Yoshio Kodama

The effects of tip clearance on the noise of low pressure axial and mixed flow fans

Abstract Results are presented of a systematic experimental investigation of the effects of tip clearance on both noise and performance of four commercially representative fans (three low pressure axial flow and one mixed flow) in a circular duct, both with and without coincidence of the fan and duct axes (fan/duct eccentricity). It is shown that with eccentricity significant pure tone noise is generated due to blade tip/duct wall interaction, with a plane wave mode component. Reducing tip clearance both improves performance and reduces noise, not only at the maximum efficiency operating point but also in an appreciable low flow rate operating region. With tip clearances small enough to optimize performance and minimize noise fan/duct eccentricity must be kept to a very small value, implying strict precautions in fan and duct manufacture, installation and service conditions.

Improvement of Centrifugal Fan on Noise With Inducer

We improved the centrifugal fan for the noise and fluid performance. The fan has the inducer that is unified to the main blade. The geometry of the inducer was obtained with the tuft method and is unique. The fluid turns from axis- to radial-direction in the inducer. The inlet angle and the chord length of the inducer vary in span position. The aerodynamic and noise characteristics of the impeller with inducer have been compared to one without inducer. The total pressure and the efficiency of the impeller with inducer are increased about 20% and 5% in all flow rate. Because the pressure rise occurs in the inducer. And the pressure loss at the inlet of the blade becomes very small. The influence of the leakage flow between the shroud and the bellmouth is restricted near the front shroud, and then the separation of the boundary layer is controlled. The wake width of the main blade becomes fairly small. The specific noise level, which presents the noise performance of the fan, can be reduced about 2–4dB in wide flow rate.Copyright

Reduction of Turbulent Noise Generated from a Counter-Rotating Fan Using a Sound-Absorbing Duct

The purpose of this study is to investigate the effects of the length and the arrangement of a sound-absorbing duct on the aerodynamic characteristics and the turbulent noise of a counterrotating fan. Moreover, we performed a theoretical investigation of the turbulent noise reduction in relation to the thickness of sound-absorbing materials and the length of a sound-absorbing duct. The results are summarized as follows. The sound absorbing duct reduces a considerable amount of noise generated from the counter-rotating fan without degrading the aerodynamic characteristics. The sound-absorbing duct set upstream of the fan only reduces the noise radiated to the upstream and has no effect on that radiated to the downstream. The duct set downstream of the fan has an opposite effect. The agreement between predicted and measured values for the noise reduction is satisfactory.