width and Location of a Ring Set in the Tip Clearance and the Blade Tip Profile

The effect of the width and the location of a ring set in the clearance between a duct inner wall and a blade tip and the profile of a blade tip on fan noise are experimentally investigated by using three rings of different widths and four impellers with different tip geometry. The results are summarized as follows : A considerable amount of noise reduction can be expected by setting a ring within a tip clearance, when its initial gap is rather large. Both the noise and the aerodynamic characteristics at the maximum efficiency point are better in the case with a larger gap on the leading edge side than on the trailing edge side. In the lower flow rate range, however, the fan with a smaller tip clearance on the leading edge side is superior